Ghost Voltage

[jeff43222]

I was troubleshooting a light fixture this morning and came across something weird. HO told me her son installed it and couldn't get it to work. I opened up the box and found a rat's nest of wires, all old cloth stuff except for one piece of 14-2 NM someone retrofitted in at some point. The leads on the light fixture were connected to the white NM wire and a pigtail connected to one of the cloth wires.

My hot stick showed the white NM wire as hot, and the cloth pigtail as not hot. My Fluke read 117V between the white NM wire and the cloth pigtail, as well as 117V from the white NM wire and the metal box. No voltage between the cloth pigtail and the box. When I connected the light's leads to the white NM wire and the cloth pigtail, I got sparks, but the light bulbs did nothing. Same thing when I connected the leads to the white NM wire and the box. Nothing happened when I connected to the cloth pigtail and the box.

I eventually was able to trace the wiring from the switch and discovered that the light was not connected to it; I found the wire capped inside the box. Once I connected the light to the wires it was supposed to be connected to, everything worked fine.

But I'm still wondering about the behavior of that white NM wire. I've never seen ghost voltage read that high, and I would think that 117V would be enough to light up a couple of 40W bulbs.

Just curious...

 

[LarryFine]

You should have checked the voltage across the lines with the bulb connected.

 

[iwire]

I have said it many times and will still keep saying it.

Forget the DMM for basic line voltage troubleshooting, all they do is make you ask more questions.

Use a basic solenoid type tester with continuity testing ability.

Had you used a solenoid tester you would have gotten done faster as you would not have been chasing the phantom voltage.

But I'm still wondering about the behavior of that white NM wire. I've never seen ghost voltage read that high, and I would think that 117V would be enough to light up a couple of 40W bulbs.

117 is enough voltage....assuming there is enough current behind it.

There is almost no current available in this situation.

Here is some NEMA info on this.

[FONT=Arial,Bold]

BULLETIN​

[/FONT][FONT=TimesNewRoman,Bold]

No. 88​

[/FONT]

October 1998
Revised February 2003​

[FONT=TimesNewRoman,Bold]

Phantom Voltages​

[/FONT]

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.​

 

[George Stolz]

I've had a question I keep forgetting to ask.

I bought one of these a while back, and have had occasion to use it quite a few times since I did. I've noticed some things I don't understand.

• Sometimes, when testing live conductors, it will indicate continuity between open air and the live conductor. When I ground the second lead, it will vibrate and indicate voltage properly.

Is this normal? How can this be?

• All conductors will indicate continuity to the EGC with the ungrounded conductor I am testing switched off. I assume it is reading through the ballasts of other connected fixtures to neutral, and getting continuity this way.

This makes troubleshooting with it very difficult. Everywhere I look I have a "short" according to the meter, since it's a yes/no answer instead of a number as with a DMM.

My confusion at using this meter almost has me carrying my DMM just to confirm or deny anything the wiggy tells me, which makes it kinda pointless.

Is my Vol-Con defective, or what?

 

[iwire]

Is my Vol-Con defective, or what?

No.

I had to think on your question for a while and I what I come up with is this.

A lot must depend on our own personal preference for troubleshooting methods.

Yes that tester will read through the load, if I am chasing an open (non-working circuit) this fact can be an aid.

If I happen to be chasing a short on a line to neutral load I will probably lift the neutral from the source at some convenient location so that it is isolated from intentional grounds.

Of the different problems the two types of meters present I would rather deal with the wiggys.

The benefits of a Wiggy is they are cheap, durable and do not 'grow legs' like DMMs can.

If you want accuracy without the phantom voltage problems you can get one of these.

I have that model but to be honest I do not use it often but IMO it would be a better meter for run of the mill line voltage troubleshooting than the average DMM.

 

[cpal]

No.
If you want accuracy without the phantom voltage problems you can get one of these.

I have that model but to be honest I do not use it often but IMO it would be a better meter for run of the mill line voltage troubleshooting than the average DMM.

There was a time when the 260 was an elite instrument disregarding the VTVOM!!!

I use a flash light with test leads to chase wire circuits (control cabinets , mag starters, etc. The 3V will not ring thru a relay coil . (but you have to isolate the the solid state)

 

[George Stolz]

Thanks for the reply.

• Sometimes, when testing live conductors, it will indicate continuity between open air and the live conductor. When I ground the second lead, it will vibrate and indicate voltage properly.

Is this normal? How can this be?

Any thoughts on this? Have you seen this happen?

 

[iwire]

Yes, but I don't give it any thought.

I just assume it works like this neon voltage tester with only one lead and no battery.

 

[George Stolz]

I think you're right.

I just wandered outside and played with it some more. It would only do it when I touched the opposite lead - as in, red lead in hot side of receptacle, and I touched the black lead with my finger, and the continuity LED came on and it starting beeping.

I must have been inadvertently touching the lead on the previous occasions.

I was kind of suprised I didn't get a shock, though. I was doing my monkeying on a GFI to be on the safe side.

It also finds continuity between my hands, when I press the probes against my fingertips firmly. That's pretty sensitive, no wonder it reads through ballasts!

 

[iwire]

It also finds continuity between my hands, when I press the probes against my fingertips firmly. That's pretty sensitive, no wonder it reads through ballasts!

Wow, that surprises me, I have not noticed that, I will have to check mine out.

Where your fingers wet or are you otherwise freakishly conductive?

 

[roger]

There was a time when the 260 was an elite instrument disregarding the VTVOM!!!

IMO the 260 is still an upper class instrument.

I still carry an analogue Amprobe in my meter bag.


Roger

 

[George Stolz]

Wow, that surprises me, I have not noticed that, I will have to check mine out.

Where your fingers wet or are you otherwise freakishly conductive?

No, my fingers were dry. A casual touch did nothing, but as I pressed more and more firmly it increased from a feeble "eee..eeee.." to a solid ring.

Found a somewhat interesting article in my search for a schematic on the tester.

That Simpson 260 does have quite the reputation, from what I've seen. A new one costs $187 from a supplier. At three pounds, it sounds a touch heavy for carrying around troubleshooting, unfortunately.

The search for the perfect meter continues...

 

[kbsparky]

....Sometimes, when testing live conductors, it will indicate continuity between open air and the live conductor. When I ground the second lead, it will vibrate and indicate voltage properly. Is this normal? How can this be?....

Look again. I'd be willing to bet that you are getting what I call "snake eyes" and not the continuity indication. Both lights are lit when you have one lead on the live wire, and the other lead hanging in the air?

Continuity is indicated by only the left side red light.

When both lights are lit in this manner, it can be the same as "phantom" or "trace" voltage. Try using it on a doorbell transformer ... you'll get the same indication of low A/C voltage present.

Try using it on a 9V battery. Then reverse the leads. The red lights will change positions as you change polarity.

There are a lot of useful features of a Vol-Con tester. You only have to get used to what it can do.

 

[George Stolz]

Look again. I'd be willing to bet that you are getting what I call "snake eyes" and not the continuity indication. Both lights are lit when you have one lead on the live wire, and the other lead hanging in the air?

Continuity is indicated by only the left side red light.

When both lights are lit in this manner, it can be the same as "phantom" or "trace" voltage. Try using it on a doorbell transformer ... you'll get the same indication of low A/C voltage present.

The "snake eyes" are readily evident when one lead is on the hot and the other grounded.

I'm certain of the "continuity" reading - When I tested it outside this afternoon, the circuit (from hot, through the Vol-Con, through my skin, through my shoes to ground, presumably to the ground rod at the pole) was resistive enough not to trip the GFI. It was just enough for some current to trickle through.

Somehow, by the design of the Vol-Con, it works it's way up to a full reading, or something, I'm not sure. A light touch to the black probe by my fingertip didn't ring as strongly as when I pressed hard, but it did ring slightly under a gentle touch under voltage. Under no voltage, it took a hard grip to make it ring from right hand to left hand.

There are a lot of useful features of a Vol-Con tester. You only have to get used to what it can do.

That's the truth. I keep playing with it under different circumstances to get used to what it's telling me.

For example, Friday morning my tick tracer indicated induced voltage onto a 277v switchleg to a light I was about to install. Other lights on the circuit were energized. I noticed the tick tracer dimmed somewhat when both three-ways were down, so I taped them both down (both within sight five feet from the ladder) and then uncapped to test for voltage.

Both LEDs lit up, and I think the neon indicator might have lit for an instant, but maybe not. Unsure of the test, I turned one of the switches on, and read a solid 277v to ground. Knowing the appropriate switch positions, I retaped the switches and installed the fixtures.

But induced voltage did set off the Vol-Con.