00zer said:
Your wireman created an arcing fault by touching the wires together, not a bolted fault. If he had solidly connected the two wires together, with the power off, and then turned on the breaker, I suspect it would have tripped very quickly. By the way, this procedure is NOT recommended as a diagnostic tool. It is difficult to determine if you damaged the conductors and/or other equipment on the circuit during such a test. Your wireman has re-discovered the reason that arc-fault circuit breakers were invented (let's not argue over their effectiveness here). An arcing fault has enough impedance that a standard circuit breaker just sees it as a load. The amount of that load would be determined by many factors, such as the distance for the arc, temperature, content of the metal to which the arc was connected, and the length of time that the arc was present. The reason that the original breaker did not trip is also probably because the original fault was an arcing fault. There may have been a loose connection at the receptacle or the springs in the receptacle itself may have been week. In any event, an arc was formed that the circuit breaker interpreted as a load. The magnitude of the load was below the trip point of the breaker, so heat was created that eventually melted the plug. Unfortunately, all of this is normal and a standard circuit breaker does not provide much protection in such a scenario.
