Incidently this is how I found my FPE breakers dont always trip.
turned off what I thought was the right breaker in the middle of a job, pushed the 2 wires together, they sparked, stuck together, and literally fried. length turned brite red and blew apart. the short loaded down the neighborhood, my neighbor was running a circular saw, it quit during the short. my neighbor saw the flash and came over and asked you still alive:( I was working in my garage, adding a outdoor light.
You try your best but when doing jobs things happen....
I've used the dead short method of touching a hot wire to a neutral, but see two problems with it:
- The wires weld together. Not badly, they can be pulled back apart with some force, but there is definitely some metal transfer that weakens at least one of the wire ends.
- The current is a quick spike of a couple hundred amps. This seems like an inadequate test of a 20A breaker -- you still don't know if it will trip at 25A as expected.
My understanding of fuses and breakers is that they typically will run at 110% of the rated capacity indefinitely, and trip at currents exceeding that. So to test a 20A breaker for instance, you would want to put a ~25A load (maybe two blowdryers on a heavy gauge extension cord) on it and wait for the trip. Any problems with this method (or is there a better one)?
It might take a while for the trip to happen... one of my books is showing that a typical 15A fuse will take 3.9 seconds to trip on 30A, and a full 31 seconds if it's a "time delay" type. Breakers have similar behavior: 150% of capacity will take a minute to cause a trip.
You are certainly right about tripping being a function of load and time. To me it seems a little risky to use wiring, receptacles, and other devices of unknown integrity for load testing of suspected breakers. I would be inclined to remove the breaker and devise some sort of load with a 12 volt car battery. I think high beam headlamps take about 5 amperes each.
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