Hi Folks
>From experience, I agree with what Ted says - especially w.r.t. his last
paragraph about setting a LOWER limit.
Failure to exceed the lower limit could also indicate a faulty EMI filter with
capacitors of lower values than specification - thus meaning you may have an
emissions issue with the units in question.
I used to sample a batch of a dozen or so units for each type of unit, and
then set higher and lower limits for the test personnel to check.
Regards
John Allen
-----Original Message-----
From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Ted
Eckert
Sent: 13 November 2008 16:51
To: Robert F. Keller; EMC-PSTC@LISTSERV.IEEE.ORG
Subject: RE: Current limit for Dielectric Test
Hello Rob,
There is no current limit specified, and there is no way a limit could
be
specified. Take a look at IEC 60950-1 section 5.1.7. You will see that there
are types of equipment where the touch current is allowed to exceed 3.5 mA.
This is typically equipment that is field wired or at least has an industrial
locking plug. The high touch current is often due to a significant amount of
filtering in the power supply including Y-capacitors. If these capacitors
(and other circuits bridging primary to ground) are allowing over 3.5 mA at
the normal operating voltage, they will pass far more current at the voltages
used for the electric strength test.
I have worked with high power telecommunication rectifiers that have a
lot of
filtering on their input. They have a very high touch current, but they are
field wired and have a solid ground connection. The risk to the user from the
touch current is quite low. The amount of current that flows during the
electric strength test is quite high.
You should know what the expected current will be during the electric
strength test. You can calculate it from the test voltage and the impedance
of the circuits between primary and ground. Giving some safety margin to this
value, you can generally select a reasonable starting point for the upper
limit on your testing. As the standard notes, a failure is a dramatic
increase in current, not a constant high current. You need to know your
equipment to determine the proper test limits.
Many types of electric strength testers will also let you set a lower
limit
for failure. This too should be set properly. If the current during the
electric strength test is too low, you may have a bad test setup or a bad
connection in your product.
Ted Eckert
Compliance Engineer
Microsoft Corporation
ted.eck...@microsoft.com
The opinions expressed are my own and do not necessarily reflect those
of my
employer.
From: Robert F. Keller [mailto:r...@cclab.com]
Sent: Thursday, November 13, 2008 8:36 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Current limit for Dielectric Test
Hello Group,
I have a question regarding the Electric Strength test in 60950-1
section
5.2. It says in that section that you have a failure or the insulation broke
down when the current which flows as a result of the application of the test
voltage rapidly increases in an uncontrolled manner. So, is there an exact
current limit at which it is considered to have "shorted"? And if so, is this
printed in any standard?
Any insight in this would be greatly appreciated.
Best Regards,
Rob Keller
Senior Engineer
Communication Certification Laboratory
Ph.: 801.972.6146 Ext.237
Fax: 801-972-8432
r...@cclab.com
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