How to Test an Electrical Outlet With a Digital Multimeter
How to safely test standard electrical outlets for irregularities that might, if undetected, damage a computer system or sophisticated home electronics.
Before you set up a computer or home entertainment system in a new location, it's a good idea to check the electrical outlets. A bad ground connection or incorrectly wired outlet can damage your sensitive electronics or cause them to misbehave in unpredictable and mystifying ways. You can do this with an inexpensive digital multimeter, one of the most versatile tools in any tech enthusiast's arsenal.
What It Is
Multimeters are sometimes referred to as digital volt meters or less commonly as volt-ohm meters. Neither term is completely accurate, because -- as the name suggests -- they're capable of making a wide range of electrical measurements. Most models have a large LCD display at the top, a set of three connections at the bottom for the test probes, and a dial in the middle that rotates between the instrument's various functions. To test an electrical outlet, you use only the AC voltage setting.
Connecting the Leads
Your meter's test leads are usually red and black. One end has a short, thick connector called a banana plug, while the other end has narrow, pointed metal probes with rigid plastic handles. Push the black lead's banana plug into the connector labeled "COM" on your meter. It's usually, though not always, the one in the middle and will probably have a minus sign beside it. One of the remaining two connectors should have a "V" for voltage, and the Greek letter omega -- which looks like a horseshoe -- alongside it to symbolize resistance. It may also show a plus sign. Plug the red lead into that connector.
Choosing a Function
Look at the dial on your meter and identify the setting for AC voltage. Some models have separate positions for AC and DC voltages, while others have a single voltage setting and a button that lets you toggle between AC and DC settings. Consult the user's manual if you're not sure. Some models have settings for different ranges or levels of voltage. Choose a setting with a minimum of 100 volts if that's the case. Most modern multimeters are autoranging, and detect the voltage level without you needing to do anything.
Testing an electrical outlet is unavoidably risky, because the power must be on. Check the test leads before you proceed. Make sure their wires aren't frayed, the insulation is intact, and the probe ends are not loose or wobbly. The probes' insulated handles should not be cracked or visibly damaged. When you're making your measurements, never touch the metal portion of the probes or let the probes brush against each other. This creates a potentially dangerous short circuit.
Testing the Outlet
Most outlets are polarized, meaning one slot is wider than the other. The wider slot is the negative or ground, while the narrower slot carries the voltage.
Insert the black lead into the wider slot and the red lead into the narrower slot. The display should show a value of 109 to 121 volts, the standard range for North America. If the display puts a minus sign in front of the number, the polarity in your outlet is reversed. That's not an issue with lamps or other simple electrical equipment, but it can cause problems for sophisticated electronics.
If you don't find a voltage at the outlet, it isn't necessarily faulty. Especially in home construction, outlets are often put on switched circuits for use with lamps. Look around the vicinity for extra light switches and turn them on. Then try your test again.
Next, remove the black probe from the wide slot and move it to the round ground connection at the bottom of the outlet. The voltage should be the same. Finally, insert one probe into the wider neutral slot and the other into the round ground connection. The voltage should read zero.
If you get a voltage on this final test, the outlet is improperly grounded and should not be used under any circumstances until it's rewired.