Grounding is a principle of electricity that sometimes puzzles homeowners. In essence, the grounding system in a residential wiring system serves a "backup" pathway that provides an alternate route for electrical current to follow back to "ground" in the case of a problem in the wiring system. To understand its importance to a home wiring system, it is important to know something about the nature of electrical energy flow.
Some Electricity Basics
The electrical current in your home's wiring system consists of a flow of electrons within metal circuit wires. The current comes in two forms, a negative and a positive charge, and this charged electrical field is created by huge generators operated by the utility company, sometimes many hundreds of miles away. It is this polarized charge than effectively constitutes the flow of electrical current, and it arrives at your home through a vast network of high-tension service wires, substations, and transformers that blanket the landscape.
The negative half of the charge is the "hot" current. In your home's wiring system, the hot current is normally carried by black wires, while the neutral wires, which are white, carry the positive charge. Both sets of wires enter your home through the utility's main service wires, run through your electrical service panel, and run side-by-side through every circuit in your home.
The physics of electrical flow are more complicated than most simple explanations can convey, but essentially, electricity seeks to return its electrons to "ground"—that is, to discharge its negative energy and return to equilibrium. Normally, the current returns to ground through the neutral wires in the electrical system. But should some breakdown of the pathway occur, the hot current may instead flow through other materials, such as wood framing, metal pipes, or flammable materials in your home. This is what may happen in a short circuit situation, where most electrical fires and shocks originate. A short circuit is when electricity strays outside the wires it is supposed to flow through—in other words, when it takes a shorter path to ground.
The Home Grounding System
To prevent this danger, your home's electrical system includes a backup plan—a system of grounding wires that runs parallel to the hot and neutral wires. It provides an alternate pathway for electrical current to follow should there be a breakdown in the system of hot and neutral wires that normally carry the current. If a wire connection becomes loose, for example, or a rodent gnaws through a wire, the grounding system channels the stray current back to ground by this alternate pathway before it can cause a fire or shock.
The grounding pathway is generally formed by a system of bare copper wires that connect to every device and every metal electrical box in your home. In standard sheathed NM cable, this bare copper wire is included along with the insulated conducting wires inside the cable. The bare copper grounding wires terminate in a grounding bar in your main service panel, and that grounding bar is in turn connected to a grounding rod driven deep into the earth outside your home. This grounding system provides a path of least resistance for electricity to follow back to ground should a break in the wiring system allow electricity to "leak" out of the preferred system of black and white circuit wires.
In most home wiring systems, evidence of the grounding system can be seen at each outlet receptacle, where the third round slot in the face of the receptacle represents the grounding connection. When a grounded appliance plugs into such a receptacle, its round grounding prong is now directly connected to the system of bare copper grounding wires inside the house circuits.
Not all homes have this elaborate and complete grounding system formed by a network of bare copper wires. While such a grounding system is standard in homes with circuit breakers that are wired with sheathed NM cable, older wiring systems installed before 1965 may be grounded through metal conduit or metal cable, not bare copper grounding wires. And even older systems installed before 1940 may not have any form of grounding at all. Such is the case in knob-and-tube wiring, where there is no grounding path of any kind. Many older systems have already been updated, and it is a good idea to have it done if your wiring is of this older generation. One clue that your wiring is old is when the outlet receptacles have two slots rather than three. This indicates the outlets may not be grounded.
Your home wiring system also includes other safety devices to help prevent disaster. Circuit breakers or fuses protect and control each individual circuit. The breakers or fuses serve two functions: they protect the wires against overheating in the event that they are overloaded by too much electrical current being drawn through them; they also sense short circuits and trip or "blow" to instantly stop the flow of current when problems occur. In a short circuit or ground fault situation, a sudden reduction in resistance causes an uncontrollable amount of current to flow, and the circuit breaker responds to this by tripping.
Finally, it is fairly common practice for the metal plumbing pipes in your home to also be connected to the grounding pathway. This offers additional protection should electricity come in contact with these metal pipes. Often, this grounding is established by a grounding wire clamped to a metal water pipe near your water heater.
Not only does your home wiring system have a grounding system for safety, but many plug-in appliances and devices do, too. Power tools, vacuums, and many other appliances are much safer when they have a third prong on the cord plug, which is shaped to fit the round grounding slot on an outlet receptacle. The presence of this third prong indicates that the appliance has a grounding system, and it is essential that these be plugged into grounded outlets. Some people have been known to cut off the grounding prong on an appliance plug in order to make it fit an outlet or extension cord that has no grounding slot. This is an extremely dangerous practice that could lead to a shock if the internal wiring in the appliance short circuits.
Most people are familiar with the plug adapters than allow three-prong plugs to be inserted into two-slot outlet receptacles. It is important to note that these offer grounding protection ONLY if the pigtail wire or metal loop on the adapter is properly attached to the mounting screw on the outlet cover plate, AND if that cover plate screw is connected to a metal box AND if that metal box is properly grounded. This is no sure thing, by any means, so three-prong to two-slot adapters should be used with great caution, if at all. The better solution is to plug three-prong plugs only in into three-slot receptacles that are grounded.
Where a grounded outlet is not possible, as in older wiring, some protection is offered by installing a GFCI (ground-fault circuit interrupter) receptacle at that location. The GFCI will sense ground faults and shut off the power before straying current can cause problems. It's important to note, though, that using a GFCI does not actually create a grounding pathway; it merely makes an ungrounded outlet somewhat safer.
Of course, not all appliances and plug-in devices have a three-prong grounded plug, and these are still safe to use since normally they have a double-insulated construction that minimizes the risk of short circuits.