Re: [PSES] Touch current in IEC 60335-1 for Household Appliances

[Richard Nute]

 

Hi Joe:

 

Some points:

 

1.  The 60-volts need not be isolated from the low voltage; it has a
common point (ground) which is better.  The lone (not both) 60-volt
conductor and circuit components should be double-insulated from accessible
conductive parts.  
2.  Please note that the dielectric strength voltage test is to maintain
insulation in the event of a power-line transient voltage from a lightning
strike or power switching.  A battery-power circuit is not subject to
power-line transients.  Clearance dimensions are based on the dielectric
strength test voltage.  A creepage distance for low voltages cannot be less
than the clearance distance.  Air does not break down at less than 330 volts
peak (Paschen's Law).  
3.  If you load the 60 volts with 1500-ohms, the current would be 40 mA
and the power would be 2.4 watts if the circuit does not collapse.  If the
60-volt circuit can provide 0.5 mA ohms, the source resistance would be
120,000 ohms. Neither condition seems reasonable from a battery source.  As
I said, I believe the circuit will collapse when tested with 1500 ohms.
This would enable you to use option 3.  

 

Best regards,

Rich

 

 

From: Joe Randolph  
Sent: Tuesday, January 14, 2020 10:10 AM
To: ri...@ieee.org; EMC-PSTC@LISTSERV.IEEE.ORG
Subject: RE: [PSES] Touch current in IEC 60335-1 for Household Appliances

 

Hi Rich:

 

Thanks, I think I am starting to better understand my options.  Following is
my current understanding:

 

1.  If the internal non-SELV circuit does not meet the requirements in
clause 8.1.4 for "protective impedance," the product will be Class 2 per
clause 3.3.10 and double insulation will be required around the internal
non-SELV circuit.  This includes meeting all the criteria for double
insulation, including creepage distance, clearance distance, distance
through solid insulation, and an electric strength.

 

2.  Even if the internal non-SELV circuit does meet the requirements in
clause 8.1.4 for "protective impedance," I still can't use the Class 3
classification per clause 3.3.2, due to the way that clause 3.3.12 is
presently worded. 

 

3.  However, if the internal non-SELV circuit does meet the requirements
in clause 8.1.4 for "protective impedance," I can use the Class 2
classification per clause 3.3.10, but also use the "protective impedance"
provisions in clause 8.1.4.  The key distinction is that with the protective
impedance provisions, the internal non-SELV circuit would not be considered
to be a "live part."  And, if it is not a "live part," no insulation is
required.  

 

What I would like to avoid is the requirement to separate the internal
non-SELV circuit from accessible parts (including the USB port) with double
insulation.  It appears to me that the physical construction requirements
for double insulation would require considerable changes to the present
design.  At present, the internal non-SELV circuit shares the same return
path as the USB circuit, so there is no isolation between the two circuits.

 

Your explanation of how an electric shock risk requires that current flow
through the human body is very helpful for understand the underlying
principles.  It would appear that with only one pole of the internal
non-SELV circuit accessible (at the USB port), it would not be difficult to
prevent current from flowing when performing the accessibility tests.

 

However, to keep a test lab happy, I also need to demonstrate compliance
with the actual wording of 60335-1.  If it turns out that I have to provide
double insulation, it is not clear to me that I can avoid placing a double
insulation barrier between the internal non-SELV circuit and the accessible
USB port.  

 

Based on the underlying principles of electric shock, it would appear that
the internal non-SELV circuit could share the same return path as the USB
port, provided that double insulation is provided between the internal
non-SELV circuit and all other accessible parts.  With this construction,
there would be no path for current to flow.  

 

I'm just not sure whether a test lab, when performing a construction review
of the required double insulation, would agree that no insulation is
required between the internal non-SELV circuit and the USB port.

 

My preference would be to use the approach outlined in item 3 above, where
there would be no requirement for insulation at all (only a requirement for
the "protective impedance").  I plan to have some tests performed to
determine whether the internal non-SELV circuit can be classified as having
a "protective impedance."  Based on just a review of the circuit diagram, I
think it has a 50/50 chance of meeting the requirement for protective
impedance.

 

If it does meet the requirement for protective impedance, do you think that
the approach described in Item 3 above would be acceptable under 60335-1?

 

 

 

Thanks,

 

Joe Randolph

Telecom Design Consultant

Randolph Telecom, Inc.

781-721-2848 (USA

[PSES] CE marking quality manufacturing requirements

[Regan Arndt]

Greetings team members,



I seem to recall somewhere either in a directive or the blue guide where it
states (or implies) that part of the CE marking requirements entails that
the manufacture must have a product quality system/production
control..and this is where I need clarification: Does it imply that
the quality system entail that *one must conduct an internal product safety
audit (or the like) *to reassure themselves that there has been no
deviation to the product design/documentation that originally supported
their CE marking testing/declaration.



*Note that I am not referring to the module conformity assessment protocol
which involves a notified body, but just the simple self-declaration
scheme. My search always seems to point me towards the notified body
modules section, where they say that the 3rd party conducts factory audits,
etc. but there is nothing that states this for the non-notified body
involvement.*



If there is no such production verification audit required, would it not be
prudent to do this? ….just like the NRTL program? My experience indicates
that most manufacturers do not conduct internal product audits relating to
CE marking unless they forced to because they also have a certification
mark on the product (i.e. NRTL program or other certification scheme via a
Notified Body).


Thanks for any help you can provide me with an official exact
clause/text/excerpt from the OJ, guides and/or directives.


Regan Arndt

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Re: [PSES] Class I vs Class II safety constructions

[Joe Randolph]

Hi Pete:

 

Over the years I have periodically been asked to troubleshoot problems with 60 
Hz harmonics appearing in the audio signals of telecom equipment.  The culprit 
is usually the type of EMC caps that you refer to.

 

The easy-to-understand case is when two conventional Y-caps are connected from 
line to the the earth ground lead in the AC mains cable of Class 1 equipment, 
and the chassis of the equipment is also connected to this same ground lead.  
If for any reason the earth ground lead is not actually connected to earth, the 
two Y-caps form a voltage divider that modulates the equipment chassis, with 
respect to earth ground, at one half the mains voltage.  Since Y-caps are 
typically limited to 4700 pF maximum, the touch current is not excessive.  I 
personally have never felt a shock from such configurations, so maybe the 
mechanism is different for the cases you have described.

 

In terms of the noise problems that I have investigated, the hard-to-understand 
case is with Class 2 equipment that has no ground lead.  The switching 
converters used in most Class 2 power supplies typically contain a single 
capacitor, in the range of 2200 pF, that directly bridges the input and output 
windings of the isolation transformer.  This causes the Class 2 output of the 
supply to ride on a common mode 60 Hz (plus harmonics) waveform, with respect 
to earth ground, equal to one half of the mains supply.

 

One would expect this common mode voltage to have no effect on the equipment 
being powered, since the equipment is isolated from ground.  However, 
experience has shown that parasitic impedances between the equipment and ground 
can cause the 60 Hz harmonics to appear in the audio of high-gain paths such as 
hands-free speakerphone microphones.

 

I can eliminate the audio problem by removing the physical capacitor that 
bridges the isolation barrier, but it is my understanding is that for most 
switching power supplies, removing this capacitor causes the power supply to 
fail conducted emissions.

 

Again, due to the limited value of the coupling capacitor, I have never 
personally sensed a shock from such configurations, so what I am describing may 
not be the cause of the electric shock problem you describe.

 

 

Joe Randolph

Telecom Design Consultant

Randolph Telecom, Inc.

781-721-2848 (USA)

  j...@randolph-telecom.com

  http://www.randolph-telecom.com

 

From: Pete Perkins [mailto:0061f3f32d0c-dmarc-requ...@ieee.org] 
Sent: Tuesday, January 14, 2020 6:14 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Class I vs Class II safety constructions

 

Scott et al,  You are correct in that there seems to be a split between 
electrical appliances and electronic equipment and that the latter is more 
Class II than the former, there is chatter about an issue that is growing among 
class II equipments.  All of the electronic equipment which has been the 
primary user of SMPS for decades typically uses EMI filters to mitigate the 
noise generated by the switching action which get fed back into the line.  
These are used on Class I equipment where the noise is capacitively coupled to 
earth/ground to be dissipated and keep much of this noise from feeding back 
into the mains/line; likewise these filters are also used on class II equipment 
for the same purpose – however the class II equipment does not have any 
earth/ground sink in which to drain the unwanted signals.  So, apparently, this 
electrical charge is fed to the equipment chassis and the chassis voltage 
builds up until it reaches some equilibrium value.  The EMI folks discover this 
when they find that the equipment discharges to the probe before they are ready 
to induce a charge into the equipment.The voltage developed on the 
equipment seems to be a hi value but limited charge (due to the limited 
capacitance of the chassis to absorb it).  

   Altho there are not yet specific numbers, this doesn’t seem to 
be a safety hazard at this point.  However, it is also unknown as to how this 
affects the filtering of the mains noise which was the desired result.  The EMC 
lab techs don’t like the equipment to zap them first rather than the other way 
around.  

   Does anyone on this thread know of a paper on this which would 
contain some specific results?  Or of some researchers who are chasing this?  
Or have personal experience with this.  It would sure be nice to get some 
feedback on this.  

   This is a great opportunity to expand our experience and provide 
a basic understanding as to the efficacy of this process in both equipment 
applications for both safety and EMC.   

 

:>) br,  Pete

 

Peter E Perkins, PE

Principal Product Safety & Regulatory Affairs Consultant

PO Box 1067

Albany, ORe  97321-0413

 

503/452-1201

 

IEEE Life Fellow

  p.perk...@ieee.org

 

Re: [PSES] Class I vs Class II safety constructions

[Pete Perkins]

Scott et al,  You are correct in that there seems to be a split between 
electrical appliances and electronic equipment and that the latter is more 
Class II than the former, there is chatter about an issue that is growing among 
class II equipments.  All of the electronic equipment which has been the 
primary user of SMPS for decades typically uses EMI filters to mitigate the 
noise generated by the switching action which get fed back into the line.  
These are used on Class I equipment where the noise is capacitively coupled to 
earth/ground to be dissipated and keep much of this noise from feeding back 
into the mains/line; likewise these filters are also used on class II equipment 
for the same purpose – however the class II equipment does not have any 
earth/ground sink in which to drain the unwanted signals.  So, apparently, this 
electrical charge is fed to the equipment chassis and the chassis voltage 
builds up until it reaches some equilibrium value.  The EMI folks discover this 
when they find that the equipment discharges to the probe before they are ready 
to induce a charge into the equipment.The voltage developed on the 
equipment seems to be a hi value but limited charge (due to the limited 
capacitance of the chassis to absorb it).  

   Altho there are not yet specific numbers, this doesn’t seem to 
be a safety hazard at this point.  However, it is also unknown as to how this 
affects the filtering of the mains noise which was the desired result.  The EMC 
lab techs don’t like the equipment to zap them first rather than the other way 
around.  

   Does anyone on this thread know of a paper on this which would 
contain some specific results?  Or of some researchers who are chasing this?  
Or have personal experience with this.  It would sure be nice to get some 
feedback on this.  

   This is a great opportunity to expand our experience and provide 
a basic understanding as to the efficacy of this process in both equipment 
applications for both safety and EMC.   

 

:>) br,  Pete

 

Peter E Perkins, PE

Principal Product Safety & Regulatory Affairs Consultant

PO Box 1067

Albany, ORe  97321-0413

 

503/452-1201

 

IEEE Life Fellow

  p.perk...@ieee.org

 

From: Richard Nute  
Sent: Tuesday, January 14, 2020 12:58 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Class I vs Class II safety constructions

 

 

Hi Josh:

 

Yes, you are correct.  Both of these standards specify the equipment be Class 
I.  

 

60335-2-38 applies to commercial-use griddles.  60335-2-75 applies to  
commercial food or drink dispensing equipment.

 

Thanks, and best regards,

Rich

  

 

From: Wiseman, Joshua mailto:joshua.wise...@orthoclinicaldiagnostics.com> > 
Sent: Tuesday, January 14, 2020 11:59 AM
To: ri...@ieee.org  ; EMC-PSTC@LISTSERV.IEEE.ORG 
 
Subject: RE: [PSES] Class I vs Class II safety constructions

 

It’s been a few years, but I seem to recall there was a particular IEC/EN 
60335-2-xx standard that required tubular sheathed heaters to be grounded.  
This would force PE to be brought in.

 

Additionally, IEC 60335-2-38 and IEC 60335-2-75 have requirements for 
Equipotential grounding.  If I remember correctly IEC 60335-2-75 does not allow 
Class II products.

 

Josh

 

Joshua Wiseman 

Systems Engineering

Staff Engineer, Product Safety/EMC

Ortho Clinical Diagnostics

 



 

From: Richard Nute mailto:ri...@ieee.org> > 
Sent: Tuesday, January 14, 2020 2:38 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG  
Subject: Re: [PSES] Class I vs Class II safety constructions

 

 

Hi Scott:

 

I have seen no safety standards or codes that specify which products must be 
Class I and which products must be Class II, except in the USA washers and 
dryers must be Class I.  As far as I know, the decision is that of the 
manufacturer.  I have been associated with a manufacturer who has made the same 
product both ways.  In my case, one of the factors in deciding Class I or Class 
II was cost (e.g., a 3-wire cord was more expensive than a 2-wire cord).  

 

I suspect a major factor is “momentum” of the manufacturer: we made it this way 
last time, and we know how to do it this way.  

 

A product with a grounding (3-wire) power cord is a Class I product regardless 
whether it has no accessible conductive parts.  Unlike a Class II product, a 
Class I product does not bear a marking attesting that it is Class I.  

 

Note that a Class I construction necessarily includes Class II construction, 
e.g., appliance inlet which is all-insulated.  We ignore the Class II 
construction portions of a Class I product.

 

I checked our electric kettle (which has accessible metal) and electric 
coffee-maker (which has the heater plate accessible metal).  Both are 2-wire.  
Neither has the double-insulated symbol.  Both are UL-certified.  

 

Re: [PSES] Class I vs Class II safety constructions

[Richard Nute]

 

Hi Josh:

 

Yes, you are correct.  Both of these standards specify the equipment be Class 
I.  

 

60335-2-38 applies to commercial-use griddles.  60335-2-75 applies to  
commercial food or drink dispensing equipment.

 

Thanks, and best regards,

Rich

  

 

From: Wiseman, Joshua  
Sent: Tuesday, January 14, 2020 11:59 AM
To: ri...@ieee.org; EMC-PSTC@LISTSERV.IEEE.ORG
Subject: RE: [PSES] Class I vs Class II safety constructions

 

It’s been a few years, but I seem to recall there was a particular IEC/EN 
60335-2-xx standard that required tubular sheathed heaters to be grounded.  
This would force PE to be brought in.

 

Additionally, IEC 60335-2-38 and IEC 60335-2-75 have requirements for 
Equipotential grounding.  If I remember correctly IEC 60335-2-75 does not allow 
Class II products.

 

Josh

 

Joshua Wiseman 

Systems Engineering

Staff Engineer, Product Safety/EMC

Ortho Clinical Diagnostics

 



 

From: Richard Nute mailto:ri...@ieee.org> > 
Sent: Tuesday, January 14, 2020 2:38 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG  
Subject: Re: [PSES] Class I vs Class II safety constructions

 

 

Hi Scott:

 

I have seen no safety standards or codes that specify which products must be 
Class I and which products must be Class II, except in the USA washers and 
dryers must be Class I.  As far as I know, the decision is that of the 
manufacturer.  I have been associated with a manufacturer who has made the same 
product both ways.  In my case, one of the factors in deciding Class I or Class 
II was cost (e.g., a 3-wire cord was more expensive than a 2-wire cord).  

 

I suspect a major factor is “momentum” of the manufacturer: we made it this way 
last time, and we know how to do it this way.  

 

A product with a grounding (3-wire) power cord is a Class I product regardless 
whether it has no accessible conductive parts.  Unlike a Class II product, a 
Class I product does not bear a marking attesting that it is Class I.  

 

Note that a Class I construction necessarily includes Class II construction, 
e.g., appliance inlet which is all-insulated.  We ignore the Class II 
construction portions of a Class I product.

 

I checked our electric kettle (which has accessible metal) and electric 
coffee-maker (which has the heater plate accessible metal).  Both are 2-wire.  
Neither has the double-insulated symbol.  Both are UL-certified.  

 

Best regards from beautiful snowy Bend, Oregon, USA,

Rich

 

 

From: Scott Xe mailto:scott...@gmail.com> > 
Sent: Tuesday, January 14, 2020 6:59 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG  
Subject: [PSES] Class I vs Class II safety constructions

 

In terms of safety level, both constructions are given the equivalent 
protection against electric shock.  In electrical appliances, Class I is used 
most whereas Class II is employed in most electronic products.  Is there any 
background for such design route?

 

In some cases such as induction cookers, the enclosure is plastic/glass - no 
any internal metal part exposes to the outside surfaces.  The product is not 
marked with a double square symbol and comes with a 3-pin plug.  Why is this 
type of product not classified as Class II rather than Class I with the 
plastic/glass enclosure?

 

Thanks and regards,

 

Scott


-

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discussion list. To post a message to the list, send your e-mail to 


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Jim Bacher:  
David Heald: 

Re: [PSES] Class I vs Class II safety constructions

[Wiseman, Joshua]

It’s been a few years, but I seem to recall there was a particular IEC/EN 
60335-2-xx standard that required tubular sheathed heaters to be grounded.  
This would force PE to be brought in.

Additionally, IEC 60335-2-38 and IEC 60335-2-75 have requirements for 
Equipotential grounding.  If I remember correctly IEC 60335-2-75 does not allow 
Class II products.

Josh

Joshua Wiseman
Systems Engineering
Staff Engineer, Product Safety/EMC
Ortho Clinical Diagnostics

[https://insideortho.orthoclinicaldiagnostics.global/wps/wcm/myconnect/add13339-9211-4e73-9f5d-229f45406f61/OrthoMailSigLogo160x17.png?MOD=AJPERES&CACHEID=ROOTWORKSPACE-add13339-9211-4e73-9f5d-229f45406f61-mm1LfFj]

From: Richard Nute 
Sent: Tuesday, January 14, 2020 2:38 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Class I vs Class II safety constructions

EXTERNAL SENDER: Verify links, attachments and sender before taking action



Hi Scott:

I have seen no safety standards or codes that specify which products must be 
Class I and which products must be Class II, except in the USA washers and 
dryers must be Class I.  As far as I know, the decision is that of the 
manufacturer.  I have been associated with a manufacturer who has made the same 
product both ways.  In my case, one of the factors in deciding Class I or Class 
II was cost (e.g., a 3-wire cord was more expensive than a 2-wire cord).

I suspect a major factor is “momentum” of the manufacturer: we made it this way 
last time, and we know how to do it this way.

A product with a grounding (3-wire) power cord is a Class I product regardless 
whether it has no accessible conductive parts.  Unlike a Class II product, a 
Class I product does not bear a marking attesting that it is Class I.

Note that a Class I construction necessarily includes Class II construction, 
e.g., appliance inlet which is all-insulated.  We ignore the Class II 
construction portions of a Class I product.

I checked our electric kettle (which has accessible metal) and electric 
coffee-maker (which has the heater plate accessible metal).  Both are 2-wire.  
Neither has the double-insulated symbol.  Both are UL-certified.

Best regards from beautiful snowy Bend, Oregon, USA,
Rich


From: Scott Xe mailto:scott...@gmail.com>>
Sent: Tuesday, January 14, 2020 6:59 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Class I vs Class II safety constructions

In terms of safety level, both constructions are given the equivalent 
protection against electric shock.  In electrical appliances, Class I is used 
most whereas Class II is employed in most electronic products.  Is there any 
background for such design route?

In some cases such as induction cookers, the enclosure is plastic/glass - no 
any internal metal part exposes to the outside surfaces.  The product is not 
marked with a double square symbol and comes with a 3-pin plug.  Why is this 
type of product not classified as Class II rather than Class I with the 
plastic/glass enclosure?

Thanks and regards,

Scott
-


This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 
mailto:emc-p...@ieee.org>>

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http://www.ieee-pses.org/emc-pstc.html

Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/
 can be used for graphics (in well-used formats), large files, etc.

Website: 
http://www.ieee-pses.org/
Instructions: http://www.ieee-pses.org/list.html (including how to 
unsubscribe)
List rules: 
htt

Re: [PSES] Class I vs Class II safety constructions

[Richard Nute]

 

Hi Scott:

 

I have seen no safety standards or codes that specify which products must be 
Class I and which products must be Class II, except in the USA washers and 
dryers must be Class I.  As far as I know, the decision is that of the 
manufacturer.  I have been associated with a manufacturer who has made the same 
product both ways.  In my case, one of the factors in deciding Class I or Class 
II was cost (e.g., a 3-wire cord was more expensive than a 2-wire cord).  

 

I suspect a major factor is “momentum” of the manufacturer: we made it this way 
last time, and we know how to do it this way.  

 

A product with a grounding (3-wire) power cord is a Class I product regardless 
whether it has no accessible conductive parts.  Unlike a Class II product, a 
Class I product does not bear a marking attesting that it is Class I.  

 

Note that a Class I construction necessarily includes Class II construction, 
e.g., appliance inlet which is all-insulated.  We ignore the Class II 
construction portions of a Class I product.

 

I checked our electric kettle (which has accessible metal) and electric 
coffee-maker (which has the heater plate accessible metal).  Both are 2-wire.  
Neither has the double-insulated symbol.  Both are UL-certified.  

 

Best regards from beautiful snowy Bend, Oregon, USA,

Rich

 

 

From: Scott Xe  
Sent: Tuesday, January 14, 2020 6:59 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Class I vs Class II safety constructions

 

In terms of safety level, both constructions are given the equivalent 
protection against electric shock.  In electrical appliances, Class I is used 
most whereas Class II is employed in most electronic products.  Is there any 
background for such design route?

 

In some cases such as induction cookers, the enclosure is plastic/glass - no 
any internal metal part exposes to the outside surfaces.  The product is not 
marked with a double square symbol and comes with a 3-pin plug.  Why is this 
type of product not classified as Class II rather than Class I with the 
plastic/glass enclosure?

 

Thanks and regards,

 

Scott


-

This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 


All emc-pstc postings are archived and searchable on the web at:
http://www.ieee-pses.org/emc-pstc.html

Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used 
formats), large files, etc.

Website:  http://www.ieee-pses.org/
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List rules: http://www.ieee-pses.org/listrules.html

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Scott Douglas 
Mike Cantwell 

For policy questions, send mail to:
Jim Bacher:  
David Heald: 

Re: [PSES] Touch current in IEC 60335-1 for Household Appliances

[Joe Randolph]

Hi Rich:

 

Thanks, I think I am starting to better understand my options.  Following is
my current understanding:

 

1)  If the internal non-SELV circuit does not meet the requirements in
clause 8.1.4 for "protective impedance," the product will be Class 2 per
clause 3.3.10 and double insulation will be required around the internal
non-SELV circuit.  This includes meeting all the criteria for double
insulation, including creepage distance, clearance distance, distance
through solid insulation, and an electric strength.

 

2)  Even if the internal non-SELV circuit does meet the requirements in
clause 8.1.4 for "protective impedance," I still can't use the Class 3
classification per clause 3.3.2, due to the way that clause 3.3.12 is
presently worded. 

 

3)  However, if the internal non-SELV circuit does meet the requirements
in clause 8.1.4 for "protective impedance," I can use the Class 2
classification per clause 3.3.10, but also use the "protective impedance"
provisions in clause 8.1.4.  The key distinction is that with the protective
impedance provisions, the internal non-SELV circuit would not be considered
to be a "live part."  And, if it is not a "live part," no insulation is
required.  

 

What I would like to avoid is the requirement to separate the internal
non-SELV circuit from accessible parts (including the USB port) with double
insulation.  It appears to me that the physical construction requirements
for double insulation would require considerable changes to the present
design.  At present, the internal non-SELV circuit shares the same return
path as the USB circuit, so there is no isolation between the two circuits.

 

Your explanation of how an electric shock risk requires that current flow
through the human body is very helpful for understand the underlying
principles.  It would appear that with only one pole of the internal
non-SELV circuit accessible (at the USB port), it would not be difficult to
prevent current from flowing when performing the accessibility tests.

 

However, to keep a test lab happy, I also need to demonstrate compliance
with the actual wording of 60335-1.  If it turns out that I have to provide
double insulation, it is not clear to me that I can avoid placing a double
insulation barrier between the internal non-SELV circuit and the accessible
USB port.  

 

Based on the underlying principles of electric shock, it would appear that
the internal non-SELV circuit could share the same return path as the USB
port, provided that double insulation is provided between the internal
non-SELV circuit and all other accessible parts.  With this construction,
there would be no path for current to flow.  

 

I'm just not sure whether a test lab, when performing a construction review
of the required double insulation, would agree that no insulation is
required between the internal non-SELV circuit and the USB port.

 

My preference would be to use the approach outlined in item 3 above, where
there would be no requirement for insulation at all (only a requirement for
the "protective impedance").  I plan to have some tests performed to
determine whether the internal non-SELV circuit can be classified as having
a "protective impedance."  Based on just a review of the circuit diagram, I
think it has a 50/50 chance of meeting the requirement for protective
impedance.

 

If it does meet the requirement for protective impedance, do you think that
the approach described in Item 3 above would be acceptable under 60335-1?

 

 

 

Thanks,

 

Joe Randolph

Telecom Design Consultant

Randolph Telecom, Inc.

781-721-2848 (USA)

  j...@randolph-telecom.com

  http://www.randolph-telecom.com

 

From: Richard Nute [mailto:ri...@ieee.org] 
Sent: Friday, January 10, 2020 10:05 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Touch current in IEC 60335-1 for Household Appliances

 

 

Hi Joe:

 

Electric shock requires both voltage and current.  If the voltage exceeds
the specified limit, the current must not exceed its specified limit.  

 

The 60335 standard as well as the 60950 standard concentrated on voltage.
But, there was some recognition that if the voltage was too high but if the
current was low, the construction was acceptable, hence the requirements for
limited current circuits.  See attached discussion of how some standards
addressed this reality.  

 

As I mentioned, I am guessing that your 60-volts is a limited current
circuit.  You can easily test this with a 1500-ohm resistor across the 60
volts and measure the voltage.  My guess is that it will be near zero.

 

More later,

Rich

 

 

From: Joe Randolph mailto:j...@randolph-telecom.com> > 
Sent: Friday, January 10, 2020 10:07 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG  
Subject: Re: [PSES] Touch current in IEC 60335-1 for Household Appliances

 

I agree that there are several opportunities to add some int

[PSES] Class I vs Class II safety constructions

[Scott Xe]

In terms of safety level, both constructions are given the equivalent
protection against electric shock.  In electrical appliances, Class I is
used most whereas Class II is employed in most electronic products.  Is
there any background for such design route?

In some cases such as induction cookers, the enclosure is plastic/glass -
no any internal metal part exposes to the outside surfaces.  The product is
not marked with a double square symbol and comes with a 3-pin plug.  Why is
this type of product not classified as Class II rather than Class I with
the plastic/glass enclosure?

Thanks and regards,

Scott

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