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I have a MUT-830 multimeter. I would like to use it to do a basic saftey check of the electric sockets in my home. Specifically, I want to verify that:

  1. there is grounding,

  2. the resistance is not too high (so that the socket should not get too hot).

How can I do this?

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    First step is to get a better meter (one with a NRTL listing/marking and CAT II rating at a minimum...) – ThreePhaseEel Feb 2 '17 at 12:47
  • I wouldn't use that multimeter with anything over 50 Volts. See Tools – RedGrittyBrick Feb 2 '17 at 15:12
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To check grounding and polarity a simple outlet tester like this would be a better tool for the DIY home owner. This type of tester will show if the outlets are grounded and can test GFCI breakers by pushing the button. With home wiring checking the resistance is not a very good test because there may be a low resistance with an unloaded circuit but under load the resistance can increase (ohm meters do not work on live circuits). you can also check the voltage on a circuit but if unloaded the same problem can arise with a circuit with no load you may see 120 volts but then a load the voltage can drop significantly. Making sure not to over load outlets (not adding power strips) and running circuits below 80% of there rated capacity is all that is needed in most homes.

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I have aluminum wiring for branch circuits (12 AWG for 15-A and 10 AWG for 20-A) so I am interested in the resistance of the connections and the voltage drops under load. I plug a short extension cord with multiple plug-ins into each receptacle under test. A put a voltmeter across the hot and neutral of one plug-in of the extension cord and plug a hairdryer in another. I read the voltage drop when the hairdryer is turned on.

(It would be convenient to have an adapter which would plug into a receptacle and securely accept the probes of the voltmeter, but I have not found one.)

This hairdryer is listed as 1600 W on the high setting and this is what I use to load the circuit. This draws about 13 A (1600 W/125 V) and is enough to cause significant voltage drops in my wiring, without tripping the a breaker. (13 A is higher than necessary and 10 A to 12 A would do.)

I am interested in finding the instances of larger voltage drops between two sequential receptacles which would indicate high resistance connections. Of course, where there is a longer run of wire between two receptacles there will be a larger voltage drop than for shorter runs and this is not a fault condition.

The above is how I test receptacles without knowing the order of the receptacles in the circuit. If the order is known, it is more informative to plug the hairdryer into the last receptacle from the panel and test the voltage drop at each receptacle going back to the panel.

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You can probably save yourself some time by just picking up an inexpensive outlet tester (less than $10 at most home improvement stores). If you're going to test all of your outlets, this method is fast, reliable, and safe. The one I've linked will let you test ground fault interrupters (GFCIs) too.

You can do the same tests (except the GFCI) with your meter, of course. It's a matter of measuring the voltage between the three conductors on each outlet. The "hot" conductor on an outlet is the smallest (narrow) slot, and obviously the ground is the larger, round pin. It doesn't matter which way you connect the meter, and of course you'd want to be set to AC Voltage.

  • Hot to Neutral: 105-125 VAC
  • Hot to Ground: 105-125 VAC
  • Neutral to Ground: 0

These correspond to the three lights on the outlet tester. I'd still recommend the tester for two reasons: You're not going to zap yourself with it (which you might if, say, you're not careful with the meter's probes or the wires connected to them). Newer outlets are often Tamper Resistant ("TR"), which means they have a shutter designed to block things like meter probes; but a dedicated tester won't have a problem with these.

Neither method will, unfortunately, tell you about resistance or other problems. This is very difficult to measure safely on a live circuit (with high resistance but no load, you'd still see normal-looking voltages) and is probably best left to a qualified electrician.

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