I have noted what you say about copying to the list address. I normally
send ONLY to the list address.
On 2020-01-15 19:02, Joe Randolph wrote:
Hi John:
Yes, solid insulation can be provided using thin sheets to solve
certain problems in confined spaces.. For the product in question,
the main challenges of adding reinforced insulation are the creepage
and clearance distances. That is the main reason I want to avoid
having to implement double insulation.
I think my immediate task is to determine whether the circuit in
question can pass the “protective impedance” requirements in 60335-1.
That’s going to take a week or so because I do not presently have a
working sample of the product. Apparently, I could use the protective
impedance classification even in a Class 2 product, so the ambiguity
that was bothering me in the Class 3 definition may be avoidable.
BTW, I noticed that the last two emails you sent to me were not copied
to the EMC-PSTC list server. For my earlier response I assumed this
was a simple oversight and I added the list server to my response. I
decided not to do that this time, just in case your omission was
intentional. On the other hand, if was not intentional, I thought you
should know that it is happening.
I very much appreciate your insights and suggestions on my touch
current question and several other topics that have come up on the
EMC-PSTC list server.
Thanks,
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected] <mailto:[email protected]>
http://www.randolph-telecom.com
*From:* John Woodgate [mailto:[email protected]]
*Sent:* Tuesday, January 14, 2020 1:27 PM
*To:* Joe Randolph <[email protected]>
*Subject:* Re: [PSES] Touch current in IEC 60335-1 for Household
Appliances
I am wondering if you are over-estimating the requirements for double
or reinforced insulation. It's difficult to be sure whether you are,
but quite a thin sheet of PET plus an air-gap provides double
insulation. A somewhat thicker PET sheet can provide reinforced
insulation.
On 2020-01-14 18:09, Joe Randolph wrote:
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?
-
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