In this article, I will show you how to test the outlet for power, grounding, polarity, and resistance, focusing on how to set the multimeter for each test and where to place which probe.
When using a multimeter to test an outlet, initially set your meter to measure AC voltage and ensure not to touch the metallic part of the probes during any of these tests. Insert the red probe’s connector into the V/Volts/+/Ω port and the black one into the COM/- port.
- To test for power, insert the red and black probes of the multimeter into the short and long vertical slots, respectively.
- If your supply is 120 volts, you should get a reading close to this.
- To test for grounding, insert the black probe on the screw or into the rounded ground slot, which should not change the reading.
- For the polarity test, invert the probes in the power test.
- To test for resistance, place both probes on the brass terminals.
Using a Multimeter to Test Outlets
Which Multimeter to Use?
Multimeters come in both analog and digital forms.
You can use either type of multimeter for testing a power outlet.
An analog-type is generally cheaper. It has a needle that points to reading on a graduated scale, but it might not always be easy to read. A digital multimeter is generally more expensive but more precise. It displays the reading on an LCD screen.
Setting the Multimeter
Insert the red probe’s connector into the port marked V, ‘Volts,’ ‘+,’ or ‘Ω’ and the black one into the port marked ‘COM,’ or ‘-,’ as shown.
Select the alternating current (AC) option on the multimeter.
You must set the multimeter to read VAC (voltage for AC circuits) for the power tests. In the multimeter shown below, this is the first voltage option available on the dial, which shows a ‘V’ with a wavy line on top, indicating AC. Don’t mistakenly set it to the direct current (DC) option, which usually has a solid and dashed line instead.
How to Use a Multimeter to Test an Outlet
I will tell you how to use a multimeter to test an outlet in four ways.
Here is a list and summary of how to conduct these tests, but continue reading for more details.
- Test a power outlet – put the red probe into the short slot and the black probe into the long slot.
- Test an outlet for grounding – put the red probe into the small slot and the black probe on the screw.
- Test an outlet for polarity – put the red probe into the long slot and the black probe into the short one.
- Test an outlet for resistance – set the multimeter to ohms and touch both probes to the outlet’s brass terminals.
Testing an Outlet for Power
You must set the multimeter to read AC voltage to test a power outlet.
If your main supply is 120 volts, the chosen VAC range should cover this value. Although you can put either probe into either vertical slot (as it’s AC), for correct polarity, put the red probe into the short vertical slot and the black probe into the long vertical slot.
You should expect a reading between 110 and 120 volts (VAC). If the multimeter shows a zero reading, try swapping the slots. Otherwise, the outlet is not receiving power. One reason might be a loose connection inside, but you must open the outlet to confirm this. Another could be that the fuse is blown or the circuit breaker has tripped.
Testing an Outlet for Grounding
To test an outlet for grounding, put the red probe into the small vertical slot and the black probe on the screw on its cover plate.
The multimeter should show current is present by giving a voltage reading. If it doesn’t, then the outlet may not be grounded.
There should be no current on the cover plate. The casing is a potential hazard if you obtain a reading greater than 0.001 volts. You will need to repair the outlet.
Testing an Outlet for Polarity
To test an outlet for polarity, put the red probe into the long vertical slot and the black probe into the short one.
The multimeter should not show current. If the wiring is in the proper order, it will not give a voltage reading. If it does, then the polarity may be wrong. If in doubt, swap them around, i.e., move the red probe into the short slot and the black one into the long one. If it gives a reading now, then the polarity is correct, and if it still doesn’t, then you will need to check the wiring for loose connections.
Testing an Outlet for Resistance
First, set the multimeter to measure ohms to test an outlet for resistance.
Then touch the multimeter’s probes to the outlet’s two brass terminals. You should get a resistance reading. If you don’t, the outlet might not receive power, so repeat the above power test.
Another thing you can do is insert a working appliance in the outlet to give it a load. The outlet probably needs to be repaired or replaced if it doesn’t work.
Why Use a Multimeter?
A multimeter is a very versatile tool that you can use to test various electrical problems.
A multimeter does these things by measuring four important electrical measures: voltage in volts (AC and DC), current in amps, resistance in ohms, and capacitance in farads.
If an outlet fails to work as expected, you can use a multimeter to check it.
For instance, if an outlet is not receiving power, you suspect it is not properly grounded, or you need to ensure the wiring is not reversed, you can use a multimeter to verify and help fix the problem.
Although you can use a current or voltage tester to do the same, a multimeter can give you exact readings.
Using a Multimeter Safely
It would be best to follow certain safety precautions when working with electricity, even when using a multimeter, because you may still be working with live current.
If you’re not careful enough, you can accidentally get a shock, burn yourself, or get electrocuted. The most dangerous thing you could do is touch the metal part of the probes while they are inserted in an outlet’s slot or otherwise in contact with a live conductor.
One safety measure you could take is to hold the two probes in one hand. This prevents creating a full circuit for current to flow.
Whenever you use your multimeter, ensure it has no loose connections and is not damaged.
While testing a live outlet with a multimeter, never touch the metal prong on the end of each probe. Another safety measure you can take is to hold both probes with one hand so that the current does not pass through your body in the event of a shock. Also, this guide is only for testing outlets externally, using their slots. If at any stage, you decide to open the outlet for further testing inside, you should first turn the power off at the breaker panel.
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