Re: [PSES] IEC60950-1 Limited Power Source via IC current limiter

[Brian O'Connell]

Mr. Nute,

Per IEV definition 192-10-06, fail-safe is:
"capable of preserving safety in the case of failure
Note 1 to entry: The safe conditions should be defined for the particular 
application."

Per IEC62368-1, fail-safe only applies to stuff in annex K (safety interlocks). 
Where the mitigation is more of an exercise in reducing energy after an 
interlock open, rather than any specific reliable performance level. There is 
no 'fail-safe' in IEC60950-1 (probably because TC108 had a Romulan spy at the 
time of the 1st edition).

As for MTTF, it tends towards a ritualized mathematical ceremony per Mil217 and 
SR322 (hooded robes are required to perform the calculations). Reliability is 
more secular, but is a 'localized' property per the scoped standard's test 
requirements. And have seen some designs that fail-safe the fail safe; that is, 
a decreased reliability, but less likely to fail to an unsafe condition. Choose 
your poison.

Avoiding the HazLoc ('intrinsic' safety) morass, the stuff in UL1310 and 
UL5085-3 resembles something that is reliably long-term fault tolerant (note 
the non-use of 'fail-safe') for limited categories of equipment.

So, it is obvious that my stupidity (and evil thoughts) prevent me from 
answering your questions, because this is another "it depends". That is, fail 
safe depends on integrating properly rated components into clever designs, and 
implementing ingenious test methods. In my little cloistered world, there are 
no fail-safe components and there are no reliable fail-safe systems. But there 
can be extensively tested products that have been subject to incremental design 
improvements, where the probability of a catastrophic life-time failure 
increasingly favors your side.

Brian


From: Richard Nute [mailto:ri...@ieee.org] 
Sent: Thursday, March 08, 2018 1:42 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] IEC60950-1 Limited Power Source via IC current limiter

In my early days in product safety, safety was prohibited from relying on 
conduction in a vacuum, gas, or semiconductor.  

Today. we rely on semiconductor current limiters and similar devices, e.g., 
GFCI control circuits.  

Are such limiters and control circuits fail-safe devices?   

Or, are they "reliable" devices and circuits where their lifetime is expected 
(proven) to be greater than the host equipment?

Rich

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Re: [PSES] IEC60950-1 Limited Power Source via IC current limiter

[Nyffenegger, Dave]

Yes new GFCIs are required to have built in automatic testing as most old ones 
installed are not getting the required periodic manual testing.

From: MIKE [mailto:msherma...@comcast.net]
Sent: Thursday, March 08, 2018 5:34 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] IEC60950-1 Limited Power Source via IC current limiter

I regard North American household GFCIs and industrial shunt trip GFCIs to not 
be fail safe because they can fail silently and therefore require period 
testing to verify continued functionality.  I believe that the UL standard for 
GFCIs has recently been updated to require some periodic self testing to 
address this issue.
Mike Sherman
Graco Inc.

Sent from XFINITY Connect Mobile App

-- Original Message --

From: John Woodgate
To: EMC-PSTC@LISTSERV.IEEE.ORG
Sent: March 8, 2018 at 3:57 PM
Subject: Re: [PSES] IEC60950-1 Limited Power Source via IC current limiter

Probably some fail-safe, some reliable. I think that this is why 'safety' 
involves risk-assessment, not just meeting standards.

John Woodgate OOO-Own Opinions Only

J M Woodgate and Associates www.woodjohn.uk

Rayleigh, Essex UK
On 2018-03-08 21:41, Richard Nute wrote:

In my early days in product safety, safety was prohibited from relying on 
conduction in a vacuum, gas, or semiconductor.

Today… we rely on semiconductor current limiters and similar devices, e.g., 
GFCI control circuits.

Are such limiters and control circuits fail-safe devices?

Or, are they “reliable” devices and circuits where their lifetime is expected 
(proven) to be greater than the host equipment?

Rich


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This message is from the IEEE Product Safety 

Re: [PSES] IEC60950-1 Limited Power Source via IC current limiter

[MIKE]

I regard North American household GFCIs and industrial shunt trip GFCIs to not be fail safe because they can fail silently and therefore require period testing to verify continued functionality.  I believe that the UL standard for GFCIs has recently been updated to require some periodic self testing to address this issue. Mike ShermanGraco Inc. Sent from XFINITY Connect Mobile App-- Original Message --From: John WoodgateTo: EMC-PSTC@LISTSERV.IEEE.ORGSent: March 8, 2018 at 3:57 PMSubject: Re: [PSES] IEC60950-1 Limited Power Source via IC current limiter
  
  
Probably some
fail-safe, some reliable. I think that this is why 'safety'
involves risk-assessment, not just meeting standards.

John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk
Rayleigh, Essex UK
On 2018-03-08 21:41, Richard Nute
  wrote:


  
  
  
  
 
In my early days in product safety, safety
  was prohibited from relying on conduction in a vacuum, gas, or
  semiconductor.  
 
Today… we rely on semiconductor current
  limiters and similar devices, e.g., GFCI control circuits.  
 
Are such limiters and control circuits
  fail-safe devices?   
 
Or, are they “reliable” devices and
  circuits where their lifetime is expected (proven) to be
  greater than the host equipment?
 
Rich
 
 
  
  -
  
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Society emc-pstc discussion list. To post a message to the list,
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at:
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Communities site at http://product-compliance.oc.ieee.org/
can be used for graphics (in well-used formats), large files,
etc.
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  (including how to unsubscribe)
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Re: [PSES] IEC60950-1 Limited Power Source via IC current limiter

[John Woodgate]

Probably some fail-safe, some reliable. I think that this is why 
'safety' involves risk-assessment, not just meeting standards.


John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk
Rayleigh, Essex UK

On 2018-03-08 21:41, Richard Nute wrote:


In my early days in product safety, safety was prohibited from relying 
on conduction in a vacuum, gas, or semiconductor.


Today… we rely on semiconductor current limiters and similar devices, 
e.g., GFCI control circuits.


Are such limiters and control circuits fail-safe devices?

Or, are they “reliable” devices and circuits where their lifetime is 
expected (proven) to be greater than the host equipment?


Rich

-


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emc-pstc discussion list. To post a message to the list, send your 
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site at http://product-compliance.oc.ieee.org/ can be used for 
graphics (in well-used formats), large files, etc.


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Re: [PSES] Isolation Transformer & Class III electrical protection

[Brian O'Connell]

0. An 'appliance' (white goods) and ITE have similar, but not same, 
requirements for Class II and Class III equipment.

1. Unknown, and your definition is incomplete. Dependent on rating of the power 
source to the Class III device. SELV output rating does not imply Class III 
equipment. By definition, for non-medical equipment, SELV is considered safe to 
touch, but is not necessarily Class III. Leakage through body not necessarily 
considered for Class III device, but for touch-current, reference the 
human-body models and test methods for leakage in IEC60950-1 and IEC60335-1. 

2. Non sequitur. Both provide galvanic isolation. Step up/down is simple matter 
of turns ratio of other than 1:1. A mains isolation transformer, depending on 
the ground reference, can be used for

- float the equipment connected to the secondary to prevent a neutral 
connection to ground
- noise mitigation
- leakage current mitigation
- controlling mains output impedance
- door stop

3. Unknown, as Class III products do not necessarily have the built-in 
protective construction required of Class I and II equipment. By definition, 
Class III equipment has no earthing; mostly because the intent is that power is 
provided by a Class II source, which implements DI, thus no protective earth 
requirement. 

For a transformer winding intended to source a Class III construction, the unit 
would have to meet inherently limited and inherently short-circuit proof 
requirements per IEC61558-1 and -2-6 and/or -2-16, and the outputs would have 
to be floated and insulated from ground, and the capacitance of the windings 
would have to be low enough to limit the touch current to any exposed metal 
surface.

In North America, the transformer should be assessed per UL5085-3/CSA No.66.3.

Brian


From: Vincent Lee [mailto:08e6c8d35910-dmarc-requ...@ieee.org] 
Sent: Wednesday, March 07, 2018 7:17 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Isolation Transformer & Class III electrical protection


Hi all,

Good day,

(1) If Class III electrical protection is defined as appliance with voltage 
output less than SELV (30Vrms based on IEC 60950 IT equipment), so can a laptop 
AC - DC power adaptor with output rating 19Vrms, 2.37A be considered as Class 
III, given that output current, 2.37A is much more than 10mA Threshold "Let-Go" 
current ?


(2) How does Isolation Transformer different from the usual Step-Down Voltage 
Transformer?


(3) And how does Isolation Transformer helps to achieve Class III electrical 
protection as stated in page 3 of 
https://www.excelsys.com/wp-content/uploads/2011/09/ApplicationNoteAN1102-ClassIvsClassII.pdf

Hope to hear from you soon. Thank you & have a nice day ahead.

Vincent

Regards, Vincent

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