“Grounding” of US household electrical systems has changed quite a bit over the years, as have the common wiring methods. Pre-1960 wiring, depending on the wiring materials, often had a “ground path,” though there was no real intent to provide one.
Growing up, I was actually present when the “electrical greats” debated various topics. I am sometimes amused when I see some creative soul today echo earlier ideas, thinking they came up with them.
What is “grounding?” What is “grounding” supposed to accomplish? Why do we “ground” things? In my life I’ve seen major changes in theory as our understanding has improved. I’ve watched these ideas reflected in code requirements. Many of the stranger parts of our codes when it comes to grounding reflect some of these earlier beliefs.
Another puzzling aspect to our code is the recognition of two very similar wiring methods: Armored Cable and Metallic-clad Cable (AC and MC). A related method is “Flexible Metallic Conduit,: or “flex” (BX). Is the spiral metal sheath of these methods a reliable grounding path? The answer, in code, is “yes” for AC, “maybe” for flex, and “no” for MC. We can attribute the different methods to competing manufacturers.
Does the metal make an effective ground path? Testing done on lightweight metal conduit (EMT) showed that even an extremely poor assembly of components would still carry a surprising amount of fault current. I guarantee that no electrician is capable of actually measuring the effectiveness of a ground path in the field.
In new work, today’s most common household wiring methods are either AC or non-metallic cable (NMC or “(Romex). Both of these require the use of a ground wire. When we add to, repair, or modify an older system that lacks a ground wire, we’re pretty much prevented from trying to add a ground. Earlier use of nearby plumbing is today defeated by the common use of plastic plumbing pipe. An old house without a ground wire will never have a ground wire. There’s no place for that third prong to go.
So how are we supposed to use modern appliances with three-prong plugs? Before we answer that, let’s ask “Why do we ground appliances?” The common answer “safety” falls flat when you realize it requires less than 1/10 amp to kill someone, and your smallest breaker / fuse is 15 amps. You’ll be well fried well before that overload ever trips.
As a compromise — a nod to reality — our rules allow such older circuits to be protected only by GFCI’s, and overlook the lack of a ground wire. The GFCI will operate in the event of a very small amount of electricity “getting lost.” This accommodation is allowed only for existing, older circuits. New circuits (complete circuits originating at the panel) must have that ground wire.
