It's unsafe to use adapters that convert three-prong plugs to two-prong ones, because as normally used, the adapters bypass the safety features of the equipment ground
If you didn't have an equipment ground
wire, attach a sticker to the cover plate saying "No Equipment Ground
." This sticker is included with most GFCI receptacles.
The equipment ground attaches to that third, round prong on a three-prong plug.
The rule requiring the equipment ground came into code about 1960, so many older homes lack equipment ground wiring and three-slot receptacles.
A GFCI (ground-fault circuit-interrupter) delivers even better shock protection than the equipment ground. And it operates dependably without the equipment ground.
Simply plug the tester into the receptacle and the combination of lights indicates whether the equipment ground is working.
Exception: Some homes have two-wire systems and an equipment ground
that consists of metal electrical boxes connected by metallic tubes (called conduit) or metallic sheathed cable (called BX) rather than a separate copper ground wire.
The equipment ground
attaches to the exposed metal parts of lamps, power saws, electrical boxes and other devices--all those metal surfaces that can carry electricity--and runs all the way back to the main electrical panel and then into the earth.
The big problem with widely separated equipment grounded
at their respective locations is the IR drops that result from currents through the lossy earth.