I highly recommend against placing a short circuit across the mains. The
only thing worse than shorting phase to phase is shorting a phase to
ground.
Circuit breakers are designed to protect from overloads, short circuits and
ground faults. However, the number of operations a circuit breaker can
withstand goes down with the severity of the fault. Although a circuit
breaker may continue to operate reliably through dozens of overload
conditions, a UL 489 Listed circuit breaker is typically only rated for two
ground faults. After that, you can no longer trust the ratings of the
breaker.
You also have to look at the immediate safety issues. A short circuit can
create a significant arc flash and blast. A good, sealed switch will
normally contain the arc, but there is always at least a small risk of the
arc flash and blast escaping your shorting switch. This will be a big
problem if there is a high short circuit current capability in your
circuit.
You also have to look at the types of overcurrent devices in the system.
What happens if the first protection upstream reacts more slowly than one
further upstream? The fault current will be well above the rating of the
device being tested and will be capable of trip the next device upstream.
You may end up shutting off power to a lot more equipment than you expect.
Even if this doesn't happen the first or second time, it will always be a
risk.
The high fault currents will also stress the wiring. It is going to get
hot and you may have some damage at connections. The damage from one test
may be small, but it will be cumulative.
If you must run this test, put your system on an isolation transformer. It
will prevent damage to other parts of your power distribution system.
Choose a transformer to properly limit the fault current. Finally, put
protective barriers around the system under test and trigger the fault
remotely.
Ted Eckert
American Power Conversion Corporation
The items contained in this e-mail reflect the personal opinions of the
writer and are only provided for the assistance of the reader. The writer
is not speaking in an official capacity for APC nor representing APC's
official position on any matter.
gmcclure@lexmark.
com
Sent by: To
[email protected] [email protected]
cc
10/05/2006 11:25 Subject
AM shorting the mains prohibited -
Help
Gentlemen,
I need your collective memory.
I have an engineer that wishes to design a protection mechanism to apply a
short circuit across the mains in order to open a protection device
upstream to stop an over-heating fault. The protection device would be in
the product, we are not talking about depending on the protection in the
service panel. I will not let them go there.
I remember somewhere in the past that one of the standards, or perhaps a
country deviation, specifically forbid shorting the mains as a means of
protection but I cannot find it. I think it is from the era when we were
using IEC 380 or 435 and UL 478 but I am not sure.
Can someone out there point me to the standard and clause? or perhaps the
deviation or an OSM decision?
I am looking for all of the arguments against this practice I can pull
together because I do not feel it is sound. It is one thing to crowbar the
output of a power supply to protect an expensive logic board from a power
supply over-voltage failure. It is quite another to short the mains input.
Many thanks,
Gregory H. McClure
Lexmark Product Safety
859 232 3240 office
859 232 6882 fax
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