Arc Fault Circuit Interrupters

An Arc Fault Circuit interrupter (AFCI) is an electrical device designed to minimize and eliminate arcing hazards on protected circuits. The AFCI does this by recognizing dangerous arcing conditions. But are AFCI's really necessary? I am going to discuss this from a theoretical standpoint.

First, we need to understand the properties of an electrical arc. An arc occurs when electrical voltage is high enough to cause a breakdown and ionization in the insulating medium, in this case air. The air now becomes a conductor, and passes electrical current. Controlled arcs are those commonly seen in lighting and welding applications.

In power distribution, electrical arcs can be as spectacular as they are dangerous:


Can dangerous arcing conditions exist in your home? To answer that question, we turn to Paschen's Law. Paschen's Law is an operation of voltage, arc gap distance and gas pressure.

Remembering back to my days in electrical training, it was taught that the minimum voltage to create any type of an arc is 350 volts. Using Paschen's Law, the arcing voltage for an arc gap of 7.5 micrometers (about the width of a red blood cell) under standard air pressure, is 327 volts. That is nearly three times the voltage present in most household applications.

What this means in simple terms, that under normal residential applications, arc faults are not even possible, let alone necessary to protect against.

The issue basically comes down to arc faults vs. spark faults which is a very real and dangerous condition.

I don't know if the issue is simply a semantic one. Saying that we are protecting against the nature and dangers of arc faults definitely sounds more pertinent than that of spark faults. Even though electrically, it appears that these terms are used synonymously(to the exclusion of "spark fault"). To this author however, it seems as more an effort for electrical manufacturers and lobbyists to use Electrical Code to legislate profits.