Originally Posted by Limey Linda
Most electro mech. breakers are heat operated. After they have popped once the bi-metal strip ( I assume) takes on a different rating. A friend of mine works for the local power service and if called out fix a problem with a breaker they always replace it.
so does the rating go up or down? if it goes up that would seem to be an unsafe design. if it goes down then you would expect the breaker to trip quicker next time, go down again and the problem to perpetuate. Their popularity suggests this cannot be a significant issue.
My understanding was the breakers in my house (which I think are fairly common) used both a thermal trip to cut out small continuous current overloads and an electromagnetic switch to activate an immediate very high overload (e.g. a short).
Is your friend changing breakers in someones house or on the electricity network? could it be that he is switching very high voltages/currents where the arcing inside the switch may damage the contacts? or perhaps the cost/inconvenience of having to return to a site/the power going down if the breaker is faulty makes it a cost effective precaution.
Sorry for drifting a bit Off Topic - but I think you were possibly doing circuit breakers a disservice - although how suitable they are on an open boat is a different question.