Question for the Sparky's

I have run into this situation a couple of times recently and not being an electrician or of an electrical backround I need some education. While checking the switched recpticles in a home recently, the middle light on my tester was dimmly lit with the switch in the off position. I put the meter on it and you can see the result, it was showing 54v. Other switched recepts in the home were the same, anywhere from 40 to 54v. Whats going on here? I did write it up as a concern that needed evaluation by a licensed electrician. Thanks for your input.

This Bulletin is intended to address the occurrence of so-called “phantom” voltages, a phenomenon detected during the testing of electrical conductors in the field.

Due to the high impedance of measuring instruments, a voltage reading may be detected on open conductors where there is no hard electrical connection to a voltage source. Conductors that are installed in close proximity to one another, and are capacitively coupled to each other, can cause this a.c. voltage reading. Such a reading could be 2 or 3 volts, or it may be as high as the voltage on the adjacent conductors.

This is what is referred to as a “phantom” voltage.

According to Underwriters Laboratories Inc., this can be a harmless reading and can be caused by the high input impedance of the measuring instrument, which places very little loading on the circuit under test. The capacitance is increased as the length of the run is increased. A 50-foot run may produce a pronounced capacitance effect whereas a one-foot sample may not produce any.

Since the “phantom” voltage is a physical phenomenon involving very small values of capacitance, it cannot energize a load or cause physiological damage to a person. Care must be taken to be sure that the voltage reading is a phantom voltage, which is caused by improper use of high impedance multimeters, and not as a result of a cable defect or improper installation, which may result in a shock hazard. In order to help minimize the likelihood of reaching a wrong conclusion from this phenomenon, NEMA recommends the use of a Listed low impedance multimeter in place of a high impedance multimeter or other high impedance measuring device for testing on open conductors where there is no hard electrical connection. Without a low impedance measuring device, a high voltage reading is an inconclusive indication of possible faults in the cable.

This was provided by NEMA…digital meters are prone to reading this " Phantom Voltage "…however it can also mean a hot wire is loose and voltage is being conducted on the netural…rather rare…try using an analog meter and you should get nothing…so be very careful on how you regard this.

Chances are with an analog meter you will get nothing…

Paul, Thank you for your explaination, makes sense now. Always a great learning experiance on here. What threw me was the light lit up even with the wall switch off.

Magentics…all voltage is created by magnetics in one way or the other…I always suggest an HI have a low cost low impedence tester like a “Wiggy” when simply detecting voltage…

Keep in mind…some conditions can use this type of effect…like an open conductor and so on…but it is best to leave that exporation to the electricians…because we will test it in all different ways…

But to settle your mind…get a cheap low impe. meter…you can get them radio shack for like $8.00 - $10.00 bucks

Good advice. I’ll pick one up tomorrow. Thanks again Paul!


Paul, I am having trouble seeing phantom voltage lighting the lamp on his 3 light tester. I suggest the possibility that the switches controlling these outlets are illuminated switches. The kind that illuminate when the switch is off. The light in these switches requires an incandecent lamp to be in series in order to glow. Ever notice when the fixture bulb burns out, the illuminated switch goes dark? Also ever notice when you change out a fixture with an illuminated switch controlling it, you can get knocked on your butt even with the switch off? I suspect his neon tester is glowing as it is in series with the neon in the switch. What do you think?

hmmm…never given that much thought Brian…let me JELL-O on it…and see what I can find.

But…we will need richard to enlighten us on IF these were illuminated switches…how goes it Richard…

Also…could be an issue of a transformer on the circuit say to a cordless phone…but I question it being the garbage disposal…I think it was…

Are you saying brian that in order for the switch to work…voltage will have to be present at the receptacle in order for the little light in the switch to function…does it not need a neutral…or are you saying it takes place within the switch…can’t say I agree on that one…but again…never thought about it…lol

We have to understand however that if the receptacle is switched…it should have NO voltage at it ( usable that is ) when it is in the off position…and properly wired…

Dang Brian…you are going to make me looking this up FUN…man I love this stuff…

<------NERD !!!

But…the digital high impedance issue and phantom voltage holds true…if anyone has been in the business long enough using a digital meter they will experience the phantom voltage…

Even if they were illuminated switches, with no load plugged into them they would not illuminate. And with the neon load plugged in, they would only glow dimly from the series circuit, probably not enough to see especially in daylight.

As soon as I have the time I am going to try this in my shop with an illuminated switch and a 3 light tester. Maybe tomorrow after work.

Do it…do it…I love experiments

You know Brian…it is a sickness…thats what they tell me…we have a sickness…:slight_smile:

I totally agree with you there. It shows up on the non contact sticks as well. Don’t know if you’re as old as me but I remember when there WERE no DMMs, only analog. Wish I still had one of those old Simpsons.

OMG…yes when I was in electrical school years and years ago…all we had was Simpsons…love them old big black boxes…:slight_smile:

Yes that’s what it is Paul. I am an avid hobbiest as a mechanic, let me tell you I once rebuilt an early Ford AOD, one of the early automatic overdrives. I couldn’t get it to shift right for about 2 weeks. I read and re read the manuals. Then, one night, I was dreaming about it and woke up at 2:00 AM with the solution clear as a bell in my head. I laid there till first light, then ran out and fixed it.
Yes, it’s a sickness all right.

Was it a dimmer switch?

I have those malfunction all the time and sometimes (depending on style) they appear to be off but are just at the lowest setting.


I hope not as switched receptacles should never be on a dimmer switch…call those out.

I know, but it wouldn’t be the first time I have found it. Replacing standard switches with illuminated dimmers etc is a common homeowner project and sometimes when I am trying to figure out one thing the obvious cause is overlooked. “Something about seeing a tree in a forest”:slight_smile:

Why should you never have a switched receptacle on a dimmer?

OK the test results are in. I used a P&S decora lighted single pole switch, non dimming. I hooked it up to the receptacle in series with the hot, ran the neutral straight to the recep. I used a lamp cord for the test, 2 wire only, so my receptacle was not grounded.

I had 3 of the 3 light testers, a GB, a Commercial Electric, and an old Calterm. Out of the three, the GB did indeed glow dimly at the center when the switch was off. The lighted switch also glowed dimly when the tester was plugged in and went out when the tester was unplugged.

Next I got out my analog meter. It read 52 volts when the switch was off.

So my conclusion is that the neon light in some testers will glow at half brightness when a lighted switch is present due to the fact that it is a series circuit, and the voltage is divided between the two loads (the switch neon & the tester neon), just like in any series circuit.

I wanted to try it with the receptacle grounded to see if two neons would glow, but it was late and I was hungry.

Also, I don’t know why the other two brands did not glow, maybe the neons are different, or maybe some are LED’s.

Anyway Paul, if you ever come to Tucson, give me a holler maybe we can get together and you can give me ten dollars for that lighted switch! :mrgreen: