Appliances with three-prong plugs are designed to operate properly when the ground plug provides a path for electrical current all the way back to the electrical panel. Surge protectors will only provide protection for electrical devices if the power source is grounded. Most questions asked about grounded electrical boxes come from homeowners with lots of two-prong power outlets in their homes.
There are a few things you can do without the assistance of a qualified electrician to safely determine whether an outlet or its electrical box are grounded. You will need a two-lead circuit tester and some basic familiarity with your electric panel. Choose a working outlet that you intend to use with a surge protector or with an appliance with a three-prong plug.
Start with the power to the circuit switched on. Use the circuit tester to confirm there is voltage between the hot and neutral terminals of the outlet. The hot wire is connected to the smaller slot of the outlet, and neutral is connected to the larger slot.
Once you have confirmed there is voltage across hot and neutral, you will use the same circuit tester to determine whether there is voltage between hot and ground. Voltage measured between the hot terminal and the ground hole is a good indicator that the outlet and its electrical box are grounded.
If you are dealing with a two-prong outlet, you can test for a ground at the cover plate screw before taking any steps to look inside the box. Many older homes with two-prong outlets were originally wired with metal boxes and armored cable, providing a reliable ground path to the panel. Still others were wired with dedicated ground wires despite the two-prong receptacles. three-prong to two-prong adapters that use the cover plate screw as a ground are still commonly available. Connect one lead of the circuit tester to the cover plate screw and the other lead to the hot slot. Measured voltage is a good indicator that your outlet and its electrical box are grounded.
If you did not find a reliable ground using the circuit tester and you want to be certain there is no ground wire inside the box, you can inspect behind the receptacle safely before calling an electrician. Turn off the circuit, and confirm there is no voltage in the hot terminal. Next, remove the cover plate and the screws securing the receptacle to the box. Once removed, you should note whether there is a bare copper ground wire coming into the box with the hot (black) and neutral (white) wires.
Once you have confirmed that your electrical box is not grounded, you should discuss your options with an electrician. Proudly serving Greater Cincinnati, Northern Kentucky and Southern Indiana, Rose Brothers & Sons can help you with anything from small electrical repairs to large electrical construction projects. Contact our experienced and professional electricians today.