On Thursday, February 5, 2015 at 11:35:42 AM UTC-5, Jeff Liebermann wrote:
Note: Some thermal circuit breakers will decrease in value if tripped
repeatedly. This is a safety feature as having the trip value
increase would eventually make the circuit breaker not trip at all.
Magnetic circuit breakers don't have this problem/feature.


I looked this up some time ago for the larger circuit breakers such as those in a main panel.

The manufacturers give specs for the number of cycles.

That number is very large for a circuit breaker being used as a switch to turn a circuit on and off. Many times lighting circuits, e.g., are operated with a breaker and don't have a switch. I don't remember offhand but I'm thinking in the range of 50,000 cycles.

Then you have the next case where the breaker "trips" on overcurrent. You have the toaster on when the refrigerator cycles, etc. You've exceed the 15 amp by enough to trip the breaker but it's not a dead short. The number of rated cycles goes down by a couple orders of magnitude but it's still high.

But now you have the dead short case, where you draw maximum fault current until the breaker trips. Breakers are only rated for 1 to 2 cycles of this kind of duty.