Circuits provide power to the receptacles and light switches in each room of your home. If a ground is bad for one or more outlets, a short which develops in an appliance or other device can cause an electrical shock. The purpose of a ground on an outlet is to intercept electrical current and cause a breaker on your circuit panel to trip.
Insert the red probe of the circuit tester into the small slot on the outlet. Insert the black probe into the large slot on the outlet. The circuit tester will light up if power is being sent to the outlet.
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Remove black probe from the large slot and insert it into the small "U" shaped ground hole. The circuit tester will light up if the outlet is properly grounded. If the circuit tester does not light up go to the next step.
Pull the black probe out of the ground hole and insert it into the large slot. Pull the red probe out of the small slot and insert it into the ground hole. If the circuit tester lights up then the outlet is grounded but the wires for "Hot" and "Neutral" are reversed.
Repeat this process to test all of the outlets in the room for a bad ground.
If the circuit tester fails to light up then the outlet is not grounded.
Outlet not properly wired or that have a bad ground should not be used until the wiring has been fixed.