Residential wiring systems installed in older homes use a two-wire system in the 110-120-volt branch circuits. One conductor is hot, and the other is neutral. The neutral may also serve as a ground, but, unfortunately, it usually does not. When this is the case, the system is ungrounded and the situation is potentially hazardous.
You can easily tell if your circuits are of this type by looking at receptacles. There are only two slots for each plug in ungrounded receptacles. Modern wiring calls for the installation of a third conductor. Receptacles used with this system have three openings: two vertical slots and a third, rounded hole centered below or above them.
©2006 Publications International, Ltd.
A plug-in polarity checker can enable
you to make sure your outlets
are installed properly.
Either two-prong or three-prong plugs can be plugged into these receptacles, but only the three-prong kind will carry the equipment grounding line to the electrical equipment. Also, one of the vertical slots is different in size from the other, so the newer types of two-pronged plugs can be inserted in only one direction. This ensures that the equipment being connected will be properly polarized, hot side to hot side and neutral to neutral.
For proper operation and safety, make sure all receptacles on each circuit are installed with the individual conductors going to the correct terminals so there are no polarity reversals along the line. Unfortunately, receptacles are not always connected this way, even in new wiring systems installed by professional electricians. Check out your receptacles with a small inexpensive tester called a polarity checker, designed for this purpose. It looks like a fancy three-pronged plug and contains three neon bulb indicators.
To check your receptacles for polarity, plug a polarity checker into a receptacle. The lights will tell you if the polarity is correct and, if not, which lines are reversed. If there is a reversal, turn the circuit off, pull the receptacle out of the electrical box, and switch the wires to the proper terminals. If the equipment-grounding circuit is open (discontinuous), trace the circuit with a continuity tester until you find the disconnection or missing link; reconnect it to restore the effectiveness of the circuit.