I had a strange thing happen today. I was working on an old house, wired partly with BX. I was attempting to pull a receptacle out of a box when one of the conductors shorted to the metal box. I saw the spark, followed by the hum, but the breaker didn't trip. It actually happened twice, with one instance for over three seconds. The breakers are all recent from a recent upgrade. Now I know BX without the bonding strip is an unreliable ground fault path. But what I can't understand is the hum. For the circuit to hum like that, I would think there has to be mega current flowing. And if there is mega current flowing, the breaker should open before three seconds. So I guess what I am really asking is, how much current has to flow to make the conductors hum quite noticeably? And if there is a huge amount of current flowing from the ground fault, the grounding path must be low resistance to allow it, right?
