Barbqranch
Senior Member
- Location
- Arcata, CA
- Occupation
- Plant maintenance electrician Semi-retired
I can't tell for sure from the picture, but it looks like the meter has a low impedance voltage setting.
HOT/NEU Reversal?
- Thread starter drcampbell
- Start date
- Status
Now that you mention it, you're correct. That's a Fluke 117 with has a built-in low-impedance measurement option. But the OP wasn't using that setting.
OP: When testing branch circuits, use the Auto-V/LoZ selection on your meter. That will eliminate phantom voltage readings. The only time you do NOT want to use low-impedance testing is when measuring voltages in electronic circuits because the meter will pull too much current through the circuit which could damage the sensitive electronic components.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
190701-2018 EDT
I am in agreement with the extension cord crowd.
The simplest quick test is to find a outlet on a different circuit, close to the main panel, and plug the extension into this as a reference point. You can use this for useful experiments. Use a high impedance DVM, and a load. A 1500 W space heater is useful for the test load. You do not want to be on a shared neutral with the circuit to be tested.
Measure the hot to neutral, and neutral to EGC at the end of the extension cord just for reference. Neutral to EGC should be small, possibly close to 0 millivolts.
At my main panel I read 0.9 millivolts from neutral to EGC as a result of whatever unbalanced load is presently on the panel. Same reading at the end of an extension cord. At one of my workbenches about 50 feet from the main panel neutral to EGC is about 65 millivolts for whatever load is on the bench.
I might suggest an outlet that has less than 100 millivolts between neutral and EGC and is not sharing a neutral with the circuit to be tested would be a good reference for the problem of this thread.
We want to use the EGC of the extension cord as the voltage reference point (black lead to the measuring VOM).
Using this reference measure the hot, neutral, and EGC voltages at the questionable outlet, and record the values. Plug the 1500 W heater, about a 10 A load when heated, into the suspect outlet. Record the above voltages again. Note that you can expect the hot voltage to jump around upwards of 1 V over short times from varying loads on the power company system.
With a 10 A (12 ohms) load you will quickly know what kind of problem may exist. This kind of test will uncover other types of problems, such as loose connections. If the voltage jumps a lot with the heater on, then there are poor connections.
.
I am in agreement with the extension cord crowd.
The simplest quick test is to find a outlet on a different circuit, close to the main panel, and plug the extension into this as a reference point. You can use this for useful experiments. Use a high impedance DVM, and a load. A 1500 W space heater is useful for the test load. You do not want to be on a shared neutral with the circuit to be tested.
Measure the hot to neutral, and neutral to EGC at the end of the extension cord just for reference. Neutral to EGC should be small, possibly close to 0 millivolts.
At my main panel I read 0.9 millivolts from neutral to EGC as a result of whatever unbalanced load is presently on the panel. Same reading at the end of an extension cord. At one of my workbenches about 50 feet from the main panel neutral to EGC is about 65 millivolts for whatever load is on the bench.
I might suggest an outlet that has less than 100 millivolts between neutral and EGC and is not sharing a neutral with the circuit to be tested would be a good reference for the problem of this thread.
We want to use the EGC of the extension cord as the voltage reference point (black lead to the measuring VOM).
Using this reference measure the hot, neutral, and EGC voltages at the questionable outlet, and record the values. Plug the 1500 W heater, about a 10 A load when heated, into the suspect outlet. Record the above voltages again. Note that you can expect the hot voltage to jump around upwards of 1 V over short times from varying loads on the power company system.
With a 10 A (12 ohms) load you will quickly know what kind of problem may exist. This kind of test will uncover other types of problems, such as loose connections. If the voltage jumps a lot with the heater on, then there are poor connections.
.
mivey
Senior Member
It's one thing to meter the wires, its another to verify the wires are on the right terminal. FWIW, I would have stuck the meter leads in the receptacle because I would not feel like going to the trouble of removing the device unless that test failed first.
Never mind, I just looked at the pics and he has a test plug set up.
growler
Senior Member
- Location
- Atlanta,GA
That test plug is the key to the whole thing.
Once I realized that he was getting his wire colors from the test plug I knew I was wrong.
I don't normally even bother with a meter because if it fails the plug in tester then I know it will either fail the electrical or home inspection. Time to pull at least a one and possibly a few receptacles to find the problem.
K8MHZ
Senior Member
- Occupation
- Electrician
We have been talking about this for what, four days? And we have yet to pull the receptacle to look at the wiring and the connections.
Wow.
Wow.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I would have done that before posting anything about it, as well as looking into other points on the circuit.
K8MHZ
Senior Member
- Occupation
- Electrician
drcampbell
Senior Member
- Location
- The Motor City, Michigan USA
- Occupation
- Registered Professional Engineer
I have been reading all your comments and giving them serious thought, but:
[1] I haven't been back to the site during those four days.
[2] I was reluctant to pull anything and expose any non-finger-safe hot terminals because I was the only person there -- nobody to call 911 for me if the need arises.
growler
Senior Member
- Location
- Atlanta,GA
Since there may be loose wires on the back of that receptacle and they may pull out when the receptacle is removed I would suggest that you turn that circuit off at least until the receptacle is removed and you can see what your dealing with.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
190703-1207 EDT
It is not necessatry to pull the receptcale to determine the likely reason for the problem.
In some ways it is possibly better to not pull the receptcale first and disturb someting that may relate to the problem.
.
It is not necessatry to pull the receptcale to determine the likely reason for the problem.
In some ways it is possibly better to not pull the receptcale first and disturb someting that may relate to the problem.
.
growler
Senior Member
- Location
- Atlanta,GA
That really depend on what you are trying to accomplish. If you just wish to discuss the theory behind the problem you can leave things as they are.
If you wish to find the problem and repair it, then move on to bigger and better things then the receptacle needs to be removed because there is a good chance that just a visual inspection will reveal the cause of the problem.
I can't think of any reason not to remove the receptacle. Receptacles get removed and replaced all the time. If the problem is not in this receptacle then it may be necessary to remove the receptacles on either side of this one ( they may even be in another room). It important to turn off the circuit and see what all is on this circuit because it can be more than one room.
MAC702
Senior Member
- Location
- Clark County, NV
Did we already determine it's the only receptacle exhibiting this issue? If it isn't, then the problem likely isn't in that box. Find where all the problems are first. Then determine where to start removing things.
growler
Senior Member
- Location
- Atlanta,GA
You can never be absolutely sure but I took the above comment by drcampbell to mean there was only one receptacle with a problem.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
190703-1504 EDT
growler:
The problem may or may not be at this receptacle.
External measurements can tell you the nature of the problem. Knowing more about the problem better prepares you to find the problem.
.
growler:
The problem may or may not be at this receptacle.
External measurements can tell you the nature of the problem. Knowing more about the problem better prepares you to find the problem.
.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
190703-1702 EDT
The following discussion is possibly useful to understand what RMS is. But with no background in calculus it is possibly just greek.
https://www.raeng.org.uk/publications/other/8-rms
AC meters are not necessarily RMS reading, and with very peaked waveforms may have errors from limited dynamic range. Also may not read correctly when an added DC component is present.
If you have no calculus background, then take equally spaced samples of the signal over one full cycle, and then divide by the number of samples.
In the RMS calculation we first square each sample, next take the average of all the samples, and last take the square root of the average.
.
The following discussion is possibly useful to understand what RMS is. But with no background in calculus it is possibly just greek.
https://www.raeng.org.uk/publications/other/8-rms
AC meters are not necessarily RMS reading, and with very peaked waveforms may have errors from limited dynamic range. Also may not read correctly when an added DC component is present.
If you have no calculus background, then take equally spaced samples of the signal over one full cycle, and then divide by the number of samples.
In the RMS calculation we first square each sample, next take the average of all the samples, and last take the square root of the average.
.
growler
Senior Member
- Location
- Atlanta,GA
OK the problem may not be at this receptacle but there is at least a 50% that it is. If it is then you spend the next 10 minutes making the repair.
What happens if the problem is not at this receptacle? If anything changes when the receptacle is pulled then the problem was at this receptacle.
Realistically, the problem could be in the wiring just beyond the outlet box and pulling the outlet could move the wires such that the problem appears to resolve itself. But we could "what if" this to death. If it were me, I would have shut off the branch circuit breaker and pulled the outlet to inspect the wiring before doing anything else. Ninety-nine percent of the time, the problem is in the box.
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