Electrical grounding is a very misunderstood topic. Here’s a quick description of the major facts about electrical grounding.
There are two distinct grounding systems in your house — the Grounding Electrode System and the Equipment Grounding System.
Grounding Electrode System
In the USA we use a grounded electrical system. This means that what we call the neutral wire is physically connected to the ground (the earth, the dirty below our feet) at various points in the system. It’s grounded at the utility transformer, at the electric meter on the side of your hose (through a buried ground rod), and at the metal water pipe coming into your house from the street.
Not all countries use a grounded system like this. But since 1913 our National Electrical Code has required this. It helps to protect us from high-voltage crossover problems at the transformer, and nearby lighting strikes. It helps to discharge static electricity, and it provides protection in case the service neutral wire coming to the house from the utility ever gets broken.
A grounding electrode is any piece of metal that creates this direct connection to the ground. In a typical house the grounding electrodes consist of the buried metal water pipe and a ground rod driven into the ground near the electric meter. Any wire that helps to create this direct connection to the earth is called a grounding electrode conductor. You should have such a grounding electrode conductor going from your main electrical panel over to where the metal water pipe enters your house from the street. (Lots of houses have plastic water service pipes instead of metal, so you wouldn’t have this part of the Grounding Electrode Conductor.)
So the grounding electrode system connects much of the metal in your house together with the neutral wires and connects it to the ground. Basically it helps to protect us from electricity that originates outside of the house – like lighting or transformer problems.
Equipment Grounding System
Equipment grounding is a very different part of the electrical system, although they are connected together. When you plug in a device with a 3-prong plug that third prong is providing equipment grounding. Equipment grounding is intended to protect us from electricity that comes from the appliances we use. And the most important thing to understand about equipment grounding is that it has nothing to do with the ground, or the earth, or the dirt beneath our feet. The purpose of equipment grounding is, in the language of the National Electrical Code (NEC) section 250.4(A)(5) “Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for impedance grounded systems.” Simple translation: if the metal case of an appliance might become electrically energized then you install a ground wire (an equipment grounding conductor) in such a way that it creates a very low resistance circuit back to the electrical panel so that the circuit breaker will trip off.
So the ground rod or metal water pipes mentioned as part of the Grounding Electrode System has nothing to do with equipment grounding. The NEC actually forbids using the earth as an equipment grounding path. NEC 250.4(A)(5) “. . . . The earth shall not be considered as an effective ground-fault current path.”
Here’s a schematic showing how electricity is wired from the utility transformer into an appliance in your house, without any equipment grounding.
Electricity wants to complete a path, so in the image above electricity wants to complete the circuit from point A to point B (in the utility transformer, where the neutral is grounded through a driven copper ground rod). The schematic below shows what happens when we close the switch (just to the right of the appliance in the schematic above) and turn on the appliance. You can follow the electrical path — follow the little red lightning bolts. Electrical current starts at Point A and goes into the house through the meter (where the neutral is connected to the ground through a driven copper ground rod), into the electrical panel (where the neutral is connected to the ground through metal water pipes), through a main breaker and a branch circuit breaker, and out to the appliance. The current goes through the appliance load and then returns through the neutral wire out to Point B at the transformer.
This is the way things were before about 1960, when there was no equipment ground. Things worked, but they weren’t as safe as they could be. And eventually we learned that an effective equipment grounding system could help to protect people, so we instituted that requirement. Here’s how we did NOT do that. This is a schematic showing a ground connection from the appliance case directly into the ground.
Now suppose that the hot wire has some torn insulation and the wire makes contact with the metal case of the appliance. That’s a ground fault. Now the case of the appliance is at 120 volts to ground and if you touch it you could be shocked. Where is the electrical current going? Remember that electricity wants to make a circuit, so the current wants to go back to point B in the utility transformer. Follow the little red lightning bolts again to see the path that electrical current takes.
There are two paths for current to return to the transformer. One is through your hand, through your heart (that’s not good!), into the ground, and then over to the ground rod at the utility transformer. Another path is through the appliance’s ground rod and over to the utility transformer. Both paths have a lot of resistance, so neither will be favored much and current will flow through both paths. (When talking about alternating current the proper term is impedance, not resistance. But I’m going to keep using the term resistance.) So a lot of current will flow through you, certainly enough to kill you. But the resistance of the earth is too high so not enough current will flow to trip off the breaker. So you’re standing here as part of an electrical path that’s sending current through your body. Terrible.
Here’s another schematic showing the proper equipment grounding configuration. There’s simply a ground (equipment grounding conductor) wire going from the appliance case back to the neutral bus bar in the main electrical panel.
Now if there’s a ground fault and you touch the appliance case, there are again two paths for current to take to get back to the utility transformer. One is through you and into the ground and back to the transformer. That’s a lot of resistance.
But another path is through the ground wire, back to the electrical panel, and then back out to the transformer through the service neutral. And this path has no load at all on it, so it has very low resistance. With low resistance this path will take most of the current — a lot of current. Electrical current favors the path of least resistance. Enough current will flow through the ground wire to quickly trip off the branch circuit breaker. And that’s what you really want — you want the breaker to trip off and de-energize the appliance.
This is the way that equipment grounding keeps us safe in the U.S.
Our National Electrical Code first required equipment grounding for receptacles in 1947, but only for laundry receptacles. In 1962 the NEC first required that all standard 120-volt receptacles be of the grounding type.