Sorry for being slow to understand this, but I can’t quite see the
rationale for exempting the USB port from having double insulation
between the USB port and the 60V circuit.
For discussion purposes, imagine a single circuit board with the
following characteristics:
1)The board contains a SELV circuit and a SELV-to-60V voltage step-up
converter. Both the SELV circuit and the step-up converter share the
same return path (loosely called “circuit ground” but not necessarily
grounded).
2)Aside from functional insulation, there is no isolation whatsoever
between the components of the SELV circuit and the 60V circuit.
3)The SELV circuit has an accessible USB connector
While it might take multiple faults to make it happen, one could
imagine a scenario where the 60V output gets shorted to one of the USB
data pins. Now we have 60V present between two pins on the USB connector.
One could argue that a fault analysis might show that at least two
independent faults would be required to make the 60V appear on the USB
connector, thereby providing two safeguards.
However, Class 2 construction does not allow the manufacturer to rely
on just a fault analysis. Class 2 specifically states that “double
insulation” be provided to prevent electric shock. At present, I do
not see a way around this if the 60V circuit is indeed classified as
hazardous.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com <mailto:j...@randolph-telecom.com>
http://www.randolph-telecom.com
*From:* John Woodgate [mailto:j...@woodjohn.uk]
*Sent:* Wednesday, January 15, 2020 2:57 PM
*To:* EMC-PSTC@LISTSERV.IEEE.ORG
*Subject:* Re: [PSES] Touch current in IEC 60335-1 for Household
Appliances
Double insulation applies only to one pole of the supply, except the
mains supply, where both poles are regarded as live. If one pole is
double insulated, clearly no shock hazard can occur due to both poles
being touched, because one can't be touched.
On 2020-01-15 19:49, Joe Randolph wrote:
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)
j...@randolph-telecom.com <mailto:j...@randolph-telecom.com>
http://www.randolph-telecom.com
*From:* Richard Nute [mailto:ri...@ieee.org]
*Sent:* Tuesday, January 14, 2020 9:45 PM
*To:* EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG>
*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 <j...@randolph-telecom.com
<mailto:j...@randolph-telecom.com>>
*Sent:* Tuesday, January 14, 2020 10:10 AM
*To:* ri...@ieee.org <mailto:ri...@ieee.org>;
EMC-PSTC@LISTSERV.IEEE.ORG <mailto: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)
j...@randolph-telecom.com <mailto: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 <mailto: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 <j...@randolph-telecom.com
<mailto:j...@randolph-telecom.com>>
*Sent:* Friday, January 10, 2020 10:07 AM
*To:* EMC-PSTC@LISTSERV.IEEE.ORG <mailto: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
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)
j...@randolph-telecom.com <mailto:j...@randolph-telecom.com>
http://www.randolph-telecom.com <http://www.randolph-telecom.com/>
*From:* John Woodgate [mailto:j...@woodjohn.uk]
*Sent:* Friday, January 10, 2020 11:43 AM
*To:* Joe Randolph <j...@randolph-telecom.com
<mailto:j...@randolph-telecom.com>>
*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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