Residual Voltage - ECN Electrical Forums

I've ran into this situation a couple of times now and was wondering if any of you had any idea as to what is causing it.

Residential - After turning off a standard single pole 15A toggle switch (i.e. no pilot light), I still get about 15 Volts coming through to the outlet in the ceiling when changing a light or ceiling fan. When the circuit breaker is turned off, the voltage drops to zero. Considering that this has happened several times in several different homes, and considering that the standard switch is breaking the connection on the ungrounded wire, then I have to conclude that this stray voltage is coming through on the grounded (neutral) conductor. But why?

I have two theories. One, is that the voltage is being induced on the wire. The other is that at least two circuits are sharing a neutral and the voltage is being back-fed. What do you think??? [Linked Image]

loose bonding connection?

How about a third theory, but really an expansion on your first one about voltage being induced.

I'll bet you are using a digital VOM. The input impedance is quite high, so high that it puts very little load on the circuit. Because of that you are reading the induced voltage. Put any load between the test leads (I would say your fingers but be careful here!) and the voltage goes to zero.

-Hal

Hal,

Correct - I'm using a Fluke digital VOM. So, in essence what you're saying is that a load on the circuit would bleed the induced voltage to zero?

You may be right. It's just aggravating not being able to simply use my voltage sensor to verify if the power is actually off. It lights up and bleeps with 15 volts just as if it were 120 volts. I guess it's probably just better all the way around if I take a few extra seconds to shut off the breaker. [Linked Image]

[This message has been edited by TNSunny (edited 06-15-2006).]

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NEMA ENGINEERING DEPARTMENT BULLETIN

No. 88

October 1998

Revised February 2003

Phantom Voltages

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.

I am very surprised meters sold to electricians don't have a setting that does put a 100 k ohm resistor <or so> across the leads to eliminate this problem. That high input impedance is handy if you are probing around in CMOS logic but it really serves little function when you are looking at line voltage.

I issue a Fluke 179 and a Square D Wiggy, the Wiggy is for LOTO verification.

I don't don't go along with induced voltages from other conductors. I think you are getting leakage resistance or capacitive effects across the switch. You have a much greater length of parallel conductors if your switch is on and your panel breaker is off so measured voltages shoud be higher. But they're not. If you have a Fluke 179, the input impedance is spec'd at >10Mohms. If you do the math, you come up with 105 volts dropping across >70Mohms to get the 15 volts that you are reading. If the resistance is infinate, <40pF of C would cause that much leakage at 60Hz. But just take the input impedance of your meter and multiply it by 7 in this case, to determine the amount of leakage resistance you're dealing with.
Joe

Joe, what do you figure the coupling is along a 40-50' switch leg and how would that show up against several megohms of input impedance? That math always boggles my crusty old mind but I do know a digital metyer is a random number generator on a floating line voltage conductor. I have had a fancy Fluke 6080 (8060?) for well over 2 decades but I never trusted it a lick on conductors that were not solidly connected to a source or a load.

Greg, I couldn't even guess how much coupling we might get. Experience in the field has probably been in the 100s of mV to couple volt range. But coupling would be intuitively greater with the switch on and running the whole distance back to the open beaker at the panel, At any rate, you wouldn't expect a change when he opened the feeder breaker if the switch looked infinately open. Plus, you gotta admit that a switch that looks like 70M when it's open, is a pretty good switch.

If you're talking about the 8060 with the buttons down the left side and dB capable, I love it. The only thing I like better on my 187, is being able to capture SCADA burst amplitudes by using the min/max record function. At least we can externally lower our input z on hi-z meters and it's a lot harder to go the other way.
Joe

No matter the source, whether capacative coupling, leakage across a device or induction. The electrician in the field needs to know if the voltage is real or "phantom". This is one of the reasons why I always carry a "solenoid" type of voltage tester. Mine is of the "Ideal" variety. They are a good test for this condition.

I have recently seen it where an unplugged meter base, with suite feeders attached and circuit breaker at the suite open, showed 120 volts to ground on the load side of the meter jaws when tested with a real good quality Fluke. I Did not have my Ideal tester with me so we used a 100 watt bulb, the voltage collapsed to 0 as soon as any load was injected.

[This message has been edited by Rick Kelly (edited 06-16-2006).]

I'm thinking that the voltage may induced into the 'disconnected' line from the grounded conductor. When you open the breaker, there is no longer any current flowing in the remainder of the circuit to induce voltage into it.

With the breaker closed, whatever else is running on that circuit will create a current flow in the parallel conductor.

If this were DC, we could use a compass to tell which way the mystery current is flowing.

I have seen up to 90 volts of 'phantom' voltage with my Fluke 87 on open conductors that share a raceway with other 277 circuits.

It is never just millivolts, the meter left unconnected shows millivolt readings. [Linked Image]

I agree with the above posters that for general line volt troubleshooting leave the DMM in the truck and use any solenoid type tester. They don't lie, assuming you check them against a known live circuit.

Bob

Thanks for the info and advice! I guess I'll be heading out to buy me a wiggy.

Kevin

I should have been more specific and said the voltage was capacitive coupled between the conductors and switch contacts rather than induced.

At any rate, your first choice when testing electrical circuits should be a Wiggy or other similar tester designed for the purpose. No real reason to use anything else like a VOM unless you need to do in depth troubleshooting. Then you need to keep in mind the high input impedance and the errors it may cause.

-Hal

I don't think I could ever leave my high quality but high-z meter back in the office in favor of one that gets me less. You can always go high to low. You can save alot of bucks with a couple feet of 1000V flexible test lead, a couple insulated gator clips, a little heat shrink, and a 27k, 2W common resistor. That would put just under 5mA load on a 120V circuit. If that's less current than GFCI specs say will kill you, it should be a good safety test when used with a hi-z meter. Just keep it coiled up in your Fluke pouch. If you get a non-zero voltage that concerns you, just clip the resitor on and measure again. Problem solved.
Joe

Joe I am not being a wise guy at all here but can you explain what kind of common line voltage troubleshooting requires such precise measurements?

Give me a solenoid tester with continuity function and I can troubleshoot most any typical lighting or power problem.

Many times (and I am guilty of it as well) I think folks just want to look 'cool' with more meter than is necessary. [Add Tim the tool man grunt here]

Of course there are times when you need to bring out the big guns.

But troubleshooting a lighting outlet with a DMM is IMO like using a sledge hammer to drive a finish nail. [Linked Image]

JMO, Bob

Bob,
A few months ago, a gentleman suggested just using a Wiggy Hot to Gnd to check a GFCI. I thought that a solenoid type tester might draw too much current to indicate proper function of a GFCI. We could search to find my old results, but I seem to recall that the current was about 8 times the trip spec. If this is the case, a solenoid type tester might well place a greater burden on the so called "phantom", or "residual", or "voodoo that you do so well" voltage, compared to a human. I know that my body will never see more voltage across it than my meter will at the same two points. Not so for a Wiggy.

Whatever TNSunny's needs might be, he already owns a high quality Fluke DMM. To invest in another tester just to put a greater load on a circuit isn't the way to go IMHO. I use just about every function on my 187 so a Wiggy wouldn't cut it for me. I might well suggest buying a Wiggy over a Fluke to someone, depending on the planned usage. Why pay the extra bucks if you don't need the extra function?

As a specific answer to your question, motor starts come to mind. What were the minimum voltages present on my phase loss monitor when the lead pump started? Do I have enough lower margin dialed in or will we just be doing repetitive pre-lubes from here on out. That UPS is non-synced. What is my input voltage and frequency? If I kill my alternate, am I free running at 60 Hz? Is my battery float at 133V and Equalize at 140V?

Finally, I like to play, "Sledgehammer" while driving finishing nails with a claw or ball peen hammer. "Solsbury Hill" goes better with troubleshooting except for the "boom, boom, boom" part. "Shock the Monkey" should probably be kept out of the rotation.
Joe

I find it rather hard to believe that seasoned Electricians are even discussing this.
I'm with Bob (Iwire) on this, look, if you get a dodgy reading on a DMM, you are obviously using the wrong test gear.
For the majority of my testing, I use a set of Duspol Solenoid testers, and I've never had a problem with "Ghost or Phantom" Voltages.
All you need is a tester that puts a bit of load on the circuit.
The Duspol (pictured below) draws 120mA with the solenoid energised, more than enough to get rid of nasty misleading Ghost voltages.

[Linked Image]

Joe it is definitely just a personal choice issue. [Linked Image]

Look at the OPs question here, there are no motor loads, there is nothing tricky.

It is just a simple branch circuit. [Linked Image]

I feel more comfortable dragging around a $30 almost indestructible meter than having a $300 fragile meter out on the job when it's not needed. (Theft, breakage etc.)

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You can save alot of bucks with a couple feet of 1000V flexible test lead, a couple insulated gator clips, a little heat shrink, and a 27k, 2W common resistor.

I don't see that as easer or better than using the right meter for the job at hand.

AS far as GFCI testing the button on the GFCI is more than up to the task.

I am not surprised a guy with a handle of 'testing engineer' would want to use overkill equipment. [Linked Image]

You could break out a Yokogawa Scope and find out even more info. [Linked Image]

But when all you need to know is;

Is it on?

Is it Off?

Is there continuity between X and Y?

It seems a little over the top.

JMO, Bob

I think it is a matter of personal choice of equipment, as for as the DMM, Solenoid debate goes. Both have benifits, and draw backs.

A lot of things I cant do with a wiggy, but dont reffer to a potential seen on a meter as "Phantom or Ghost", IMO belief in such things in that context can only lead to trouble. It may not be usable under even the smallest load, but it is there. Likewise, just because a solinoid is popped up in a certain range, there is a big difference in 120 vs. 150v or 90v that is not totally obvious on a Wiggy. Both instruments are subject to interpitation to some degree. But whatever the choice in tool is, you need to know the limitations of it. I have a Fluke 322 that I use for most things, it does the job, and I like the way it does it. Not to look cool.

On the original question, "One, is that the voltage is being induced on the wire. The other is that at least two circuits are sharing a neutral and the voltage is being back-fed. What do you think???"

Could be either, depending on the circuit and layout of it, and what it is measured against.

"I still get about 15 Volts coming through to the outlet in the ceiling when changing a light or ceiling fan. When the circuit breaker is turned off, the voltage drops to zero."

15volts is obviously not a usable voltage for the intended circuit, but does not mean it is not there. Think about the circuit path, where you are measuring you could be about 100' from the panel. Wind all that wire into some neat coils and you could end up with more, and it could be usable to boot.

I have to agree with the mods on this one, wiggy type testers for basic resi branch circuits & DMMs for in-depth troubleshooting......like on a PLC where a wiggy can actually turn stuff on ! (which is handy sometimes)

or a DC drive where the SCRs don't hold up to the "kick" from a wiggy

OK guys, I see I'm not convincing anyone here so please humor me. If a Wiggy type solenoid tester is your final arbitor of the safety of delving into a box or panel, PLEASE trust but verify. Firmly grasp the neutral or ground but give those "tested dead" hots the back of your hand first. Half the time, it's the surprise of the tingle that causes the most damage. And when the impedance of the tester is so much lower than the impedance of the user, these things shall happen. I'd rather be accused of beating a dead horse than hear of a dead electrician.
Joe

My, my, my....it seems that I've opened a can of worms here! [Linked Image]

Just like Tim "the tool man" Taylor, I can never have too many tools -- or boy toys, as my wife calls them. [Linked Image]

I can see a benefit to each of them, so I'll keep both on the truck. I don't recall the model number of my Fluke, but it is a clamp-on ammeter. I do use it quite often to check for proper line voltages as well as current draw for motors, etc. At the same time, I like to test open branch circuits to ensure that they are off before I connect a light fixture or other device. So, a wiggy would come in handy for these "on/off" tests (which is what I had been using my volt-tic for, but apparently it isn't too trustworthy).

As far as what looks "cool" is concerned, the average person is just as impressed with my volt-tic as they are my DMM. Others are more love-struck by my drills than any specific electrical tools. I think a Wiggy looks pretty cool too. Kind of retro, but cool. [Linked Image]

Thanks again for the abundance of information.

Kevin

Kevin, just so you don't feel bad- I might have related this story awhile back.

My shop is next to a utility room. I go into the utility room and flip the wall switch and the one fluorescent fixture doesn't light. Simple problem, should be able to take care of it with my eyes closed.

I change the tubes then I check the breaker- no dice. I take the reflector off the fixture and disconnect the feed to see if there is voltage present.

Now, since I'm basically a lazy fellow and my Fluke was sitting a few steps away on the bench and my Wiggy was out in the truck guess what I used? Yup and it measured 120 volts.

Ahaa, bad ballast! I replace it. STILL doesn't work. So after spending time and money finally my brain kicks in. I check the voltage again this time with my Wiggy. Zilch.

I check the switch and find that it wasn't closing, pitted contacts that presented a high resistance. High enough in series with the Fluke's high impedance to still show 120 volts. Put a load on it and it goes to zero.

Just goes to show ya. The right tool will save you time and money.

-Hal