1

I have a multimeter and I don't want to spend more money on other electrical test devices.

How do I use a multimeter to perform the same functions as a:

  1. circuit tester
  2. GFCI outlet tester
6

I have a multimeter and I don't want to spend more money on other electrical test devices.

While, I understand your thought process, keep in mind that multi-meters, when combined with house wiring are not nearly as safe to operate as devices designed to perform these tests. You are paying not only for utility but also for safety (which is far more important).

circuit tester

I guess you mean outlet tester specifically.

  • Set your Cat II or better rated multimeter to a high voltage AC range. If your meter has a Low-Z range intended for 120 V AC (or 230 V AC), use that.
  • Double check that you have the leads in the right sockets for voltage testing.

  • Measure the voltage

    • between ground and neutral
    • between ground and hot
    • between hot and neutral
  • Check that the results match what you expect (e.g. 0, 120, 120).

GFCI outlet tester

You can't test this with just a multimeter. You need a load that will pass about 8mA at 120V (or >30mA at 230V for UK/EU readers) between hot and ground.

(Note, according to NEMA, a GFCI "test" button uses a 15kΩ resistor with a suitable voltage rating. This means the test current is nominally 8mA at 120V (or up to 9mA for a 10% tolerance resistor)

A typical multimeter has a 10MΩ impedance on its AC voltage ranges, so it passes less than 12 micro-Amps at 120V - too small to trip a GFCI. A "low-Z" range might use a 3kΩ impedance and pass 40mA at 120V - this is more than is needed for safely testing a GFCI. Note that around 10mA is the let-go threshold for humans and so currents above that can be problematic.

Fabricating something that is safe for use with 120 V AC is going be difficult to do for less than the cost of a purpose built tool.

  • 2
    I prefer using a multimeter over simple testers for measuring voltage because I like to see the actual reading and have confidence in its accuracy. I'd recommend a Cat4 meter though so you can go on to use it in other areas. But don't be a cheapskate on the gfci tester. buy one thats made for the job. – Billy C. Jul 7 '16 at 9:52
  • Actually, I wonder if a H-G measurement with a low-Z meter range/mode could be used for GFCI testing? – ThreePhaseEel Jul 7 '16 at 11:48
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    It would have to draw over 6maA to trip the GFCI that sounds a bit high. The built in testers provide a specific load to trip the GFCI. I have a very expensive unit that starts at 0 and goes to 150MA so I can certify the ones used in chemical systems. I understand wanting to know the trip value but think the plug in testers are better and safer for the average DIY person. + rgb – Ed Beal Jul 7 '16 at 13:36
  • @ThreePhaseEel, I haven't tested this but I think Flukes Low-Z impedance is 3kΩ. That would pass 40mA at 120V. So it should trip a GFCI. – RedGrittyBrick Jul 7 '16 at 14:53
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    As the owner of several multimeters including at least one fairly expensive one - a GFCI outlet tester (which was all of $5, IIRC) is in my kit. It's quick, it's safe, it works - it would cost me more than that to replicate its GFCI-test function alone. The idiot lights are therefore cheaper than free, and faster than poking the probes in 3 ways. – Ecnerwal Jul 7 '16 at 15:38

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