Electrical Fault Finding

A series arc happens along the length of a circuit, at a loose termination or a broken strand, and the current passing through it is limited by whatever load is downstream. It can be a small current and still run hot enough to ignite wood, and because it does not exceed the breaker rating, a conventional breaker never sees a reason to trip.

A parallel arc is a fault between conductors or to ground, which draws a much larger current and usually does trip something, though not always fast enough where the fault is intermittent and self extinguishing.

Arc fault breakers exist because the series case was invisible to everything else in the panel. They look at the electrical signature rather than the magnitude, which is why they can catch a damaged cord or a nicked conductor that a standard breaker would carry happily until the wall caught.

A three prong receptacle in an old two wire house has to come from somewhere, and often it came from somebody replacing the device without a ground present. That is an open ground, and a plug in tester reports it honestly. The device works, the surge protector plugged into it does nothing at all, and nobody notices until something faults.

The dangerous version is a bootleg ground, where a jumper ties the ground terminal to the neutral at the receptacle. A plug in tester reads that as correct wiring, which is the whole problem. It is not correct. If the neutral opens anywhere upstream, every grounded metal case downstream becomes energized through that jumper.

Finding them takes actual testing rather than a three light tester, and they turn up regularly in houses that have been added onto over the decades. The honest repair is a real equipment ground or a ground fault device labeled as having no equipment ground, not a jumper.