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Old 15th Sep 2012, 4:01 am   #11
dinkydi
Retired Dormant Member
 
Join Date: Feb 2011
Location: Sydney, Australia
Posts: 277
Default Re: Failing Schaffner IEC Inlet Filters

Fuses and circuit breakers have a rating at which they can pass current on a continuous basis. However, currents that are orders-of-magnitude higher can be supplied in the short term.

In the investigation of sudden short-circuits on a supply line, when you have two or more breakers of different capacity in series supplying the load, you may find that a larger breaker trips but not a smaller. What does this tell you?

Plainly the fault current must have been in excess of the larger breaker, but also, at this current the larger breaker must have been faster than the smaller breaker. As David notes, the manufacturers supply current-versus-time characteristics for their breakers. (In general, breakers get faster with larger current.) The characteristics enable you to determine the minimum current at which the larger breaker is faster than the smaller one.

In a real case with a 20A and a 50A breaker in series with only the 50A tripping, examination of the characteristics shows that the minimum short-circuit current that could cause this situation is 300A. The actual short-circuit current must be greater than this. (For reference, note that the peak current in some "solid" short circuits may reach 1000A, though usually only for a limited time!)

In my experience of Schaffner filters tripping breakers and QQVO6/40's experience with Rifas, I think my estimate of "100s of Amps" is realistic. (By the way, QQVO6/40, at the risk of being called ghouls, Wellington and I would love to see photos of your unfortunate accident.)

Peter
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