Re: Leakage current test conundrum

[Rich Nute]

Hi Charles:


   a) Does anyone one know the genesis
   of the requirement to lift the Neutral
   AND the Ground simultaneously 
   during a leakage current test??
   (I am referring to UL6500)

While I am not familiar with UL 6500 per se,
I believe I can comment on the lift of the
neutral.

In times past, non-polarized products used a 
single-pole power switch.  Since the products
were non-polarized, the single-pole power
switch could be in the neutral rather than in
the phase conductor.  When the switch is in 
the neutral and is open, the leakage current
essentially doubles compared to the value when
the switch is closed (assuming the switch is
ahead of the line filter).  (Draw the circuit
and you can readily see why the leakage current
doubles.) 

Certainly, leakage current should be measured
(and controlled) for both the ON and OFF
positions of a power switch, especially a
single-pole power switch.

   b) What human body model is appropriate
   for UL6500? Can I use the one in UL1950??

Since you are referencing UL standards, the
correct measuring circuit (i.e, human 
model) is the ANSI circuit (C39.5?).  

In practice, just use an ordinary ammeter.
This will give you a slightly (~5%) pessimistic
measurement.  So, unless you are very close to
the leakage current limit, you need not use the
humand body model for leakage current 
measurements.

For more details on both these questions, see
my article in the Product Safety Newsletter,
Volume 7, Number 1, January-February 1994.

You can download a pdf version of this PSN:

http://www.ewh.ieee.org/soc/emcs/pstc/psn/

The file name is:

94v07n1.pdf


Best regards,
Rich





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Methenamine timed burning tablet

[Rich Nute]

Some fire tests use a methenamine timed burning
tablet.

This tablet is specified in USA government
standards for furniture flammablity testing.  
(See 16 CFR Part 1630.4 or CPSC standard FF 1-70.)
It is also specified in ASTM D2859.  I have also 
seen it used to test fire enclosures
in Europe.

Until February of this year, Eli Lilly supplied
this tablet as Catalog No. 1588.  In February, 
2002, Eli Lilly discontinued the tablet.  
According to Eli Lilly, no other US manufacturer 
has picked up this tablet; they did not know if 
a Euro manufacturer was making the tablet.

I am looking for a source for this methenamine
tablet.  If you can provide some pointers, I would
appreciate it.


Thanks, and best regards from San Diego,
Rich








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minimum distance for PWB inner layers (was creepage)

[Rich Nute]

Hi Tom:


   In 60950, applicable clauses are 2.10.5.3 and 2.10.5.1 (I'm not
   reading genuine IEC/EN 60950, and I'm sorry if I'm wrong) -
   the former states that distance through insulation would be applied
   in such situation, and the latter set minimum thickness of 0.4mm
   for supplementary and reinforced insulation for peak working voltage
   greater than 71V.

Agree.

   However, I think applying minimum distance for pollution degree 1 or
   that for coated PCBs (clause 2.10.6) for inner layers when designing
   PCBs are not bad idea, although both of them wouldn't be applied here.
   The latter set, for example, minimum distance of 1.2mm for reinforced
   insulation for working volgage up to 250V on coated PCBs.

Anyone may, of course, use any distance through insulation
that he chooses, provided the distance is at least 0.4 mm.

However, I would caution against rationalizing the distance 
for conductors on inner layers of printed wiring boards by 
applying criteria that are not in any way applicable to the 
construction.  

These requirements (for creepage distances based on 
pollution degree and for coated printed wiring boards) are 
based on reasonable principles of engineering and physics.  

Requirements for pollution degree 1 creepage distances apply 
to the interface between solid insulation and air insulation,
not to inner layers of multilayer boards where creepage does
not exist.  The inner layer conductors of an ideal multilayer 
board are imbedded in solid insulation.  The requirements for
creepage distances presume a long-term accumulation of 
pollution, which does not occur to the inner layers of a
multilayer PWB.

Likewise, requirements for coated printed wiring boards apply
to coated PWBs, not to inner layers of multilayer boards.
Thin coatings are not of uniform thickness, and bend around
the surface conductors, both of which contribute to highly 
localized voltage stresses on the coating.  The requirements
for coatings account for these localized stresses, which do
not occur to the inner layers of a multilayer PWB.

Rationalizing larger distances by applying these requirements
to inner layers of PWBs is contrary to the basis of the
requirements.  Such rationalization generates confusion among 
designers and among fellow safety professionals.  

Indeed, this thread originated because of just such a
confusion!

If the belief is that the 0.4 mm distance is too small for 
inner layers of PWBs, then a greater distance should be
specified.  But, please don't rationalize the larger 
dimension by applying inapplicable requirements.


Best regards,
Rich





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Re: Creepage on PCB Internal Layers

[Rich Nute]

Hi Chris:


 To me, it's sort of funny in that it just says that the Creepage and 
 Clearance distances do not apply on inner layers of void free PCBs.   
 That's nice; but I can't find where a distance is specified.  I mean, I 
 would think that there should be some minimum distance between an AC line 
 and a 5V SELV line on an inner layer of the board
 
 Our layout guy has a military specification that specifies insulation for 
 300-500V circuits.  It specifies .100 on external layers and .010 on 
 internal layers...which would work out to a factor of 10 reduction between 
 surface layers and inner layers

Ideally, the conductors of inner layers of printed
wiring boards are imbedded in void-free solid
insulation.

For Basic Insulation, there are no requirements for
distance through solid insulation.  

For Supplementary Insulation, IEC 60950 and maybe 
other standards specify a minimum distance of 0.4 mm 
(regardless of voltage) through the solid insulation.

The principal property of any insulation is that of
electric strength.  Electric strength is the maximum
volts per unit distance at which the insulation will
not fail (break down).  The electric strength of 
solid insulations are typically at least two orders 
of magnitude greater than that of air insulation. 
Consequently, the distance between conductors of 
inner layers of a printed wiring board can be quite
close together compared to those conductors on the
surface of the printed wiring board.

One theory of solid insulation failure is that of 
partial discharge in air-filled voids.

The problem of thin solid insulation is that the
volts per unit distance can be quite high.  Recall
that an electric field exists between any two
conductors.  The electric field is defined by a set
of equipotential lines within the solid insulation.  
The thinner the insulation, the closer the 
equipotential lines.  

Air breaks down when the absolute potential exceeds
about 300 V rms AND the potential per unit distance
exceeds about 1500 V rms/mm.

So, if the voltage across an insulator exceeds 
300 V rms, and the equipotential lines are closer 
than 1500 V/mm, then partial discharge can occur 
in a void in solid insulation.  If the partial 
discharge is allowed to continue, then the air in 
the void will break down (arc).  

The heat of the arc will decompose the insulating
material, resulting in a carbon path through the
void.  This effectively shorts out a part of the
solid insulation.  The equipotential lines are
re-distributed, and the remaining solid insulation
has a higher V/mm, at least near the void.  The
higher V/mm can cause other voids to partial 
discharge and arc.  We then have a cascading effect,
with the end-result being a carbon path through the
solid insulation.

(I've assumed the insulating material is organic;
inorganic materials don't necessarily have this
same failure mechanism, which is why many high-
voltage insulators are porcelain.)


Best regards,
Rich








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   Date: Tue, 13 Aug 2002 16:09:32 EDT
   Subject: Re: Creepage on PCB Internal Layers
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   In a message dated 8/13/2002, you write:
   
   
To me, it's sort of funny in that it 

Re: Creepage on PCB Internal Layers

[Rich Nute]

Hi Chris:


 To me, it's sort of funny in that it just says that the Creepage and 
 Clearance distances do not apply on inner layers of void free PCBs.   
 That's nice; but I can't find where a distance is specified.  I mean, I 
 would think that there should be some minimum distance between an AC line 
 and a 5V SELV line on an inner layer of the board

To answer this comment, we need to look at what 
a creepage is and its role in the scheme of the
product.

Almost all product constructions employ solid 
and air insulations, both in parallel and in
series, between conductors.  We call solid 
insulation solid insulation.  We call air
insulation clearance.

We call the interface between solid insulation
and air insulation creepage.

Note that solid insulation and air insulation
are truly electrical insulations.

Creepage is NOT an insulation.

Creepage is not a material. It is simply a 
surface at which solid and air insulations 
meet.

The surface of solid insulation is subject
to deposition of airborne pollution.  
Typical products provide little or no control 
of airborne materials to prevent deposition 
of the polluting material onto the surface 
of a solid insulating material.

Polluting material is a solid, uncontrolled 
(i.e., not a known insulating) material in 
parallel with the solid (and air) insulations.  
The polluting material bridges the solid 
insulation, and therefore could jeopardize 
the safety function provided by the solid 
insulation.

When sufficient polluting material accumulates
on the surface of the solid insulation, the
voltage across the insulator and the pollution
causes micro-arcs in the pollution.  These
micro-arcs are high temperature, and cause 
thermal decomposition of the surface of the 
solid insulation.  When organic materials 
decompose, they free up the carbon atoms,
leaving a tiny carbon resistor on the surface
of the solid insulation.   

Each tiny carbon resistor is in parallel with
the adjacent solid insulation beneath the
surface of the solid insulation.  So the tiny
carbon resistor is shorting out a small part
of the surface of the solid insulation.

This phenomenon is known as treeing due to
the tracking pattern of the carbon path on the 
solid insulation surface.  

To account for the effect of pollutants on
the surface of organic solid insulations, we
require the creepage distance to be larger
than the air distance (clearance).  Because 
some materials are more resistant to tracking 
across the surface, the creepage distance is 
a function of the relative tracking index 
characteristic of the insulation.

Where a solid insulation is not subject to
pollution, there is no requirement for a
creepage distance.  Many standards specify 
that a hermetically sealed assembly is not
subject to creepage distance requirements.

Likewise, the inner layer of a multi-layer
printed wiring board is not subject to 
pollution and therefore is not subject to
creepage distance requirements.


Best regards,
Rich




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Re: Marking Languages for Canada

[Rich Nute]

Hi Kris:


   The markings on the power supply are: (also as per UL file)
   Date code
   model number
   electrical ratings
   Class 2 transformer
   
   The additional markings are:
   CAUTION, Risk of electric shock and Dry location use only and Do not
   expose to liquid, vapor, or rain
   
   As I said before: only in english, not in french, but approved for:
   - UL1310, 4th ed. 1994 , including revisions through April 4th, 2000 and 
   - CAN CSA C22.2. No. 223
   by UL and having the C UL US logo.
   
   As I understand from some people: I am allowed to import my product with
   this power supply in the US but not in Canada.
   Is this interpretation still correct?
   If YES, I'm probably one of the many mftr's in this nice situation.

Having checked the 

Official Languages Act, 
Consumer Packaging and Labelling Act, 
Consumer Packaging and Labelling Regulations,
Standards Council of Canada,

I have concluded that the language requirement is
imposed by CSA in their the standard, CAN CSA C22.2 
No. 223.

As near as I can tell, none of the above references 
requires a product warning be marked in both official
languages.  (However, the product identifier must be
marked in both French and English, at least on the
packaging.)

Based on the information so far, my guess is that UL
overlooked the language requirement of 223.

Having said that, Kris asks about importation into
Canada.  The cUL mark qualifies the product for 
importation.  The fact that UL *may* have made an
error in evaluating the power supply against the
standard does not negate the requirement that the
unit be certified.  All we can say is that an error
*may* have been done in the evaluation.

I don't believe the issue is importation, but 
connection to the public power network.  The local
hydro authorities are the folks who require safety
certification of electrical equipment.  I'm not 
sure that the hydro authorities have control over
Canadian customs for inspection of cert marks.  I
know that not all electrical equipment is required
to be safety certified.

If you are looking for advice as to what action 
you should take...

I would notify the power supply manufacturer that
UL *may* have made an error in that 233 requires
warning markings be in both French and English.  I
would ask them to verify that with UL, and take
any corrective action that may come from such an
inquiry.  In the meantime, I would import the 
units into Canada as the units do have the correct
safety certification mark.

I feel it is important to separate the issues so
that they can be dealt with.  I would not self-
impose a no-import position based on our/your
reading of the requirements.  If you were not 
familiar with the language rules, and only 
familiar with the certification rules, then the
unit is certified and is acceptable in Canada.
This is really no different than a constructional
error that might exist within the power supply
where you could not see it.  I suggest you ask
the question of your supplier, and load the 
monkey onto his back.


Best regards,
Rich



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Re: Marking Languages for Canada

[Rich Nute]

Hi Kris:


   A product from manufacturer X is delivered with a (direct plug-in) power
   supply from mftr Y to customers in Canada.
   Regarding the power supply, it is UL approved + UL listed and has the UL
   marking with C and US.
   The warning marking on the power supply is only in english.
   Question:
   Is it required that the marking is also in french (en francais) when the
   product is sold in Canada?
   If YES,  is there any reason why the marking is not in both languages if
   there is an approval for US+Canada?

Your question generates a number of questions:

1)  Is the warning marking required by the
applicable standard, or did the manufacturer
voluntarily apply the warning?

(We, too, use UL/cUL direct-plug-in power 
supplies.  To my recollection, there are no
warnings on our power supplies.  So, I 
question whether or not the warning is 
required by the standard.) 

2)  Is the warning marking required by the US 
standard?  If yes, then the warning is for
the US and not for Canada.

3)  Is the warning marking required by the
Canadian standard?  If yes, then does the
standard require the warning marking to be 
in French?

In my experience with UL, UL requires compliance
to the standard.  

If the standard does not require a specific 
language, then UL does not require a specific 
language.  

If language is a requirement of a national law
(and not a standard), then compliance to that
law applies to the manufacturer, not to UL.

UL does not require compliance to national laws 
of countries other than the USA.  However, when
UL is aware of a law that might specify
requirements in addition to those in the 
standard, UL will call this to the attention of 
the manufacturer, usually by letter.  

This is a very common UL practice, especially
when UL issues a CB Report.  The cover letter 
to the CB Report will often mention that the
language of the instruction manual, for example,
was not evaluated.  The cover letter will state
something to the effect that certain countries 
may require the language of the manual to be the 
local langauge.  (To my knowledge, IEC standards
do not specify language except in the very
broadest terms, e.g., the language of a warning
must be in a language acceptable to country
authorities.)


Best regards,
Rich





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Re: Compliance Primer

[Rich Nute]

It has been some time since I have had to explain
or justify product safety activity to a high-level 
manager-type.  As others have said, it is fraught 
with difficulties.

Success depends on first determining the mindset 
of the person asking the question.  I believe I
would first ask a number of questions to find out
where the person is coming from, why he is asking,
and what his objective is in learning about 
product safety activity.  Then, I would enter into
a conversation where there is a lot of back-and-
forth so that I could continuously read the person 
as to what he wants to know.

For a business, product safety, EMC, and other
regulatory or compliance activity usually represent 
a cost without a benefit, a cost without an 
associated income.  No wonder management will 
occasionally inquire as to what happens in the
compliance department.

There is no income derived or guaranteed from 
having a set of bumper-stickers on your product.  
In some cases, those bumper-stickers may comprise 
a passport for the product, but in themselves, they 
generate no revenue.  Indeed, some organizations 
can and do get by without the bumper-stickers, but 
usually not for the long term.

Making a product safe, or complying with EMC and
other regulatory issues can prevent fines, and
can prevent a government-ordered product recall.

One management question is:  How much money do I
spend to prevent a recall?  And, does spending 
that money guarantee no recall?

As a general rule, the cost of a recall exceeds
the per-unit profit.  Its a money-loser.

And, even the best of us cannot foresee every
product safety event.  A product safety recall
is almost inevitable at least once in the lifetime
of a company.  Consultants universally advise that
each company should have a product recall plan in
place before the recall.

I address the question of Why product safety? 
by stating that a company has a moral (as well as
legal) obligation not to injure its customers.

Depending on mindset, management may only agree
with this principle for major injuries, not for
minor injuries (and management decides which 
injury is major and which is minor).

Do I sound pessimistic?

Scott raised another issue in that we don't have
such things as primers on compliance and 
similar subjects.  Nor do we have papers on more
complex subjects (in the field of product safety). 

Some years ago, we had the Product Safety 
Newsletter.  We used this newsletter as a means
for publishing papers on safety topics (although
none was published on this subject).  

With thanks to Jim Bacher, many of the old PSNs 
are now available for download from: 

http://www.ewh.ieee.org/soc/emcs/pstc/psn/

While the IEEE EMC society has several 
publications, the product safety folks have 
nothing.  We need to develop both authors and a
publication medium.  We have the medium, the
mindcruiser web site.  While it is not perfect,
it is usable.

http://ieeepstc.mindcruiser.com/

We intend this web site as an electronic version 
of the PSN.  But, we haven't yet developed a 
cadre of authors who would post papers to this 
web.  

This is an open invitation to post papers of 
general interest to the product safety, emc, 
and telecom communities to this web site.  

We're looking for the equivalent of an editor 
to oversee this function.  Volunteers please 
contact me or Jim Bacher.


Best regards,
Rich







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Re: medical-grade transformers

[Rich Nute]

Hi Ged:


   In transformers with REINFORCED INSULATION or DOUBLE INSULATION the
   insulation between 1ary and 2ary windings shalll consist of -
   -  1 insulation layer having thickness at least 1mm, or
   -  at least 2 insulation layers with total thickness not less than 0.3mm, or
   -  3 layers provided each combination of 2 layers can withstand the
   dielectric strenght test for REINFORCED INSULATION
   
   Can anyone explain ther rationale behind these requirements ?

Thin insulation is presumed to be subject to 
pin-holes.

If 1 layer of insulation, then the insulation
must be thick rather than thin, hence 1 mm 
thickness.  We presume no pin-holes in thick
insulation, and the insulation comprises
reinforced insulation.

If 2 layers of thin insulation, then presume
a pin-hole in one layer, and that the pin-hole
will not overlay a pin-hole in the other layer.  
If that pin-hole is 0.15 mm, then we know that 
the air in that pin-hole will at least withstand 
the mains voltage.  Therefore, the construction
still has two insulations in series, one being
the 0.15 mm air of the pin-hole, the other being 
the 0.15 mm thick solid insulation.  The 2 
insulations in series comprise double insulation.

If 3 layers of thin insulation, then presume 
a pin-hole in one layer, and that the pin-hole
will not ovelay a pin-hole in the adjacent
layer.  The remaining 2 layers comprise double 
insulation.  

(In IEC 60960, the requirement for 1 layer in
a 2-layer system is 0.4 mm thickness.)


Best regards,
Rich


ps:  I'm not certain of my explanation for 2
 layers.  This rationale came from 60950,
 and I have applied it to 60601.





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Re: Bonding Impedance

[Rich Nute]

Hi Ron:


Two years ago, Jim Bacher presented my paper, 

Equipotentiality and Grounding -- Deriviation 
 of Grounding Resistance for Equipment 

at the IEEE EMC Symposium Product Safety Workshop.

This paper is available for download from the IEEE
PSTC Mindcruiser web site:

http://ieeepstc.mindcruiser.com/

On the left side, click on Infopieces, then 
Latest postings.  Scroll down to the bottom, and 
click on the title.  Then, click on download.

This paper discusses the 0.1 ohm value for equipment
and how this value performs in terms of prevention of 
electric shock.

You will see that values up to 0.5 ohm will also
provide the same degree of protection against 
electric shock.

As for the 25-amp test current, I have no answer for
the rationale.  We do know that a circuit-breaker
will operate for no more than 1 minute at twice rated
current.  So, for a 15-amp branch circuit, 30 amps
would be a rational test current (and is the required 
current under CSA standards).

I have conducted tests that show that the 25-amp test
current will not show any flaws or defects in the
current path that an ohmmeter will not show.  The 
test will not find a loose connection!  And, a short
length of #22 AWG wire will pass the test.  I believe 
the 25-amp test is of no value.  

The high-current circuit path must be visually 
inspected for robust construction techniques.  Wires 
must be at least 18 AWG.  Screwed connections must 
use lock-washers that provide gas-tight connections 
between metal parts.  Crimp connections must be 
according to crimp parts manufacturer's instructions.  
Etc.  


Best regards,
Rich






   From owner-emc-p...@majordomo.ieee.org Wed Jul 24 16:17:44 PDT 2002
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   Hello all,
   
   I recent issue has come up for me regarding why some safety standards 
 specify 100 milliohms as the acceptable impedance for a protective earth 
 ground bonding test. I am curious as to what the rationale is for this 
 specific value and why 25 amps is chosen as the default test current. 
   
   Regards,
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Admin message: posted advertisement

[Rich Nute]

Today, a message was posted that violates IEEE 
emc-pstc rules against advertising.  Here is 
our rule:


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The listserver is provided as a service by the IEEE, whose policies
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Richard Nute
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c/o Hewlett-Packard Company
San Diego

Tel: 858-655-3329
e-mail:  ri...@ieee.org






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Product safety job at Hewlett-Packard San Diego

[Rich Nute]

Hewlett-Packard Company San Diego site is looking
for a high-level product safety engineer.

The major job function is that of advising product
designers on 

1)  innovative safeguards for new products, and
2)  safety certification requirements for new
products.

The ideal candidate would be capable -- in time --
of taking over the HBSE course support, and would 
be capable -- in time -- of taking over the 
technical advisor position to TC 108.  Note that I
used the word capable; the job may never include 
these functions.  I used this description to give
an idea of the term high-level.

If you are interested in this job, go to the HP
jobs web site:

http://www.jobs.hp.com/

Click on search jobs in hp.

In the next window, scroll to the bottom and 
enter:

808093

in the keyword box.  This will bring up the job
data.  At the bottom of this page, click on Safety
design engineer.  This will bring up the complete
job description.

If you want to apply, follow the instructions on
the web pages.  You can also send a resume directly
to me.

If you want more details on the job, I have a 
detailed job description I can send you.  I urge
you to evaluate yourself against this detailed job
description before applying.

After we review your resume, the next step for
qualified candidates will be a phone screen 
interview.  Successful phone screen candidates
will be subject to further interviews, probably
by a team of HP engineers.

Feel free to call or e-mail me about the job.


Best regards,
Rich

Tel: 858-655-3329
e-mail:  rich_n...@hp.com










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Re: TOUCH CURRENT LIMIT

[Rich Nute]

Hi Xing Weibing:


 We can read from table 5A of IEC60950:1999:
 
 Touch current limit for accessible parts and circuits not connected
 
 to protective earth: 0.25 mA
 
 question 1 : How does it (0.25mA) come from and what  it is based on?

For grounded (Class I) equipment for the home, the UL leakage current 
is 0.5 mA.  (This is NOT a 60950 limit.)

UL reasoned that if 0.5 mA arises from both real and stray capacitance,
then, for double-insulated (Class II) equipment, the current should be
one-half of 0.5 mA because the standard requires the same insulation
to be applied again (i.e., the second half of double insulation), thus 
halving the capacitance.  Half of the capacitance results in half of 
the current, therefore 0.25 mA.

UL carried this concept from their generic double-insulation standard
into IEC 60950.

The UL reasoning is faulty because in real life the two capacitances
need not be equal.  And, this is certainly not the case for reinforced
insulation.

Furthermore, the current, 0.5 mA, is deemed acceptable in terms of 
the safe current that can be applied to the body.  

Therefore, the 0.25 mA limit is not related to the effects of the 
current on the body, but presumes a product construction comprised of 
double insulation where each insulation system provides exactly the 
same capacitance.



Best regards,
Rich


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EMC-PSTC now functional

[Rich Nute]

The EMC-PSTC listserver was down from Friday
afternoon until today, Monday, late morning
(Pacific Daylight Time).

If you attempted to post a message during this
time, the message was lost.  Please re-send.

We apologize for the inconvenience.

If you have any questions or comments about 
this outage, please contact me or one of the
other administrators off-line.


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Chief Administrator, IEEE emc-pstc listserver

 


   Richard Nute
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Changing our safety standards (was 0.1 uF discharge)

[Rich Nute]

Hi Gert:

   I also tried the discharge between two fingers, and found the result to
   be unpleasant at least.
   Time to change standards...

Now we must ask the question:

Is the purpose of the standard to prevent 
injury or to prevent an unpleasant experience?

I presume that you would want to adjust the various
values of current (in our standards) so that either
there is no sensation, or the sensations of the
various conditions are the same.

If you try the same test with 3.5 mA leakage 
current, I believe you will also find the result
to be unpleasant.

Here's another test:

Using a 2-wire external power supply with a
barrel-type output connector (such as the type
power supply used with a laptop computer), rub 
the metal barrel lightly on the inside of your 
forearm.

I believe you will find this unpleasant.

Simply, we cannot eliminate the sensation of 
electric current -- from an ac voltage source --
through a capacitance.  Somewhere, someone will 
sense the current.  We *may* be able to eliminate
the sensation of electric current from a charged
capacitor by selecting small-value capacitors.

But, is that the purpose of our safety standards --
to eliminate sensation of electric current?  What 
should be the body response criterion used in
safety standards?


Best regards,
Rich




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Changing our safety standards (was 0.1 uF discharge)

[Rich Nute]

Hi Gert:

   I also tried the discharge between two fingers, and found the result to
   be unpleasant at least.
   Time to change standards...

Now we must ask the question:

Is the purpose of the standard to prevent 
injury or to prevent an unpleasant experience?

I presume that you would want to adjust the various
values of current (in our standards) so that either
there is no sensation, or the sensations of the
various conditions are the same.

If you try the same test with 3.5 mA leakage 
current, I believe you will also find the result
to be unpleasant.

Here's another test:

Using a 2-wire external power supply with a
barrel-type output connector (such as the type
power supply used with a laptop computer), rub 
the metal barrel lightly on the inside of your 
forearm.

I believe you will find this unpleasant.

Simply, we cannot eliminate the sensation of 
electric current -- from an ac voltage source --
through a capacitance.  Somewhere, someone will 
sense the current.  We *may* be able to eliminate
the sensation of electric current from a charged
capacitor by selecting small-value capacitors.

But, is that the purpose of our safety standards --
to eliminate sensation of electric current?  What 
should be the body response criterion used in
safety standards?


Best regards,
Rich




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Re: Question: Discharge capacitance 0.1 uF

[Rich Nute]

Hi Tom:


   So, for voltage up to 450V d.c. (i.e. up to 318V a.c.), capacitor
   up to 0.1uF will become a Limited Current Circuit, hence the voltage
   is not Hazardous Voltage (1.2.8.4) - no additional condition would
   be required for the capacitor connected to the primary circuit.

Electric shock (or electrically-stimulated
sensation) is a function of BOTH voltage 
and current.  For an electric shock to 
occur, the source must exceed, say, 30 V
rms and 0.5 mA rms.

As a general rule, we say that any voltage
not exceeding 30 V rms is not hazardous,
regardless of current.  We identify this 
voltage as ELV or SELV.

Likewise, we say that any current not
exceeding 0.5 mA rms is not hazardous,
regardless of voltage.  We identify this 
current as Limited Current.

Furthermore, Limited Current addresses
capacitors charged to voltages exceeding 
42.4 V dc.  In this case, we control the
capacitance, or the charge, or the stored
energy, and deem the transient discharge 
current equivalent to a steady state 
current.

In the case of an across-the-line 
capacitor installed on the supply side of
the power switch, the capacitor can be 
charged to the peak of the line voltage 
if the plug is disconnected at the peak
of the line voltage.

As has already been noted in this forum,
the discharge is not a pleasant sensation,
and may very well result in an involuntary
reaction, depending on the individual.

By comparison, the sensation from 3.5 mA
steady-state leakage current (allowed by
some standards for Class I equipment) is
also not a pleasant sensation and may
result in an involuntary reaction.

The initial discharge current of a 
charged capacitor is limited only by the
body resistance.  Regardless of capacitance
value, the initial current is the same.
The effect of capacitance value  is the 
duration of the current.  If the current 
has a short duration, the body will not 
sense it.  Assuming all bodies have the
same resistances, then the duration of the 
current is a function of the value of the 
capacitance.

The standards happen to draw the line at 
0.1 uF.  Any value less than 0.1 uF is
deemed acceptable.  Any value greater than
0.1 uF is deemed unacceptable.  

Consequently, for capacitors less than 0.1
uF, the discharge time is short and the
body is less likely to sense the current.
If the capacitor is more than 0.1 uF, the
discharge time is long, and body is likely
to sense the current.

Another factor, of course, is the magnitude
of the voltage to which the capacitor is
charged.  If the capacitor is charged to
the peak of the 230 V mains, the discharge
is almost always sensed.  The same 
capacitor charged to the peak of a 120 V
mains is barely detectable.  The lower the
voltage, the lower the initial current.

The fact that the initial discharge 
current is limited only by body resistance
also applies to leakage current through 
Y capacitors.  At the moment the body is
inserted into the circuit, the initial
current is limited only by the body
resistance.  Thereafter, the current is
the steady-state current due to the
capacitive reactance.  However, the Y
capacitors are much smaller in value,
and therefore the discharge is much
shorter in time.  (You can test this by
putting a switch in series with the leakage
current; if the switch closes at the peak
of the line voltage, you will feel an
initial sharp sensation.)

By the way, the sensation of leakage 
current from a Y capacitor is greater 
than the sensation of leakage current 
from a resistor whose value is equal to 
the value of capacitive reactance.  This
is due to the same phenomenon, namely the
initial discharge of the capacitance.


Best regards,
Rich




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Re: Question: Discharge capacitance 0.1 uF

[Rich Nute]

Hi Tom:


   So, for voltage up to 450V d.c. (i.e. up to 318V a.c.), capacitor
   up to 0.1uF will become a Limited Current Circuit, hence the voltage
   is not Hazardous Voltage (1.2.8.4) - no additional condition would
   be required for the capacitor connected to the primary circuit.

Electric shock (or electrically-stimulated
sensation) is a function of BOTH voltage 
and current.  For an electric shock to 
occur, the source must exceed, say, 30 V
rms and 0.5 mA rms.

As a general rule, we say that any voltage
not exceeding 30 V rms is not hazardous,
regardless of current.  We identify this 
voltage as ELV or SELV.

Likewise, we say that any current not
exceeding 0.5 mA rms is not hazardous,
regardless of voltage.  We identify this 
current as Limited Current.

Furthermore, Limited Current addresses
capacitors charged to voltages exceeding 
42.4 V dc.  In this case, we control the
capacitance, or the charge, or the stored
energy, and deem the transient discharge 
current equivalent to a steady state 
current.

In the case of an across-the-line 
capacitor installed on the supply side of
the power switch, the capacitor can be 
charged to the peak of the line voltage 
if the plug is disconnected at the peak
of the line voltage.

As has already been noted in this forum,
the discharge is not a pleasant sensation,
and may very well result in an involuntary
reaction, depending on the individual.

By comparison, the sensation from 3.5 mA
steady-state leakage current (allowed by
some standards for Class I equipment) is
also not a pleasant sensation and may
result in an involuntary reaction.

The initial discharge current of a 
charged capacitor is limited only by the
body resistance.  Regardless of capacitance
value, the initial current is the same.
The effect of capacitance value  is the 
duration of the current.  If the current 
has a short duration, the body will not 
sense it.  Assuming all bodies have the
same resistances, then the duration of the 
current is a function of the value of the 
capacitance.

The standards happen to draw the line at 
0.1 uF.  Any value less than 0.1 uF is
deemed acceptable.  Any value greater than
0.1 uF is deemed unacceptable.  

Consequently, for capacitors less than 0.1
uF, the discharge time is short and the
body is less likely to sense the current.
If the capacitor is more than 0.1 uF, the
discharge time is long, and body is likely
to sense the current.

Another factor, of course, is the magnitude
of the voltage to which the capacitor is
charged.  If the capacitor is charged to
the peak of the 230 V mains, the discharge
is almost always sensed.  The same 
capacitor charged to the peak of a 120 V
mains is barely detectable.  The lower the
voltage, the lower the initial current.

The fact that the initial discharge 
current is limited only by body resistance
also applies to leakage current through 
Y capacitors.  At the moment the body is
inserted into the circuit, the initial
current is limited only by the body
resistance.  Thereafter, the current is
the steady-state current due to the
capacitive reactance.  However, the Y
capacitors are much smaller in value,
and therefore the discharge is much
shorter in time.  (You can test this by
putting a switch in series with the leakage
current; if the switch closes at the peak
of the line voltage, you will feel an
initial sharp sensation.)

By the way, the sensation of leakage 
current from a Y capacitor is greater 
than the sensation of leakage current 
from a resistor whose value is equal to 
the value of capacitive reactance.  This
is due to the same phenomenon, namely the
initial discharge of the capacitance.


Best regards,
Rich




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Re: safety testing in the USA

[Rich Nute]

Hi Rob:


   I am in discussions with a potential supplier of IT equipment, Its our 
 usual policy to request testing to a listed standard
   such as UL 60950 for safety in North America.
   
   The supplier has replied that this is not mandatory.
   
   Is he correct? what  compels safety testing for IT equipment in this 
 geography? is it mandated by law?

I am going to interpret your statement of safety
testing as meaning safety certification.

In the USA, there are two sets of regulations that
are mandated by law:

1.  Workplace regulations (OSHA).

2.  Electrical installation regulations (NEC).

Between these two sets of regulations, almost 
every sales situation is covered.

Each of these has two alternatives:

1.  Test each unit in place.

2.  Third-party certification.

Technically, your supplier is correct.  Third-party
safety certification is NOT mandatory (because 
testing each unit in place is the alternative).

Practically, your supplier is wrong.  Third-party
safety certification avoids testing each unit in
place.

In the USA, governments cannot mandate testing by
a private party (which is the case of third-party
safety certification houses).  Consequently, the
regulations promulgated under various laws prescribe
testing in place, with an alternative of third-
party certification to a national standard, i.e., 
an ANSI standard.


Best regards,
Rich




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Re: safety testing in the USA

[Rich Nute]

Hi Rob:


   I am in discussions with a potential supplier of IT equipment, Its our 
 usual policy to request testing to a listed standard
   such as UL 60950 for safety in North America.
   
   The supplier has replied that this is not mandatory.
   
   Is he correct? what  compels safety testing for IT equipment in this 
 geography? is it mandated by law?

I am going to interpret your statement of safety
testing as meaning safety certification.

In the USA, there are two sets of regulations that
are mandated by law:

1.  Workplace regulations (OSHA).

2.  Electrical installation regulations (NEC).

Between these two sets of regulations, almost 
every sales situation is covered.

Each of these has two alternatives:

1.  Test each unit in place.

2.  Third-party certification.

Technically, your supplier is correct.  Third-party
safety certification is NOT mandatory (because 
testing each unit in place is the alternative).

Practically, your supplier is wrong.  Third-party
safety certification avoids testing each unit in
place.

In the USA, governments cannot mandate testing by
a private party (which is the case of third-party
safety certification houses).  Consequently, the
regulations promulgated under various laws prescribe
testing in place, with an alternative of third-
party certification to a national standard, i.e., 
an ANSI standard.


Best regards,
Rich




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double-pole switching and fusing

[Rich Nute]

Hi John:


   I read in !emc-pstc that Crabb, John jo...@exchange.scotland.ncr.com
   wrote (in B6CD5947CF30D411A1350050DA4B75FF03C2341C@sgbdun200.scotland.n
   cr.com) about 'South Korean Power System', on Wed, 20 Feb 2002:
   Typically I would expect IT equipment to be safe if line and neutral
   were reversed, except for the issues relating to a single pole
   disconnect device and fusing.
   
   See Tables 2E and 2F in IEC/EN60950:2000. Double-pole switching and
   fusing is almost always necessary in practice.

Tables 2E and 2F are invoked only by the Note 2
to Sub-clause 2.7.4.

A Note is informative, not a requirement.

Note 2 specifically addresses the situation where
protection devices are integral to the equipment.
It further states that the examples are not 
necessarily valid for protective devices in the
building installation.

For earth faults in plug-and-socket-connected 
equipment, the equipment need not include double-pole
fusing as the protective device in the building
installation provides the earth-fault protection.
This is the basis for the robustness requirements
of the equipment protective earthing circuit.

So, in practice, double-pole fusing is seldom
necessary.

Double-pole switching, on the other hand, is almost
always necessary in practice.  The exception is 
where both the supply and the equipment connection
to the supply are polarized, in which case a single-
pole switch in the phase conductor is acceptable.


Best regards,
Rich




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Re: Using PCB traces as transient voltage suppressor

[Rich Nute]

Hi Chris:


   1.  Can anyone else verify the breakdown voltage of 1Megavolt/meter for
   air?  Seems different than what I can remember; but I don't have a
   reference handy.  It also seems to me that this would be very dependent
   upon humidity and pollution degree?

In a separate message, I will send you the air
breakdown voltage curves from IEC 664.

Humid air has a very slightly higher electric
strength than dry air.  (Water vapor, a gas,
has quite different properties than water as 
a liquid.)  I believe air temperature has more 
effect on electric strength than does humidity.

Pollution affects the electric strength of
the scheme because it is deposited on the 
electrodes.  This tends to reduce the electric 
strength between the two electrodes.  This is 
a larger effect than humidity.

The biggest factor affecting the electric 
strength of air is air pressure.  Pollution,
temperature, and humidity have relatively 
low effects.


Best regards,
Rich




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Re: Safety of Indicating LEDs

[Rich Nute]

Hi Scott:


   The scope of EN 60825-1 says LEDs are included whenever the word laser
   is used, meaning they are to be evaluated the same way lasers are.  Does
   this apply to status indicating LEDs (non-lasing)?  If so, are
   manufacturers expected to test every status-indicating LED on the
   product as if it were a laser?

Yes.

At least one certification house demands measurement 
data for each indicator LED.  Emission class must be
identified on the product or in the manual.  (Note
that emission class is determined under single-fault
conditions in the driving circuit.)

However, in practice, other certification houses use 
a get-out for indicator LEDs.  Usually this is in the
form of a not tested, but may be required by some
authorities statement in the report.  

Most indicator LED manufacturers do not know of EN 
60825-1, and have no idea how to test.

Measurement is not easy, especially the determination
of the aperture.

Most indicator LEDs will open before achieving Class
2 emission levels.

The above does not apply to automotive LEDs or to
traffic signal LEDs.


Best regards,
Rich




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Re: South Korean Power System

[Rich Nute]

Hi Ed:


With few exceptions, most power distribution
systems have one pole of the supply, the 
neutral, grounded.  (Indeed, the definition 
of neutral for single-phase systems is the 
grounded conductor.)

In the IEC scheme of the world, a power
distribution system where the neutral is
grounded is known as a TN or TT system.

The first letter identifies the grounding
scheme for the neutral wire.  The second
letter identifies the grounding scheme for
the protective wire.

T  =  terra (a ground rod)
N  =  neutral

In North America, the scheme is TN.  The
neutral is connected to a ground rod at
the service entrance.  The protective wire
is connected to the neutral in the breaker
panel.  

I believe Korea uses the TN system.  So,
at any socket-outlet, one pin will be at
the phase voltage, 220, and the other pin
will be at the neutral voltage, 0.

Polarity is a separate issue.  By polarity,
I mean that the neutral identification is
maintained through the plug/socket-outlet
scheme.

Polarity is not maintained where the plug
can be reversed in the socket-outlet.  The
SCHUKO plug is a plug that can be reversed
in the socket outlet.  Not only that, but
the socket-outlet is symmetrical, so the
wiring to the socket-outlet cannot be such
that the neutral is always wired to the same
pin.

Among the world's plugs/socket-outlet 
combinations, relatively few maintain the
polarity through the system.  These are:

the British 13-A plug/socket-outlet in
UK, Ireland, Hong Kong, Singapore, etc.
the old British 15-A plug/socket outlet
in South Africa and India, etc.
the Australian plug/socket-outlet
the North American grounding plug/socket-outlet
the North American two-wire plug/socket-outlet
with one wide blade

The French plug/socket-outlet with its 
grounding pin scheme COULD be polarized, but 
is not so wired.  Likewise, the Danish, 
Chilean, and Chinese plug/socket-outlet 
could be polarized, but I cannot say if they 
are.

Polarized plugs and socket-outlets always
bear markings indicating the pole of each
pin.  For North American plugs and socket-
outlets, the white or silver-colored screws
or terminals are the neutral pins.  Other
plugs and socket-outlets bear molded in 
letters such as L, N, E or PE or G.

Virtually all safety standards include the
requirement that the neutral wire within the
equipment shall be treated as if it was at
mains voltage rather than at zero voltage.
This is because, in many installations, 
errors may occur in the wiring of the socket-
outlet.


Best regards,
Rich






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Japan mains frequency

[Rich Nute]

   A good source is http://kropla.com/electric2.htm
   http://kropla.com/electric2.htm .

   According to that site: 
   Japan100V50/60 Hz*   

   *Eastern Japan 50 Hz (Tokyo, Kawasaki, Sapporo, Yokohoma, and Sendai)
   *Western Japan 60 Hz (Osaka, Kyoto, Nagoya, Hiroshima)

According to a history told to me by a GE old-timer...

Japan has two frequencies because, in the early days
of electric power in Japan, Western Japan bought its
first generator from General Electric (60 Hz), and
Eastern Japan bought its first generator from Russia
(50 Hz).

So, this is an easy way to remember what part of the
country is 60 Hz and what part is 50 Hz:  

The western part is closest to North America (60 Hz), 

and 

the eastern part is closest to Russia (50 Hz).

(However, the cities listed above seem to be in the
opposite parts of Japan than as listed.)


Best regards,
Rich





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Admin message -- format of posted messages should be plain text

[Rich Nute]

A request from your list administrators...

   Except... I (and other classic Compuserve users) won't see these messages
   at all, unless and until they appear in someone else's REPLY in a form
   Compuserve can send. It LOOKS great - but please don't send HTML to the
   list.

Cortland is not alone.  Some mail systems simply don't 
show messages in formats other than plain text or ASCII.
UNIX mail (which I use) is another scheme that has 
difficulty displaying some message formats.

For this reason, we ask that all messages to the listserver
be posted in plain text/ASCII mode.

Please set your mail program to send messages to the listserver
in plain text or ASCII mode, not in rtf, htm, or html mode.  

(If you need help in doing this, please contact your local IT 
folks, or send e-mail to Jim Bacher, our e-mail admin guru.)

In Outlook, when in the reply (or send) mode, click on format 
and plain text before sending.  

(If Outlook won't let you change the format of a reply, then 
initiate a new message and copy the original text into your 
reply.)

In Outlook, if you have emc-pstc in your address book, select
the address, 

click on properties, 
click on smtp-address,
click on send options, 
click on I want to specify the format...
click on MIME and plain text.

This will automatically set your postings to plain text (ASCII)
mode.

If you want to discuss this request, please do so off-line so
that we don't contaminate our technical discussions with admin 
stuff.


Thanks for observing this protocol,
Rich






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Re: OEM

[Rich Nute]

Hi Richard:


   We may soon be privately labeling our products for a particular customer,
   and I have a question concerning the EU Declaration of Conformity. We will
   be shipping products to our customer's distribution center, and they will be
   responsible for sales to their customers. By shipping the product to our
   customer's distribution center, have we placed the product on the market and
   must therefore issue a Declaration of Conformity?

When we buy products from an OEM (whether or not
they go to our distribution center), we ask the
OEM for their DoC (and copies of reports supporting
the DoC).  

Using the OEM DoC (and reports), we issue our own 
DoC as if we designed and manufactured the product.

(If we have all the applicable reports, then we can 
issue our own DoC regardless of whether the OEM has 
issued a DoC.)

This doen't answer your question as to whether 
shipping the product to your customer's distribution
center constitutes placing the product on the market.

However, in using this process, the question does not
arise as a DoC has been issued by both organizations.

I guess the answer is:  If you issue a DoC, then you
have met your obligation.  If you do not issue a DoC,
then you may or may not have met your obligation.

I doubt your customer would want to ship your DoC 
with his product.


Best regards,
Rich




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Re: My subscription may have terminated

[Rich Nute]

Hi Dan:


   I have received no emails today.  Did my subscription cease?

No, your emc-pstc subscription is not terminated.

Today is the most unusual day in the history of
emc-pstc in that there were no postings to the 
listserver between 5:30 PM PST Tuesday and 11 AM 
PST Wednesday.

I, too, was a bit alarmed that the server might
be down or there was some other problem.  We 
checked, and everything is in working order.  We've 
simply had no postings.

Today, up to the time of your message, there was 
only one topic posted to the listserver.


Best regards,
Rich
Administrator, emc-pstc listserver




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Re: Flammabilty requirement for cloth used on loudspeaker / UL6500

[Rich Nute]

Hi Pierre:


   Unfortunately, Hexamine is temporarely unavailable from IMSPLUS, and,
   furthermore, this product cannot be sold outside the US.
   
   If somebody knows another source, he's welcome !

I did a Google search and found at least one more source:

http://www.omahas.com/store/commerce.pl?product=Messkits

I believe there are some Euro sources in the Google results, 
but I did not check all the listed sources.


Good luck and best regards,
Rich



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Re: Flammabilty requirement for cloth used on loudspeaker / UL6500

[Rich Nute]

Hi Pierre:


   Reading the standard, I understand that the cloth used on the front of the
   loudspeaker (external to fire enclosure) has to be tested according the
   tablet test (see Table 13).

As I recall (from the UL standard), the tablet is 
hexamine.

I have purchased tablets from:

IMS Plus (International Military Surplus)
http://www.imsplus.com/ims28.htm

I purchased the tablets on-line.  My first order 
was lost, but they sent the second order without 
charging me for the lost order. 

   Do you know this test ?

No.

(Some years ago, I required some means of igniting
something.  I did some research of the UL standards
and found that UL used the hexamine tablet for 
igniting speaker cloth.  Since the tablet was an
established source of ignition, I've used it since
then whenever I want to test for ignition of some-
thing.)

   Do you know if this information can be provided by the cloth manufacturer ?

You might check to see if the cloth is UL-Recognized.
If yes, then the cloth manufacturer submitted the
cloth to UL for testing.


Best regards,
Rich




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Re: Teslars???

[Rich Nute]

Hi Joe:


   We have a customer that is concerned about how our product, laboratory
   equipment, will respond to electromagnetic disturbances from a high speed
   train that runs close to their lab.  The customer states that the
   disturbance will be around 0.7-1.2 m Teslar.

I believe you mean tesla (with a lower-case t), not
Teslar.  

The symbol for tesla is upper case T.

According to ISO Standards Handbook 2, the tesla is a
unit of magnetic flux density.

The magnetic flux density is an axial vector
quantity such that the force exerted on an element
of current is equal to the vector product of this
element and the magnetic flux density.

1 T  =  1 N/(A*m)
1 T  =  1 Wb/m^2
1 T  =  1 V*s/m^2

I'm sure your customer is referring to an electric 
train.  

I recall a suburban electric train line where the
stations had color TV monitors to announce the train
and its schedule.  A few minutes before the train
arrived in the station (and usually before you could
see the train), the monitor's colors would go awry.  
The degradation would increase until the train 
arrived, at which time the normal colors were 
restored.

Upon departure, the colors would again go awry, and
then gradually return to normal.

The color degradation was due to the magnetic field
around the overhead wire.


Best regards,
Rich







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Re: TUV NRTL

[Rich Nute]

Hi George:


   However, for the U.S. OSHA establishes acceptable NRTLs, and the
   CFR uses the words NRTL, so I do not see how an electrical
   inspector could object to what the Federal Government has sanctioned.

The inspector must abide by the certifications 
approved by the local building/electrical code
jurisdiction.

It would be more accurate to substitute local
jurisdiction for inspector.

OSHA NRTL and local code processes are independent
of each other.  I agree that it would be prudent
of the local jurisdiction to accept OSHA NRTLs 
rather than establish other criteria which might
exclude one or more NRTLs.

I believe TUV-R has indeed applied for acceptance
in most or all local jurisictions.  (Don't rely 
on my belief.)


Best regards,
Rich




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Re: CE Marking - Prototypes

[Rich Nute]

Hi Gregg:


   My understanding if that:
   
   where OSHA is applicable then UL or equivalent is mandatory.

True.  However, replace UL or equivalent with NRTL.

   where OSHA is not application then Local Code is mandatory - and LOCAL 
 Code
   requires NRTL (UL or equivalent)

Not true insofar as the conditional statement, where 
OSHA is not applicable...

Compliance with the local code is mandatory and 
independent of OSHA.

Here is the difference:

OSHA rules are for the protection of employees in
the workplace.  They apply to products and installations
used by employees.

Local codes/rules are part of the local BUILDING
code.  They apply to ALL products and installations 
within the local jurisdiction.  Local codes apply 
equally to residences, commercial buildings, factories, 
etc. -- everywhere except electric utility circuits.  

OSHA and local codes are *not* alternatives.

For every place of employment, *both* OSHA rules and the
local electrical code rules apply.

For residences and schools and similar non-employment
and government sites, only the local electrical code 
applies.  (OSHA does not apply to government employees.)


Best regards,
Rich





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Re: Telecommunications Vs. ITE Product

[Rich Nute]

Hi Gregg:


   Let me start by asking one simple question - WHAT IS THE PURPOSE OF
   CERTIFICATION?
   
   1-  to meet the legal requirement
   2-  help market the product
   3-  do the 'right thing'
   4-  not to kill anyone
   5-  make sure that the product is nor recalled
   
   
   Most companies will answer (4) to make sure the product is not recalled.
   This is a GOOD answer - it may not optimized, in terms of immediate cost,
   but this is the one that I would opt for - particularly if I wanted to sleep
   at night.

This is what the certification house would like you
to believe.

However, the process of certification is a simple 
matter of checking off requirements.  (Have you 
looked at a CB Test Report lately?)  There is very
little further looking for hazards, (and there is 
a great deal of nit-picking for compliance).

So, certification means compliance with the standard.

To accomplish #4, the standard must be omniscient.

Mere mortals put the standard together.

We wouldn't certify products if there wasn't a legal
requirement to do so.  But, I agree that we expect a
lot more from certification than meeting legal 
requirements.  However, we don't get more than 
compliance to the standard.


Best regards,
Rich




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Re: Harmonic current emissions

[Rich Nute]

Hi John:


  But whichever method is adopted, the customer pays the bill eventually 
 and I
  have more confidence in the highly competitive world of electronic 
 products
  to come up with a cost-effective solution in a timely manner.
   
   One of the USA's major objections to EN61003-2 is
   that remedying the load repeats with each new product
   that is introduced, while remedying the source is a
   one-time remedy.  EN61000-3-2 requires continous 
   cost to the consumer with each product.  (The cost
   is NOT trivial -- nearly double the cost of the power
   supply.) 
   
   No, that's certainly an exaggeration. We have been told various sums
   from USD1 to USD5, and I suspect that the lower value is nearer the
   truth.

The additional cost for a PF-corrected SMPS is 
not a constant adder; it is proportional to 
power output.  One must use higher power PF
components for higher power output.

Having actually purchased production quantities
(1995) of the same SMPS in both non-PF-corrected 
and PF- corrected schemes, the additional cost 
for PF-corrected ranged from 50% to 75% higher 
than the non-PF-corrected supply.  

More recently (2001), the cost difference was 
indeed lower -- only about 25-30% premium.

I suspect your sources wanted to sell PF-corrected
SMPS's and exaggerated on the low side.


Rich







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Harmonic current emissions

[Rich Nute]

Hi Keith and Ghery:


There are a number of effects of harmonic current 
emission from non-linear loads.

1)  When a large number of loads rich in triplen
harmonics are supplied from a 3-phase source,
the neutral current can be as high as root 3
of the phase current.  (This effect does not
exist on a single-phase distribution system,
or on a 3-phase system where each phase has
its own neutral.)

Where the neutral wire is sized for a balanced
load, some authorities allow the neutral wire 
to be one size smaller than the phase wire.  
Such a wire is likely to be overheated by the 
triplen currents.  Indeed, it is possible to
overheat the neutral wire when it is sized the
same as the phase wire.  In the USA, authorities
now require (for such loads) the wire to be
larger than the phase wire, or two, parallel
neutral wires.

2)  Consider that the non-linear load generates 
current at harmonics of the mains frequency and 
injects it into the mains distribution system.  
This current must circulate in the distribution 
system and return to the source (load).  Often, 
this current circulates in the delta primary of 
the first upstream delta-wye distribution 
transformer, and causes the transformer delta 
winding to overheat.  (This effect is likewise
mostly due to triplen harmonics.)

In the USA, distribution transformers are 
specially designed to dissipate this power
without overheating.  Such transformers include
a K-factor rating, which is a measure of the
transformer to accommodate the current.

3)  Depending on the source impedance, a large 
number of non-linear loads can cause voltage
waveform distortion.  Voltage distortion is
caused by all of the harmonics, not just the
triplen harmonics.

Voltage waveform distortion can cause motors to
overheat.

Each of these effects is a separate and independent
issue.  They should not be lumped as a single issue.

For each effect, there can be one or more remedies.
The remedy can be either in the load or in the 
source.

EN 61000-3-2 arises from the voltage distortion 
effect.  Mr. Van den Bergh's comments (as quoted by
Keith) appear to address voltage distortion, not the 
other effects.  

Because of the difference in the design of power
distribution systems, voltage distortion is more of
a problem in the EU than in the USA.

   I suspect the real reason for the computer industry's denial of harmonics
   problems, or else blaming them on a poor distribution system, is that US
   computer manufacturers simply want to make one model they can sell 
 world-wide
   so they want whatever is permitted in their main market (the US) to be
   permitted everywhere else.
   Would you agree with this?

I believe this is an oversimplification of the 
manufacturer's dilemma of addressing this problem.

The USA computer industry has been quite forward in 
addressing effects 1 and 2.  The computer industry 
was the force behind a series of academia-based 
seminars on the causes and solutions to effects 1 
and 2 that resulted in changes to the USA National 
Electrical Code and to distribution transformer
testing and ratings.  (I presented in some of those 
seminars.)

This is NOT denial.

   But whichever method is adopted, the customer pays the bill eventually and I
   have more confidence in the highly competitive world of electronic products
   to come up with a cost-effective solution in a timely manner.

One of the USA's major objections to EN61003-2 is
that remedying the load repeats with each new product
that is introduced, while remedying the source is a
one-time remedy.  EN61000-3-2 requires continous 
cost to the consumer with each product.  (The cost
is NOT trivial -- nearly double the cost of the power
supply.)  Indeed, this has forced manufacturers to
develop one supply for the EU, and one supply for the
remainder of the world.  And, forced two products for
the world instead of one.

(One of the benefits of EN61000-3-2 has been a real 
effort at power reduction so that more and more 
products are below the 50-watt exemption limit.)

   I have some knowledge of power-factor correction techniques in switch-mode
   supplies, and some of them can cost very little indeed. So I really don't
   know why the US computer industry is making such a fuss about controlling
   harmonic emissions.

I certainly have not seen low-cost PF correction
techniques.  My experience is that the cost is
nearly twice the cost of a non-PF corrected power
supply.


Best regards,
Rich


ps:  EU power suppliers are taking an interesting
 approach to their customers.  If the customer
 has a linear power factor problem, we will
 correct it.  If the customer has a non-linear
 power factor problem, we will not correct for
 it, and we will not sell power to you if the
 effect is too great.  Clearly a monopolistic
 view.

Re: Company close down due to EMC phenomena

[Rich Nute]

Hi Amund:


I suggest you and your client and the regulatory 
authority meet to address the situation.

Here are the issues as I understand them from your
message:

1.  In idle mode, the emissions are comfortably 
below the maximum allowed emissions.

2.  In transmit mode, the emissions exceed the 
maximum allowed emissions.  The duration of
the transmission is 25 ms.  The repetition
rate of the transmission is about 1/hour.

3.  The emission level is probably related to the
nature of the system, i.e., transmission via
the power distribution network.

I feel that 25 ms/hour is the important parameter.

I suspect that most motor-starting events exceed
the emission level, and for more than 25 ms.  Like-
wise, I suspect most igniter events also exceed the
emission level and for more than 25 ms.  Does
your regulatory authority permit such emissions?

Set up the system for normal operation.  Ask the
regulatory authority to measure the emissions.  The
emissions will be in compliance for 1 hour.  The
regulatory authority must be observing the emissions
at the moment of the 25 ms transmission in order to
determine if the emissions exceed the allowed level.

I suspect this is a difficult measurement.  The
receiver or SA must be tuned to the transmit 
frequency during the 25 ms transmit interval.  This
probably requires advance knowledge of the transmit
frequency, and therefore the measurment is not an 
agnostic measurement.  (If you were submitting the 
equipment to the regulatory authority, do you have 
an obligation to inform them of expected performance 
of the unit, especially the specifics of the transmit 
mode?)  And, there must be some means of capturing 
the data during the event.  Short of staring at the 
SA screen for an hour or more, I'm not sure this can 
be done except with exceptional effort and additional 
equipment.

The preceding paragraph is something of a game to
play with the regulatory authority.

   So, why should the company close down ? Because if the national authority
   gets what they want, there will be one sale. Logical, but is it a correct
   prohibition the authority call?

The company SHOULD have known that its product would
exceed conducted emissions.  The company SHOULD have
taken appropriate steps with the regulatory authority
to know whether or not its product would be approved
BEFORE it invested in the product development.

This is not a fault of the regulatory authority, but
a fault of the company to not understand the
regulations BEFORE it developed the product.


Best regards,
Rich




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Administrative message -- posting formats

[Rich Nute]

Regarding postings, here is a re-statement of our 
guidelines that were sent to you when you
subscribed:

1.  No attachments (because many of our subscribers
use dial-up modems for which message size 
determines the download time).  

If an attachment is appropriate or necessary to
your message, then 

a)  offer to e-mail it separately upon request, or 
b)  make it available on an FTP site, or 
c)  post it to a web site and provide the URL in
your message.

2.  Post in ASCII plain text.  Do not use RTF, HTML,
or similar formats.  Not all mail readers are
compatible with these formats, but all mail 
readers are compatible with ASCII plain text.

In most cases (especially Outlook), you can set 
your mailer to always use ASCII plain text for 
messages sent to emc-pstc.

Also, please don't write messages without 
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handle long lines, so the line is truncated and 
part of the line disappears.  If you use CR 
(ENTER) at the end of each line, then each 
reader will see the same format as the one you
wrote.

3.  Please don't re-post the entire message string
when replying to a message.  

Instead, pick out the passages to which you want 
to respond, enter your response, and delete the 
other text.

This makes your point easier to understand, and
helps keep the message size down for our modem-
connected colleagues.  

Don't forget to delete the emc-pstc footer!  

:-)  

(Each posted message gets the footer attached; 
multiple footers provide no useful information,
and just make downloads longer.)

4.  If appropriate, when responding to a message, 
change the subject line to agree with the major 
point or content of your contribution.  You can
append was 'original subject' to your subject
if you want to reference the original subject.

This will help us when the subject matter strays
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5.  If you want to attach a signature,  please do
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ASCII), not the business card format that is 
gaining popularity.


Best regards,
Richard Nute
Administrator, emc-pstc listserver

Tel: +1-858-655-3329
FAX: +1-858-655-4374
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docopocoss

[Rich Nute]

docopocoss 

This word was unknown to me.  I checked an
American dictionary and could not find it.

Then, I called up the Google search engine
and entered the word, hoping to find an 
English dictionary.

Google immediately came back with the 
definition.


Rich




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Re: Value of Using Non-NRTL Engineering Firms?

[Rich Nute]

Hi Chris:


OSHA rules apply to employers.

OSHA electrical rules require employers (1) to 
use only electrical products that have been 
certified by an NRTL, or (2) in the case of 
custom products, to test the product in place.

If you sell a non-NRTL-certified custom product 
to an employer who is subject to OSHA rules, then 
that employer must test the product in place, and 
file a suitable record of the testing.  

Few employers choose the test-in-place alternative.

A listing by a non-NRTL is useless to an 
employer subject to OSHA rules.  He can't use it
for proof that the product meets OSHA rules.

At the employer's discretion, you may be able to
convince the employer (your customer) that your 
listing test report will provide a suitable 
record of testing to OSHA requirements.  

See OSHA rules, Sub-part S for complete treatment
on OSHA electrical rules.  See especially 1910.303(a) 
Approval and the respective definitions.

http://www.osha.gov/pls/oshaweb/owastand.display_standard_group?p_toc_level=1p_part_number=1910p_text_version=FALSE


   Statement: Non-NRTL laboratories can provide Listings and publish the 
 customers (thus Listing) however, based OSHA law, NEC requirements, 
 Retailer specification, and other MOU/MRA with Canada/EU, it would not seem 
 to be a significant accomplishment if not an NRTL.

Except for NEC, I would agree with this statement.


Best regards,
Rich





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Re: OFF is zero

[Rich Nute]

Hi Stephen:


  I think I missed the beginning of this thread, so
   please forgive me if I rehash old ground.
   
  The 'vertical line' and 'circle' you refer to, are
   actually a 1 (One) and 0 (Zero); as in digital
   logic on/off respectively.

My original posting on this subject was with regard to
the ETYMOLOGY of the IEC 60417 symbols 5007 (a vertical
line) and 5008 (a circle).

The IEC 60417 symbols 5007 and 5008 are SYMBOLS.  They
are NEITHER numbers NOR letters.

The ETYMOLOGY I quoted shows that these symbols were 
originally the numbers zero for OFF and any number one 
or greater for ON.


Best regards,
Rich






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Re: OFF is zero

[Rich Nute]

Hi Serge:


   O = Out of Circuit
   I = In Circuit
   
   Note: while many people believe the symbols are O  1, they are in fact
   O  I

IEC 60417 Symbols 5007 and 5008 are neither:

the numbers 1 and 0, nor 
the letters I and O.

They are the symbols depicted in the 60417 drawings,
specifically a vertical line and a circle.

My comment is that the ETYMOLOGY of the 5007 and 5008
symbols are the numbers one and zero.


Best regards,
Rich





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Re: OFF is zero

[Rich Nute]

Hi Neil:


   The origin may be true, but IEC60417 is quite clear. ON is a vertical line
   (symbol 5007) and OFF is a circle (symbol 5008).

Agreed.

And thank you for the clarification.


Best regards,
Rich







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OFF is zero

[Rich Nute]

A week or so ago, I asked about the origin of the
0 and 1 symbols for off and on.

I mentioned that it was my recollection that the
the symbols came from early safety standards where
the off and on functions were by means of a
rotary switch where the 0/zero position was off.

I found some very old standards that I still have 
on my bookshelf.

IEC 380, First Edition, 1972:
Electrical Safety of Office Machines

Clause 7, Markings  
Sub-clause 7.10:

If figures are used for indicating different 
positions, the off position shall be indicated 
by the figure 0, and the position for greater 
input, speed, cooling effect, etc., shall be 
indicated by a higher figure.

The figure 0 shall not be used for any other 
indication, except that this shall not preclude 
the use of the figure 0 for the identification 
of an alphabetic or numeric key on an office 
machine.

I believe this text is virtually the same as the text
in CEE-10 mentioned in my previous message.  CEE-10 
was the predecessor to IEC 380 and addressed safety 
of office machines.

I conclude that the 0 and 1 symbols came from the 
number zero and the number one.

Today, the figure/symbol 0 is REQUIRED as the OFF 
symbol and the figure/symbol 1 is REQUIRED as the 
ON symbol.  

What have we done to ourselves?


Best regards,
Rich




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Origin of power symbols 0 and 1?

[Rich Nute]

I am looking for the origin of the power symbols
0 (off) and 1 (on).

The earliest standards that I have are CEE 10 
and similar where the standard specifies something 
like:

   Where a rotary switch is used for power,
   the 0 position shall be off and the 1 and
   higher position shall be on.

Do you have the specific wording?  Do you have
something earlier than the CEE standards?


Thanks for your help!
Rich


   Richard Nute
   Hewlett-Packard Company
   16399 West Bernardo Drive
   San Diego, California 92127-1899

   Tel: 858-655-3329
   FAX: 858-655-4374
   e-mail:  ri...@sdd.hp.com




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Re: Creepage

[Rich Nute]

Hi Richard:


   What is a good source that explains the rational for the values for creepage
   distances?

There was a good deal of research done in the late 
'70s and early '80s on creepage distances and the
deterioration due to deposition of foreign materials
and the effects of humidity.

As I recall, the results of this research was 
published as working papers of the IEC TC that did
the research (TC 64?).  I suppose you could ask the
TC for copies of this research.  

In a nutshell, the voltage across the contaminants
causes micro-arcing.  The heat from these microarcs
causes tracking on the surface of the insulator.
Eventually, the micro-arcs develop into trees, and
the leakage current becomes significant.

The phenomenon is a long-term one, so the effective
voltage across the insulator is the significant
parameter.


Best regards,
Rich




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Re: Class III ID mark?

[Rich Nute]

Hi Tom:


With a bit further search, I found:

http://w3.hike.te.chiba-u.ac.jp/iec417/html/doc/5180.html

This site specifies the use and the referring 
standard:

Application: On any kind of equipment. To 
identify equipment meeting the safety 
requirements specified for class III 
according to IEC Publication 536 : 
Classification of electrical and electronic
equipment with regard to protection against
electric shock. 

So, with thanks to you, my questions are answered!


Best regards,
Rich





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Re: Class III ID mark?

[Rich Nute]

Hi Tom:


   I guess what you think is IEC 60417 No. 5180, which shows III
   in diamond shaped frame (45 degree rotated square).
   (http://member.nifty.ne.jp/tsato/terms/iec417-5180.gif)

Yes, this is the mark that was mentioned 
to me.

   However, I don't know which standard require the mark.
   I thought none of 60335, 60950 and 61010 require to put the mark
   on Class III equipment, although they require double-square mark
   on Class II equipment.

Now, I need to confirm that this mark indeed
indicates Class III equipment, and does not
indicate something else.

And, I need to determine what standards require
this mark.

Can anyone provide this info?


Thanks, and best regards,
Rich





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Class III ID mark?

[Rich Nute]

A colleague recently showed me a Class III
identification mark.  However, he had no
further information about the mark.

(The Class III mark would be used to identify
a Class III product similar to the way the
Class II mark square-within-a-square is used
to identify a double-insulated product.)

Have you seen this mark?  

If so, can you tell me its origin and provide
me with a copy of the mark?

Also, can you tell me the standards in which
it is identified and requirements for its use?

I suspect this mark originated at VDE. 


Thank you for your help,
Rich








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Re: EN60950-1 Sect 2.5 Limited Power sources

[Rich Nute]

Hi Tom:


   Is it permitted to use PTC as an overcurrent protective device
   for power sources not inherently limited?
   I don't have EN60950-1 here, but I remember it must be a fuse
   or other non-autoreset device.

I said:

The trick is to determine if your test houses can 
 class the PTC as an 'overcurrent protective device.'

Another subscriber said that the PTC is a resistor and
therefore a PTC-protected LPS must be inherently limited.

I suggested the PTC as an overcurrent protective device
because as its resistance goes up, the PTC provides
protection against overcurrent.  


Best regards,
Rich





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Re: Green/Yellow Earthing Leads

[Rich Nute]

Hello from San Diego:


One of our subscriber colleagues has pointed out that
my assertion that Europe uses yellow as the color of
the earthing wire is incorrect.

I recall when the green-yellow was agreed to for 
power cords and internal equipment wiring.  My
recall was that both yellow and green were used as 
earthing wire colors, hence the green-yellow 
compromise.

My recollection is in error.  I apologize for 
misleading you.


Best regards,
Rich





 In North America, the earthing conductor in
 building wiring is green.

 In Europe, the earthing conductor in building
 wiring is yellow.

 For power cords and equipment earthing conductors,
 a compromise was agreed to for green/yellow.

 Note that in North America, power cord and
 equipment earthing conductors may be green/yellow
 or green.

 In Europe, power cord and equipment earthing
 conductors may be green/yellow or yellow.


 Best regards,
 Rich




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Re: electric strength test

[Rich Nute]

Hi Greg:


Why does True SELV require basic insulation
between SELV and earth?

What is the hazardous voltage source, and what
is the current path through the body if that 
basic insulation should fail?


   
   I believe (and I could be wrong) that it's a philosophy of keeping SELV 
   intact under single fault conditions.   If there's a single fault bridging 
   mains and earth, then an SELV circuit is exposed to mains voltage unless 
   there is basic insulation protecting it.  Since it is estamited that close 
 to 
   50% of the homes in the US have improperly grounded outlets, the earth 
   connection itself is not taken into consideration as a reliable means of 
   protection.  In other words, for the purposes of the standard, an open 
 earth 
   connection is not considered a fault.

Thanks for the explanation.  

One implication of this explanation is that
an open earth connection is not a fault 
condition, but a normal condition.  (Failure
of basic insulation is the fault condition.)

This would suggest that it would be better to
double/reinforce insulate the mains against
earth, in which case SELV could be earthed.

(This also solves the problem of accessible
earthed parts becoming live when used on a
system with no earthing connection.)

But, unfortunately, the requirement remains
that true SELV cannot be connected to earth,
and must have basic insulation between it and
earth.


Best regards,
Rich









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Re: EN60950-1 Sect 2.5 Limited Power sources

[Rich Nute]

Hi Alex:


   We use a linear (basically a transformer with a PTC thermistor) class II
   SELV O/P stand alone power supply rated 10Vac, 5A. One of the labs tested
   the O/P Isc with a direct application of the sc. The other lab tested the
   O/P Isc by applying a load of 8A. The results were therefore interpreted
   differently.

   1.  What is the correct method to test for a Limited Power source in
   determining what enclosure is required?

Limited Power Source is a combination of a number
of criteria:

output voltage;
output current;
output volt-amperes.

These are independent criteria.

Note that Isc is defined as:

Maximum output current after 60 s of operation with 
 any non-capacitive load, including short-circuit.

Both labs are wrong.

If a power supply shuts down due to a short-circuit, then
it is necessary to determine the maximum current where the
power supply does not shut down.  Typically, this is about
twice rated output.  Since your power supply is rated 5 A,
then I would expect the overpower to take effect at about
10 A output.

When testing a power supply output for LPS, start at rated
load and gradually increase the load.  If the output 
exceeds 8 amps or any other criteria, then the output is
not LPS.

Then, introduce the single, worst-case fault in the
regulating circuit, and repeat the test.  In your case,
this means shorting the PTC, in which case the output will
certainly exceed 8 amperes.

By the way, your output also fails the output volt-amperes
criteria.  If the open-circuit output voltage is 10 volts, 
then the maximum VA is 50, or 5 amps.  

Since one lab only did the output short-circuit test, the
results are not correct.

Since the other lab did a single test at 8 amps, this is
more-or-less okay, but for the wrong reasons. 

Neither lab did the VA test!  The VA test would have proved 
the unit failed at 5 A output.

Neither lab introduced a fault in the regulating network!
(But, the didn't need to since the unit failed under normal
operating conditions.)

Both labs are wrong, but one came up with the correct
determination.  (Of course ONE had to be correct!)  :-)

If you use a PTC, then your output need not necessarily be 
considered inherently limited.  In this case, your 
maximum output current is 100 amps and your maximum VA is 
250, i.e., 25 amps (with the PTC shorted).  The trick is 
to determine if your test houses can class the PTC as an 
overcurrent protective device.  

A colleague claims that very few test houses really know
how to do LPS testing.  I concur, and your report confirms
this again.


Best regards,
Rich







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Re: Green/Yellow Earthing Leads

[Rich Nute]

Hi Peter:


   What is the percentage of color over a surface area for the insulated
   green/yellow earthing conductor. Is it better to have more yellow than green
   or more green than yellow. I know North Americans prefer more green than
   yellow and Europeans like more yellow than green. Any historical reason? I
   hope it has nothing to do with the Boston Tea Party! Can the group we come
   up with some compromise?

In North America, the earthing conductor in
building wiring is green.

In Europe, the earthing conductor in building
wiring is yellow.

For power cords and equipment earthing conductors, 
a compromise was agreed to for green/yellow.

Note that in North America, power cord and 
equipment earthing conductors may be green/yellow 
or green.

In Europe, power cord and equipment earthing
conductors may be green/yellow or yellow.


Best regards,
Rich




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Re: electric strength test

[Rich Nute]

Hi John:


   True SELV . requires double or reinforced insulation from hazardous
   live parts/parts at hazardous voltages [different expressions used for
   the same things] and basic insulation from earth. '950 SELV' allows SELV
   circuits to be earthed (see 2.2.3.3 of IEC60950:1999 or EN60950:2000 or
   of IEC/EN 60950-1).

Why does True SELV require basic insulation
between SELV and earth?

What is the hazardous voltage source, and what
is the current path through the body if that 
basic insulation should fail?

This is a question that has bothered me for a
number of years.


Thanks, and best regards,
Rich




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IT power distribution and Norway (was electric strength test)

[Rich Nute]

Hi John:


   You may come across some distributors in Norway asking for DI or RI
   between PRI-EARTH, even for Class 1 power supplies.
   
   That's because Norway uses the 'IT' power distribution system, where
   neither mains conductor is earthed directly, but one is earthed through
   a quite high impedance to prevent common-mode charge build-up.

I fail to understand how, in a Class I product on an IT 
power distribution system, PRIMARY-EARTH double or 
reinforced insulation provides protection against 
electric shock while, on a TN power distribution system, 
basic insulation is sufficient to provide protection 
against electric shock.

If the product is earthed, then in the event of a fault
of basic insulation, the voltage on accessible parts 
will remain close to earth potential.  This applies 
equally to IT and TN power distribution systems.

My understanding of the Norwegian unwritten requirement
was because many of the socket outlets, while of the
earthing type, do not have the earth terminal wired to 
earth.  

Under this condition, double or reinforced insulation
makes sense.


Best regards,
Rich



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Re: electric strength test

[Rich Nute]

Hi John:


   That depends on which variety of SELV you mean. True SELV, as opposed to
   '950 SELV', requires double or reinforced insulation from earth

Why?


Best regards,
Rich




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Re: electric strength test

[Rich Nute]

Hi John:


   Y-caps have enough margin to easily withstand 
   the 4300 V dc without damage.  
   
   Without immediate failure, no doubt. But such a huge overstress may
   cause latent damage that later results in failure, and, since IEC 60384
   doesn't call for such an over-voltage test, the failure might not be a
   fail-safe failure.

According to my copy of 384 (1996), a Y1
cap is proof-tested at 4000 V ac.

4300 V dc should not be a problem.

However, a Y2 cap is proof-tested at 
1500 V ac, and this might be a problem
just as you suggest.

Note that I suggested this overvoltage
not as a production test, but as a type
test during product evaluation -- to
learn the weakest point in the insulation
system.  If the cap should fail, then you
know that the cap is the weakest part.

I then remove the cap, and repeat the
test to determine the next weakest link.

Knowing the weakest link in an insulation
system can be quite valuable when a 
production problem arises.


Best regards,
Rich



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Re: electric strength test

[Rich Nute]

Hi Brian:


   What I've always wondered about, at least for class 1 construction, is just
   what is really being tested by the pri/sec hi-pot, when the customer chooses
   to ground the power supply's return. 

(These comments presume the secondary is SELV.)

When you use a ground for safety purposes, the
ground circuit must be capable of carrying the
fault current.  (This construction is called
bonding.)

Typically, the construction of the secondary
ground circuit does not meet the requirements
of bonding.  Consequently, the ground cannot
be relied upon for safety.

Therefore, pri-sec insulation must be double/
reinforced.

So, the pri-gnd insulation is basic and is
backed up by grounded/bonded parts.  And, the
pri-sec insulation is basic backed up by 
supplementary (or is reinforced).

(Most switching-mode transformers don't use a
grounded shield between pri-sec as this 
reduces coupling and does other things that
reduce the effectiveness of the transformer.
Therefore most switching-mode transformers
use double or reinforced insulation between
pri-sec.)

By the way, another reason for not using the
secondary ground for safety is that the sec
winding may not be capable of carrying the 
fault current, and will open.  The problem is 
that fault current is passing through the part 
of the winding that is grounded, that part of 
the winding opens, and the high side of the 
SELV secondary is now connected to primary.

   Also,as some of our output-to-chassis Y caps are just 100V decouple caps,
   and because SELV to P.E. spacing and insulation requirements would never
   withstand reinforced test levels, how does the end-use installation ever get
   pri/sec hi-pot to pass without removing the unit from the chassis?

There is no requirement for SELV-to-PE spacings 
and insulation.  After all, both the SELV and 
the PE are accessible parts and need not have
any safety insulation or spacings between them.

If your SELV is floating, then the pri-sec hi-pot
test is simply pri-sec.  No connection to chassis
or ground.  This will, however, damage your sec
decoupling caps because there will be a capacitive
voltage divider from primary to Y-cap to chassis
to decoupler cap to sec.  To prevent overvoltage
of the decoupler, either short it out or open it.


Best regards,
Rich




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Re: electric strength test

[Rich Nute]

Hi Brian:


   I am attempting to comply with both the letter and spirit of 60950:2000, cl
   5.2. Note that 5.2.2 allows for separate testing, according the type of
   insulation required. When testing a (class 1) power supply, the withstand
   level for primary to chassis is Basic; and for primary to secondary is
   Reinforced. Typically, I will apply approx 2500vdc for Basic and 4300vdc for
   reinforced. But to pass primary to secondary test, 60950 says that I can
   allow for following: 
   
   care is taken that the voltage applied to the reinforced insulation does
   not overstress basic 

Good requirement and good advice.  But, very 
difficult to apply in practice.

   Also
   
   to avoid damage to components or insulation which are not involved in the
   test, disconnection of ICs or the like and the use of equipotential bonding
   are permitted.

Again, good advice but very difficult to apply
in practice.

   For Pri/Sec testing, the screw connecting the Y-caps to the chassis is
   removed, insulation is inserted between the screw insert and chassis.

Well... I would just lift the cap and let it
hang in air for the test.  

Inserting solid insulation between the Y-cap 
terminal and the chassis means that you must 
also consider:

the creepage and clearance around the solid
insulation, and 

that most of the hi-pot voltage will appear 
across the solid insulation.  

   A (new) agency engineer says that inserting a piece of insulator defeats the
   purpose of the test. For class 1 construction, it is just not possible for
   me to pass 4300vdc test levels without inserting my little piece of valox,
   unless I physically remove all Y-caps (and the unit will not operate
   reliably with y-caps). I have inspected units from several other companies
   and have determined that it was not possible to have ever passed pri-sec
   test levels without isolating these circuits.

Inserting the solid insulator DOES NOT defeat 
the purpose of the test.  While it is true that
the inserted solid insulation gets tested in
parallel with the pri-sec reinforced insulation,
the test results apply to the pri-sec insulation.
If a failure should occur, then it would be 
necessary to sort out whether the failure was
your inserted solid insulation or the pri-sec
insulation.

If your secondary is not grounded, then you can
open the protective ground and test pri-sec
without lifting the Y-cap.  (Put the unit on an
insulating surface, and be careful not to touch 
the unit during this test!)

Our products are designed in the same way.  We
don't lift the Y-caps or anything else when we
hi-pot pri-sec (secondary is functionally grounded).  
Y-caps have enough margin to easily withstand 
the 4300 V dc without damage.  In other words, 
our pri-gnd meets 4300 V dc.

In general, solid insulations have electric
strengths in the neighborhood of 10 kV or more.  
The required spacings have electric strengths 
in the neighborhood of 5 kV.  Y-cap electric
strength usually is greater than the lead
spacing.

You should expect a system electric strength
approaching 5 kV rms.

During design, we always hi-pot test to failure
or max voltage of the hi-pot tester, whichever
is lower.  In our designs, the weakest point 
(4500-5000 V rms) in the power supply is the pwb 
terminals of the Y-cap on the back side of the 
board (where the sharp points are located).

The hi-pot test is a pass-fail test, and gives
no data.  Hi-pot to failure identifies and 
measures the weakest link.  This is useful data
for future evaluation of production-line hi-pot
failures.


Good luck, and best regards,
Rich



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Re: Criteria for UL V0 plastic enclosure

[Rich Nute]

Hi Scott:


   In the case of LPS, wouldn't the plastic enclosure still be considered 
 decorative
   plastic outside a fire enclosure and required to meet HB flame class (per 
 UL
   60950, 4.7.3.3)?

Yes.

Virtually all plastic materials meet the HB flame class,
hence my comment that he may use any plastic.


Best regards,
Rich


   Rich Nute wrote:
   
The choice is yours.  If your product is supplied by a
Limited Power Source, then you may use any plastic for the
enclosure.  If your product is not supplied by a Limited
Power Source, then you must use V1 or better plastic for







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Re: Don'r Get Caught Running a Red Traffic Light!

[Rich Nute]

Hi Doug:


   I've only heard shortening the yellow light cycle 
   as an unconfirmed rumor. But I've been watching. 
   grin 

According to our local newspaper, the San Diego Union-
Tribune, not only did the contract photo cops shorten
the yellow light, they also moved the sensors.

The contract photo cops get a percentage of the fine.
The fine is $271, and the photo cops get $70 or so.  
The contract photo cops choose the intersections and 
set up the system.  Only the intersections with known 
high-volume red-light runners were set up.  The whole
operation was based on maximizing revenue (in the name
of vehicular safety) for the photo cops, a private 
business.

The contract photo cops do the whole job without
supervision from real cops.

A class-action lawsuit has shut down the cameras.

You may be able to find some articles on the UT web
site.  The hullabaloo was about a year ago.


Best regards,
Rich




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Re: ventilation holes

[Rich Nute]

Hi David:


   You probably already know this, but don't forget that the product will need 
   to pass temp tests during abnormals i.e., blocked vents.

I'm not sure if pass temp tests means compliance with
the heating requirements or the abnormal requirements,
so I offer this clarification.

Under abnormal conditions (blocked vents), there are no 
temperature limits per se.  According to Sub-clause 5.4.1, 
under abnormal conditions the unit must remain safe.


Best regards,
Rich




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Re: UL Material flammability requirements for enclosures less than .3

[Rich Nute]

Hi Paul:


   Can anyone give me any information on whether a box with less than .33 cubic
   feet needs to meet UL Flammability ratings?
   I have been told this is true, but I can not find the location of that
   information.

I believe the requirement you are referring to is

Sub-clause 4.3.3, UL1950/IEC 60950
Sub-clause 4.7.3.3, UL 60950-1/IEC 60950-1

This requirement exempts materials and components within
an enclosure of 0.06 m^3 from flammablity requirements.

The enclosure itself must be of metal and must have no
ventilation openings.

However,

0.06 m^3   =  2.12 ft^3
0.33 ft^3  =  0.01 m^3

so the volume you quote does not agree with the volume
in the requirement.

With this exception, and as far as I know, requirements 
for material flammability ratings are based on the circuit 
parameters, not on volume of the box.


Best regards,
Rich




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Re: ventilation holes

[Rich Nute]

Hi George:


   I have a safety question for the group.   I  have a small  (about 8 x 10 x 2
   inch)  piece of ITE equipment, SELV, enclosed in a plastic case,  powered by
   an external 12 volt brick from AC mains. 
   
   The problem is, the device tends to run a little hotter than desired.   One
   proposed solution is to cut some vent holes in the top.   These would be
   roughly (1/2) inch long by  (1/6) inch wide, spaced (1/4) inch apart,
   running across the top near the front of the device. 
   
   I haven't seen many (any?) devices with vent holes in the top, so I'm
   wondering if there is a basic reason why not, such as the cover must shed
   water, etc.
   My questions are,  what considerations arise and what sections of EN 60950
   apply to this, either to allow it or to exclude it.

Top openings are specified in Sub-clause 4.6.1, 4.6.4, and 
Annex T of IEC 60950-1.

There are no restrictions on openings if the source (brick)
is SELV and LPS and the unit is intended for indoor use, 
i.e., does not need to be protected against ingress of water.

You told us the source is SELV, but you did not tell us
whether the source is also LPS.  

If the source is both SELV and LPS, then there are no 
restrictions on opening sizes.  Indeed, there is no 
requirement for an enclosure.  (With SELV and LPS supply,
there is no shock or fire hazard, and the enclosure is not
required to prevent access, ingress of foreign objects, or
containment of fire.)

If the source is SELV but not LPS, then the top openings 
cannot exceed 5 mm maximum dimension, or 1 mm maximum width.

Generally speaking, physically small devices are also low-
power devices.  Consequently, they require little or no
cooling that would require ventilation openings.  So, for
physically small products, there are no openings in the
enclosures.


Best regards,
Rich





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Re: double-pole switching and fusing

[Rich Nute]

Hi John:


   I read in !emc-pstc that Rich Nute ri...@sdd.hp.com wrote (in
   200202202227.oaa19...@epgc264.sdd.hp.com) about 'double-pole switching
   and fusing', on Wed, 20 Feb 2002:
   Tables 2E and 2F are invoked only by the Note 2
   to Sub-clause 2.7.4.
   
   A Note is informative, not a requirement.
   
   Yes, well, Note 2 is weasel-worded. It claims to be 'informative' but
   some test houses hold that if Table 2E or 2F is not observed, the
   'essential requirements' of the LVD are not met.

Sigh... I agree with your assessment.  

I suppose such test houses do not understand
that the LVD does not necessarily require 
compliance to a standard?  


Rich




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Re: [PSES] UL 60950-1 clause 2.2.3

[Rich Nute]

Hi John:


I believe the authors of 60950-22 expected that user-accessible parts would
comprise SELV circuits rather than grounded parts.  See 2.1.1 and 2.2 in 
60950-1.  

Grounded parts would be subject to 5.1 in 60950-1.


Best regards,
Rich




- Original Message -
From: John Cochran jcoch...@strongarm.com
Date: Sunday, September 7, 2014 1:42 pm
Subject: RE: [PSES] UL 60950-1 clause 2.2.3

 Actually I need to meet the standard for outdoor enclosures, UL 
 60950-22, and clause 6.1 refers back to UL 60950-1 clause 2.2.2 and 
 2.2.3.  The difference is the voltage limits are reduced, due to 
 contact resistance of the body being reduced when subjected to wet 
 locations.  The clause states the voltage difference under a single 
 fault must not exceed 30 V ac, without any mention of current.  Am 
 I reading this correctly?
 
 John Cochran
 STRONGARM Designs
 425 Caredean Drive
 Horsham, PA 19044
 PHONE: 215-443-3400 X193
 FAX: 215-443-3002

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Re: [PSES] UL 60950-1 clause 2.2.3

[Rich Nute]

HI John:


As mentioned in my first response, you will have the same
problem (half the mains on the enclosure) if you use a 
Class II power supply.  

If you talk with your NRTL, I'm sure he will accept your 
construction based on 60950-1, 5.1 rather than 60950-22.

Your equipment is not unsafe with the ground open if it
complies with 5.1.  (And it will!)


Best regards,
Rich




- Original Message -
From: John Cochran jcoch...@strongarm.com
Date: Monday, September 8, 2014 8:04 am
Subject: Re: [PSES] UL 60950-1 clause 2.2.3

 It appears that I can only meet this requirement for outdoor 
 enclosures by either using a Class 2 power supply, or require the 
 installer to use the external ground connection in addition to the 
 internal PE ground.  The system is safe under normal conditions, 
 but only has hazardous voltages on the enclosure when the PE ground 
 is broken.  Are there any opposing opinions?
 
 John Cochran
 STRONGARM Designs
 425 Caredean Drive 
 Horsham, PA 19044
 PHONE: 215-443-3400 X193
 FAX: 215-443-3002
 
 -Original Message-
 From: John Woodgate [mailto:j...@jmwa.demon.co.uk] 
 Sent: Monday, September 08, 2014 2:42 AM
 To: EMC-PSTC@LISTSERV.IEEE.ORG
 Subject: Re: [PSES] UL 60950-1 clause 2.2.3
 
 In message f6d1e59f218e.540c8...@bendbroadband.com, dated Sun, 7 
 Sep 2014, Rich Nute ri...@bendbroadband.com writes:
 
 I believe the authors of 60950-22 expected that user-accessible 
 parts 
 would comprise SELV circuits rather than grounded parts.  See 
 2.1.1 and
 2.2 in 60950-1.
 
 Grounded parts would be subject to 5.1 in 60950-1.
 
 Thank you. So what can the OP do? Does this need to be taken into 
 account in a revision or replacement of 60950-22?
 --
 OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk 
 Quid faciamus nisi sit?
 John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
 
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Re: [PSES] input current rating

[Rich Nute]

Dear Bostjan:


One of the uses of current rating on equipment is to determine that the 
equipment, with all of the other equipment, would not overload the mains 
circuit.  If the actual current exceeds the marked current, then the equipment, 
with all of the other equipment, can overload the mains but there is no means 
to identify the culprit equipment.

Note, however, that there is no limit on lower current than rated current.  I 
suspect this is because much equipment has an idle mode during which the input 
current is minimum.  


Best regards,
Rich

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