Understanding Two-Wire and Three-Wire Systems

A two-wire system means you don’t have a grounding conductor. This is the bare copper or green-insulationclad wire attached to a grounding electrode outside your house. Alternatively, the grounding conductor can be attached to a water pipe near your electrical panel.

Your first clue to determine whether your system has a grounding conductor comes from your receptacles. A two-pronged receptacle usually doesn’t have a ground; a three-pronged outlet should have a ground.
An older house might well have three-pronged outlets, but there is no guarantee that they were installed with grounds (another peril of uninspected remodeling work). To be certain, you must either …

• Test each outlet with a circuit tester, which will indicate the presence of a ground wire.
• Take the cover plate off each receptacle and look for the ground wire.

Of the two methods, using a circuit tester is easier. If your system isn’t grounded, it doesn’t mean you’re in grave danger and should refrain from turning on the lights ever again. It does mean, however, that it’s a dated system that lacks a modern safety feature—a safety feature that people lived without until the 1960s. Unless there’s been a deep conspiracy to cover up massive electrocutions of homeowners over the years, you can still live with an old two-wire system (but it’s always a good idea to upgrade).

A grounded receptacle connects any exposed metal sections of an appliance or lamp to the house grounding system. This means an errant current shouldn’t pass through the metal shell of your washing machine and turn your wash day into something unexpected. A three-pronged, grounded receptacle is the made-to-fit receiving end for a threepronged plug. Remember, electrical systems are nice and logical. If you have a twopronged outlet and a three-pronged plug, they don’t go together, no matter how hard you push on the plug. Before you ask, cutting off the grounding pin from the plug is a bad idea. Buy an adapter, which is available at any hardware store, instead.