Hi Chris:
Consider a product with two, independent protective
grounding/earthing connections. This may be by
means of two power cords (as is done for uptime
reliablity by employing parallel power supplies) or
by means of one power cord and a separate ground
connection (as, for example, by mounting in a
grounded rack).
> Safety standards call for single fault testing. For Class I equipment, one
> of the single fault test conditions is removal of the ground connection.
Agreed. The disconnection of one ground is a single-
fault condition.
> I'm curious how most test labs would reconcile the two statements above. My
> guess is that they would interpret removal of ground to mean removal of all
> ground connections. So putting on an extra ground wire wouldn't help. It
> would just make the safety engineer disconnect another wire to perform the
> test.
I don't agree. The requirement is that of a single-
fault condition. If normal operation employs
redundant grounding, then a single-fault condition
is that of failure of one ground connection.
> "What if the product is used in a building or environment with an unreliable
> ground?" or "How can you garantee that the product's ground potential will
> always be equal to the potential of the floor where the user is standing"
If the ground within the building installation is
subject to failure, then the fault is that of the
building installation, not of the product. So, it
would be nonsense to require a product single-fault
"no-ground" test on that basis.
A faulty ground in the building installation allows
cumulation of leakage currents from all equipment to
be available on each and every grounded equipment,
a truly dangerous situation because the cumulative
leakage current could be in the hundreds of
milliamperes!
(Ironically, the equipment with a faulty ground would
be the only safe equipment in such a situation!)
I was recently invited to comment on the subject of
single-fault testing requirements for products with
multiple power cords. My argument was based on the
idea that any product with multiple power cords is
"professional" equipment where the advantage of such
equipment is only achieved by connecting to multiple
power sources. So, this is normal operation. A
single-fault test is with one ground open (a meaningless
test when there is a second ground in place). I
recommended a leakage current test with one power
cord connected which would simulate the situation
where the redundancy was not used, and there was a
fault in the grounding system.
Best regards,
Rich
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