Working with live wires is always dangerous. Exercise extreme caution when testing them. If the wires are exposed, make sure they don’t come into contact. It’s also a good idea to switch off the power until you are ready to test for voltage.
A multimeter is the best tool for testing the voltage of live wires. It not only tells you that the wires are live (meaning there’s current flowing through them), it also tells you how much voltage is present.
This can help diagnose an electrical problem or check that an outlet or switch is working properly.
Read on to learn more about how to use a multimeter to test voltage of live wires.
Check if the Wires are Live
Before you test voltage, check if the wires are live. Use a voltage tester. A non-contact voltage tester is great especially for wires that are hidden behind insulation or a socket.
If the wires are exposed or you can reach the metal terminals, you can use a contact voltage tester.
A voltage tester does not tell you the exact amount of voltage. It simply measures if there’s any voltage at all by lighting up. Some voltage testers also produce a buzz or chirp.
If the wires are live, now you can use a multimeter to measure voltage.
Setting up the Multimeter
Before you can use a digital multimeter, you need to set it up properly.
Turn the rotary dial to the section of the scale marked V with an alternating current symbol (~) next to it. Within that section, several numbers that indicate the voltage range.
Set the dial to the lowest number that’s higher than the expected voltage in the wires. In the UK, this is 230V. Set the multimeter to the next number above 230V.
Selecting the right voltage range increases measuring resolution and accuracy.
Not all multimeters require this step. If your multimeter has no numbers on the scale, it’s likely an auto-ranging multimeter. All you need to do is set the dial to V~ and the multimeter will set the right range as it measures voltage.
Next, connect the probes into the multimeter. The black probe goes into the slot marked COM while the red probe goes into the slot marked VΩ.
1. For a Socket
If the live wires are connected to a plug socket, it’s easy to test the voltage.
Insert one of the two leads into the neutral left slot of the socket and the other into the live right slot. Check the reading on the multimeter.
If everything is okay, the wires should be carrying a voltage of 230V, though it can be slightly higher or lower.
You can also test the voltage of the neutral wire using the multimeter. Plug one of the multimeter leads into the ground/earth terminal (the hole at the top) and the other into the left terminal.
Check the multimeter for the voltage reading. If the socket is wired properly, it should read zero or close to zero. If it’s a big number, there’s a wiring problem.
If it is 230V or close, the socket wiring may be reversed.
2. For a Switch
If the live wires lead to a switch, unscrew the switch to access the exposed terminals
Making sure the switch is on, touch one multimeter lead on the ground wire or terminal and the other on one of the live wires. Check the voltage reading.
While keeping one lead on the ground terminal, touch the second lead onto the other live terminal or wire.
Similar to a plug socket, the live wires of a switch should both read 230V or thereabouts.
3. For Exposed Wires
You can also measure the voltage of exposed live wires. Just be extra-careful.
To measure the voltage of each wire, touch one of the multimeter leads on any metal object nearby. It can be the radiator or a metal pipe. That object will act as the ground reference.
Then touch the other multimeter lead on each of the exposed wires and check the voltage of each.
For most home 3-wire circuits, only one wire should register a voltage of about 230V. This is the live wire.
The neutral and earth wires should indicate a voltage of zero or close to zero.
If you don’t have a metal object nearby to use as a ground reference, you can measure the voltage difference across the wires.
Measure the neutral wire and live wire to get the voltage of the live wire (use the sheath colours to identify which wires are neutral, live and earth). Measuring earth and live also gives you the voltage of the live wire.
Measure the ground and neutral to see if there’s any voltage in the neutral wire.
That’s all from me! Any other tip on how to use a multimeter to test voltage of live wires? Please leave it in the comment section below.