Hi Rich:
Thanks for the feedback. Right now it appears that my Option 3 would
be the simplest approach because it does not require double insulation
anywhere. I don’t presently have a working sample of the product, but
I plan to perform the “protective impedance” tests after I obtain a
working sample. As I noted earlier, just from an inspection of the
circuit diagram and knowledge of the load that the 60V circuit is
driving, I think it has a 50/50 chance of meeting the requirements for
protective impedance.
If the outcome is that the circuit does */not/* meet the protective
impedance requirements and double insulation has to be provided, I
have a question about your statement that that it is okay for the 60 V
circuit to share a common return with the USB circuit.
Somewhere (possibly in relation to using 60950-1), I recall that ports
for connection to SELV circuits were treated as being accessible even
if the connector could not be reached by the accessibility probe.
This suggests that the USB connector would be treated as an accessible
part, even if it is recessed and guarded to make it inaccessible to
the probe.
If such reasoning applies for 60335-1, it would appear that double
insulation must be provided between the 60V circuit and USB connector.
I suspect that a fault analysis could show that at least two faults
would be required to place the 60V output on the USB port, but “double
insulation” is “double insulation,” and creepage/clearance
requirements would seem to apply between the 60V circuit and the USB port.
Is there a compliance path through the standard that would not require
double insulation between the 60V circuit and the USB port? So far I
have not found such a path.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected] <mailto:[email protected]>
http://www.randolph-telecom.com
*From:* Richard Nute [mailto:[email protected]]
*Sent:* Tuesday, January 14, 2020 9:45 PM
*To:* [email protected]
*Subject:* Re: [PSES] Touch current in IEC 60335-1 for Household
Appliances
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 <[email protected]
<mailto:[email protected]>>
*Sent:* Tuesday, January 14, 2020 10:10 AM
*To:* [email protected] <mailto:[email protected]>;
[email protected] <mailto:[email protected]>
*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)
[email protected] <mailto:[email protected]>
http://www.randolph-telecom.com
*From:* Richard Nute [mailto:[email protected]]
*Sent:* Friday, January 10, 2020 10:05 PM
*To:* [email protected] <mailto:[email protected]>
*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 <[email protected]
<mailto:[email protected]>>
*Sent:* Friday, January 10, 2020 10:07 AM
*To:* [email protected] <mailto:[email protected]>
*Subject:* Re: [PSES] Touch current in IEC 60335-1 for Household
Appliances
I agree that there are several opportunities to add some interlocks
(physical or electrical) that would power down the high voltage
circuit under certain conditions. If that were all that is necessary
to comply with 60335-1, it would probably be quite manageable.
i
The main problem I’m concerned about is the apparent requirement in a
Class 2 appliance to provide double insulation between the high
voltage circuit and accessible parts. In particular, the creepage and
clearance distances required for the double insulation would be a
challenge to achieve without major changes to the physical design.
Presumably, with a sufficient number of interlocks, it would be
possible to ensure that the high voltage circuit is disabled under
certain circumstances, and this might possibly help to avoid double
insulation in certain areas.
However, the fact remains that when this handheld product is actually
in use (no battery charger connected, battery compartment closed, and
user-replaceable module installed), there is a non-SELV voltage being
generated within the device, and 60335-1 seems determined to require a
double-insulation barrier around that circuit.
In other standards such as 60950-1 and 62368-1, circuits with non-SELV
voltage but limited current can be classified as a “limited-current
circuit” (60950-1) or “ES2 circuit” (62368-1). Circuits that qualify
have no accessibility restrictions whatsoever. In terms of
accessibility, they are treated like SELV.
For the product in question, this would be a much easier way to
demonstrate compliance. I think that clause 8.1.4 in 60335-1 is
intended to provide this option, but the unusual wording of clause
3.3.12 appears to prevent this option.
I agree that it can take a very long time to get an official
interpretation issued or, worse yet, get a standard changed.
Fortunately for me, the product in question is not my product and I
was not involved in designing it. I’ve simply been asked to evaluate
the product for compliance with 60335-1.
When I made my initial post on this topic, I was just trying to find
out if clause 3.3.12 had been (or might soon be) revised to remove the
apparent contradiction in the 2009 edition. From some preliminary
feedback I have received, it sounds like the answer is “no.”
In the meantime, the responses I have received here have identified
some useful approaches for trying to demonstrate compliance as a Class
2 appliance per 60335-1, and I plan to think carefully about those
approaches. Right now, the main sticking point is the apparent
requirement to provide double insulation, with its attendant creepage
and clearance distances.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected] <mailto:[email protected]>
http://www.randolph-telecom.com <http://www.randolph-telecom.com/>
*From:* John Woodgate [mailto:[email protected]]
*Sent:* Friday, January 10, 2020 11:43 AM
*To:* Joe Randolph <[email protected]
<mailto:[email protected]>>
*Subject:* Re: [PSES] Touch current in IEC 60335-1 for Household
Appliances
Can that be done while the product is powered-up? If not, how long
does it take after power-down for the 60 V to disappear from the
accessible poles?
Can you cover up the accessible poles? Basic insulation might be
enough, but reinforced insulation might not be too bulky.
It would almost certainly take years to get 60335-1 modified. There is
a very small chance of an Interpretation Sheet being agreed, but even
that takes about 18 months minimum. I suppose you are in USA. If so,
go to this page on the IEC web site and send an email explaining your
problem to Ms R K Myers and ask for advice how to try to get a
clarification:
https://www.iec.ch/dyn/www/f?p=103:29:13970009729823::::FSP_ORG_ID,FSP_LANG_ID:1236,25#3
You will probably be referred to the chair of the ANSI committee.
You can still do that if you are not in the USA. but there is a
different path if you are not.
On 2020-01-10 16:03, Joe Randolph wrote:
4)Lastly, there is a user-replaceable module that, when removed,
allows both poles of the high voltage supply to be accessible.
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