Dead 120 VAC circuit - troubleshoot?

plugged in a

machine base.

electrical panel

turned off

circuit and

Ground is

You sure there isn't a GFCI upstream of the outlet?

First thing is to check for voltage right at the screw terminal on the breaker. Maybe the short damaged the breaker?

Ned Simmons

There are also some breakers which require the handle to be pushed further "OFF", then "ON" to reset after tripping. Just flipping this type to "ON" will not reset it. Don Young

And do you know where the wiring from the breaker goes before it gets to the outlet? Some do-it-yourself electrician may have left a loose wire nut somewhere.

Dennis in nca

As others have noted, check to see that you have not toasted the breaker. There are some real trash manufacturers of breakers and panels, some of the breakers will only take a few hard shorts before they act like a fuse and burn out. Use a voltmeter to check for 120 at the breaker terminal.

Check to see if you lost the hot or the neutral. On the bad outlet check the voltage from the hot terminal (black or the narrow blade on the outside) to ground (round pin). If you get 120 volts or so, you lost the neutral side.

If that checks ok, you likely opened up a connection **upstream** from the offending outlet. Classic case of problem and symptom in different places. Map your circuits by turning off the offending circuit, find ALL the outlets affected by the one in question. Then open up the closest good outlet to either of the bad outlets. You may have to try several to find the culprit.

Look for a wire nut that has one loose or burned wire, it could be either the black or white/grey. If the outlet is wired as a through point, you should see one pair of wires to upper set of terminals, the other pair of wires to the lower set. If these are screws, tighten them. If they are just pushed in the back, (ACK!!!, I HATE it when people use those!), push them in and see if they wiggle.

I fixed a dead outlet in the (professionally wired) basement by finding the junction where the electrician had nicked the wire. It broke, leaving the outlet dead. It was a PITA to find, at least for me.

I doubt that's the exact problem here, but if it was wired using the push-in holes of receptacles it could have burned one of those in another box.

Best regards, Spehro Pefhany

Be aware of a common GFCI "mousetrap." If the GFIC is wired backward, i.e., power connected to the output side and downstream outlets connected to the input side, it will protect the downstream outlets, but not itself. In other words, if it trips, the downstream will be dead, but it will have power itself, leading one to conclude it is not the problem. If you have GFCIs, it is a real good idea to hit the "test" button to trip it and then check to make sure it is, itself, off.

Jerry

Thanks for all the suggestions.

As it turns out there is a GFCI in a bathroom adjacent to the garage that had tripped and which apparently feeds the garage outlet. The GFCI outlet is not used so we never noticed that it was tripped.

I got off on several tangents in trying to troubleshoot this. This is a townhome that we purchased new and it looks like the electricians did a fairly decent job (wires screwed to receptacle terminals) and there were no worries about what another owner might have done. Aside from not remembering the bath room GFCI, the main problem was that I couldn't figure out which breaker was supplying power to the garage outlet and didn't want to pull the receptacle until I was sure that the wires were unpowered. The main panel breakers are labeled but a bit more obtusely than I would have liked. There is a "garage" CB but it turns out to be on a different circuit and the correct one is labeled only "GFI". I got sidetracked by a GFCI in a utility room which seemed to be a logical tie point for the garage outlet parttially because I noticed my wife had partially painted over the recptacle slots. That was a red herring.

At one point I decided to check voltages from the outlet to hots and neutrals on a known good circuit. I was getting 12-13 VAC on the hot & neutral from one pair of wires to the outlet and 68-72 VAC from the other pair of hot/neutral wires. Is that normal?

As an unexpected bonus, I did find one GFCI in the house that filed to test properly. It's located directly below the main electrical panel between two window wells. I replaced that one today.

Some time spent mapping CBs and circuits is probably in order against the next time something like this happens.

Mike

didn't want

unpowered. The

Did you use a digital voltmeter to measure this by any chance?

Yep, it was a digital VOM - would analog have given different results?

Yes, an analog volt meter is definitely more reliable than a lot of DMMs for AC volts.

With a DMM sitting on a table, not connected to any circuit, you might notice that if you only hold a single meter lead tip with your fingers, that the reading jumps around.

A quick check for the relative quality of a DMM is to set it on the lowest voltage range with no leads attached. The display should be zero or a very low reading that doesn't fluctuate. Fluctuation can indicate that there is no shielding, or that it's inadequate, and some low-end meters are the worst.

Depending upon the design of the DMM meter's input and conversion circuitry, the readings can be very unreliable, especially when checking for AC voltages.

Generally, the only analog AC meters that will give unusual-looking readings, are the amplified ones that are intended to measure very small potentials in the micro or millivolt potentials (not for AC house/building wiring troubleshooting).

WB ..............

"Ghost" AC voltages can often be seen with analog as well as digital VMs. The situation is related to the extremely high impedance of most "service type" VMs. They are designed to impose very small loading on the circuit under test. This is fine when testing electronic devices but can be misleading on ordinary AC power circuits. The cause is leakage. The source of most home power is a center tap (grounded) transformer with 120VAC circuits taken from both ends. Insulation leakage anywhere in home distribution, esp. across "open" breaker contacts, can give erroneous readings on ordinary voltmeters.

Bob Swinney

Glad you isolated your problem. Just a comment about digital meters: A low battery can give bad readings.

Ace

In this same vein those neon circuit testers are worthless and potentially dangerous because they can give erroneous readings. Capacitive coupling and leakage give these. There was an article in Electrical Contracting and Maint. several years ago. It's gist was that they were only good for night lights.

Chuck P.

They can also be connected between your fingers and the wire to identify the hot wire in knob-and-tube wiring in the attic where no ground is within reach. That function is probably not very useful nowadays. Don Young

And a hundred thousand ohms or two of resistance between the leads usually kills the spurious readings - keeps Zero at Zero. Take the resistor off when you need the high accuracy and impedence.

Bought a neon tester some years ago - screwdriver with insulated shaft, exposed blade, neon tube in the handle connected between the shaft and the pocket clip. I doubt that these are available now. Gerry :-)} London, Canada

I guess I expected something other than absolute zero VAC, but 14 and 70 VAC had me a bit concerned. Without cause apparently.

Your "no lead" test was interesting. The Tenma (import from Gaingers, probably) that I was using most of the time is an autoranging DMM and reads around 1-2 MV. An old Heathkit (an actual kit) from around 30 years ago reads 0.1-0.2 MV reads exactly 0 on all of it's ranges. Guess I'll use the Heathkit next time and maybe look around for an analog in good shape.

Is Simpson a good brand?

Mike