I'm working on a 480V distribution relay protection project that
requires the application of a shunt trip device at the time of an
overload condition.  I need to detect when the load exceeds 500A, and at
that time apply a short across the 480V source in order to force the
circuit's 2000A main breaker to trip.  This is similar to using a
grounding switch as the initiator for a transfer trip scheme in the
electric utility world.  Crude, but simple and effective.  I have argued
for other methods but this method has been dictated by the client, the
government client.
 
Detecting the current is no problem.  My effort now is to determine the
best method to apply the short.  The available fault current is in the
neighborhood of 50kA.  A near-zero ohm short would be ridiculous.  A
fault current of approximately 3000A is sufficient and it can be limited
to that magnitude if I use a 425uH (160mohm) inductor for the shorting
conductor.  Such an inductor would be roughly 15cm in diameter and 30cm
long, with 90 turns of wire that is capable of withstanding the 3000A
surge current for a maximum time of about 10 cycles, or 160 ms.
 
The question now is:  What is the best way to switch the inductor across
the 480V supply?  Traditional clapper or mercury wetted relay?  Triac?
SCRs?  Whadayathink?
 
Thanks.  M. Peterson
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