“Reverse polarity” is the term used to describe a situation where electrical wires are connected to the wrong terminals of a receptacle. It can be hazardous if the “hot” side of your electrical system gets connected to certain types of lamps or equipment.
It’s a common condition because it’s an easy mistake to make when hooking up wires and because receptacles work fine even when the polarity is reversed. You generally don’t know the condition exists unless you look for it. If you suspect your home’s wiring is a bit haphazard, it’s a good idea to check for reverse polarity.
You can buy a simple circuit tester for less than $20. Just plug it into all duplex receptacles; test both the top and bottom.
If you discover reverse polarity:
1Shut off the circuit breaker that serves that receptacle (the tester’s lights will go out).
2Unscrew the cover plate from the receptacle, and use a voltage tester to be sure that none of the wires in the electrical box are still hot.
3Unscrew or release the wires from the receptacle and re-fasten them to the proper terminals–white to the silver (neutral terminal) and black to the brass (hot terminal). The bare or green wire should connect to the green screw.
4Put the cover plate back on, turn the circuit back on, and test the receptacle again. If the tester indicates reverse polarity but the wires are hooked up to the proper terminals, call an electrician.