The National Electrical Code requires that all receptacles installed in all 15- and 20-amp 120-volt circuits be grounded, but if your house wiring predates the adoption of this requirement, you don't have to replace your ungrounded receptacles with grounded ones. That doesn't have to mean living without grounding protection in your bedroom or kitchen outlet. Replacing an ungrounded outlet with a ground fault interrupting receptacle protects you from shocks, but it won't protect your electronic equipment from power surges.
Ground That Circuit!
Up until the mid-20th century, residential circuits were ungrounded, which means they had only two wires. Loose connections, worn insulation and broken internal parts made it easy to get shocks; whenever you touched a device in which residual current was present, it would travel through your body to ground. Not only that, residual current caused metal parts to overheat, making fires more frequent. To counter this, the NEC began requiring all electrical panels to include a ground bus connected physically to earth by means of a ground rod. Today, all circuits controlled by the panel must connect to the ground bus by means of a third conducting wire, and all devices on the circuit must have terminals for this ground wire.
But...My Circuits Aren't Grounded
Because the NEC regulations applied only to new construction, homeowners with existing two-conductor wiring weren't required to make changes. If you live in an older home, and your outlets have only two wires, your panel may not be grounded. In that case, the best course of action -- although an expensive one -- is to upgrade your panel and replace all the two-conductor cables with three-conductor ones. You probably don't want to go to that expense just to ground a single outlet, though, and if not, you have a workable option. First, though, it's important to consider what you shouldn't do.
An outlet can have two slots and still be grounded. If the slots are are different sizes, the outlet is grounded and polarized.
What Not to Do
Don't replace a two-prong outlet with a three-prong outlet: Simply installing an outlet with a third slot that allows you to plug in your three-pin coffeemaker is illegal and dangerous. It doesn't provide grounding, and although you know it isn't grounded, someone else might not and could be injured as a result.
Don't ground a grounded outlet to a metal electric box: Connecting the ground wire of a grounded outlet to a metal electrical box will energize the box in the event of a short circuit. The box could overheat and start a fire, or someone could get a shock from touching it.
Don't rely on metal sheathing or an exposed ground wire: If your electrical cables have metal sheathing that runs all the way to the panel, the sheathing can serve as a ground path. Alternatively, you might consider connecting the ground terminal to a water pipe by means of an exposed ground wire. Two problems make the safety of these approaches questionable:
- The connection is often unreliable.
- The sheathing or wire gets energized in the event of a short circuit, potentially overheating and causing a fire or giving someone a shock.
The GFI Option
A GFI outlet monitors the relationship between the current coming into it and the current leaving, and a breaker trips whenever the difference exceeds a preset limit. This provides the same shock and fire prevention that a ground wire provides, but it doesn't protect your electronic equipment from a power surge, because surge protectors work by shunting excess current through the ground wire to earth. To replace your two-prong outlet with a GFI:
Turn off the breaker controlling the outlet circuit and unscrew the outlet from the electrical box.
Disconnect the hot wire -- which is black -- from the existing outlet and connect it to the brass LINE terminal on the GFI. It must be connected to this terminal -- not to the LOAD terminal -- or the GFI won't offer any protection.
Disconnect the neutral wire -- which is white -- and connect it to the chrome LINE terminal.
Leave the ground terminal disconnected. Replace the outlet and put a label on it that reads "No Equipment Ground." This label is usually supplied with the outlet, and it warns people against relying on the outlet for surge protection.
You can wire a three-prong outlet to the GFI by connecting it to the LOAD terminals. That outlet will get ground fault protection from the GFI, and it must also have a label that says "No Equipment Ground."