Very strange phenomenon

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aaknj

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We recently used a 15' length of 12/3 Romex to connect two circuits in a residential garage. One circuit was dedicated to a new phone system and the second was to be used as a GFCI utility circuit. In the course of making the final connections, we noticed that we had voltage on one of the legs whose breaker at the main panel was turned off. We decided to measure the voltage and found that we had about 24 volts to either neutral or ground with the breaker turned off and the other leg powered. The bare ground and neutral were connected to the appropriate bars in the main panel. At first we suspected an induced voltage, but could not find any source for that. We disconnected the outboard end of the circuit and found the same condition existed for either leg with the other legs breaker turned on. There was no path since nothing was connected and hence no load. We then thought that perhaps there was a high resistance short somewhere in the feeder and subsequently removed it and then replaced it with another piece of three wire, but the same condition resulted. We checked the utility ground to the ground rod and found both at the same potential.

There are no florescent fixtures nearby, although there are several other parallel two wire runs to other parts of the home installed near our new feeder.

We were not successful in isolating the reason for this condition and would appreciate any sage advice you could offer.

Thanks
 
Capacitance:

Capacitance:

I would suspect capacitive coupling between the ON and OFF hot wires. A high impedance meter could easily measure such "stray" voltages. Measure it again with a low impedance meter like my 5Kohm/volt Heathkit VOM.
 
Or simply place a load, like a 60-watt bulb, on the dead circuit and measure the voltage across it.
 
Saw this many years ago and determined for that case anyway, that the parallel conductors were causing the problem. Since then, it became the practice of running all GFCI circuits as 12/2 (no sharing of neutrals) and making sure they did not run in close proximity to any other conductors, and all crossing points were perpendicular.
 
kingpb said:
Saw this many years ago and determined for that case anyway, that the parallel conductors were causing the problem.
Click to expand...
But is it really a "problem"?

The extremely high input impedance of some test instruments permits the "measurement" of tiny amounts of energy.

Tiny amounts.

In the macro world of normal power and light wiring in a "residential garage" this energy is of no consequence.

IMO, it is less than a static charge that might get the hair on my arm to stand, but not have enough potential to even jump an air gap.
 
al hildenbrand said:
But is it really a "problem"?

The extremely high input impedance of some test instruments permits the "measurement" of tiny amounts of energy.

Tiny amounts.

In the macro world of normal power and light wiring in a "residential garage" this energy is of no consequence.

IMO, it is less than a static charge that might get the hair on my arm to stand, but not have enough potential to even jump an air gap.
Click to expand...


I don't think it is a problem either. The input impedence of modern multimeters can be almost infinite. On "volts", the positive lead may be wired to the gate of a "Metal Oxide Semiconductor" (MOS) transistor. That transistor actually has a thin insulating layer between the the gate and the rest of the transistor. So you are trying to measure the voltage of something that is not electrically connected to anything. It is basically acting as an antenna, and the voltage you are measuring is the signal it is picking up.

Get any voltmeter with an analog display (the kind with a needle that moves), and I'll bet you measure 0 volts.

Steve
 
KingPB, I would have thought that a shared neutral downstream of a GFCI alone would be the cause of it tripping, and not the fact that they were parallel.
 
Uh-uh!

Uh-uh!

steve66 said:
I don't think it is a problem either. The input impedence of modern multimeters can be almost infinite. On "volts", the positive lead may be wired to the gate of a "Metal Oxide Semiconductor" (MOS) transistor. That transistor actually has a thin insulating layer between the the gate and the rest of the transistor. So you are trying to measure the voltage of something that is not electrically connected to anything. It is basically acting as an antenna, and the voltage you are measuring is the signal it is picking up.

Get any voltmeter with an analog display (the kind with a needle that moves), and I'll bet you measure 0 volts.

Steve
Click to expand...

Steve,

In practice MOS gates are never left open. It happens sometimes; then you have a "floating gate" problem which is often difficult to track down. You still want a high Z meter though for probing MOS circuitry.
 
Make sure that you have installed the GFCI so that it does not "see" any neutral current form the other circuit, or it will always trip. The alternative is to use a 2-pole GFCI breaker to feed the multiwire circuit and protect both circuits.
 
haskindm said:
Make sure that you have installed the GFCI so that it does not "see" any neutral current form the other circuit, or it will always trip. The alternative is to use a 2-pole GFCI breaker to feed the multiwire circuit and protect both circuits.
Click to expand...
The openning post (OP) is not concerned with a GFCI trip. Kingpb mentioned the tripping as part of a similar situation. . .just not so similar ;) .

The OP is talking about a high impedance voltmeter reading on floating conductors.
 
So this phenomenon is as old as digital meters.
I use a 20 year old Fluke 77.
It doesn?t have to be 60 hz. but it probably is.
A higher frequency would induce a stronger voltage with a short run like that.

Has any meter manufacturer ever addressed this problem?
Maybe add a 10k ohm/Volt button to a digital meter.
Heck, it wouldn?t even have to be per volt would it?
 
johnny watt said:
So this phenomenon is as old as digital meters.
I use a 20 year old Fluke 77.
It doesn?t have to be 60 hz. but it probably is.
A higher frequency would induce a stronger voltage with a short run like that.

Has any meter manufacturer ever addressed this problem?
Maybe add a 10k ohm/Volt button to a digital meter.
Heck, it wouldn?t even have to be per volt would it?
Click to expand...

The problem is in the cockpit, not the meter The sensing circuitry determines the input impedance, not the display. Years ago there were VTVMs, Vacuum Tube Volt Meters that is. One could use an analog meter or a digital display (remember "Nixie tubes?) for the readout, but the impedance was still in the order of 10megs.

Of course, the classic Tripletts and Simpsons used a d'Arsonval meter movement for sensing and display, analog that is.

It is up to the user to understand the phenomenon and either use a low impedance meter or put an external load on the phantom voltage.

As the youngsters say, "No prob".
 
I don?t see why they shouldn?t make meters a little easier to pilot.

Adding a load to the circuit to be measured is an extra step that the meter manufacturers could do for us with one button and a resister.

A large portion of meter users are not concerned with voltages that can only drive micro amps.
 
johnny watt said:
Maybe add a 10k ohm/Volt button to a digital meter.
Click to expand...

I think that is an excelent idea. It's not easy to add a resistor in parallel with your test leads, and then hold them in place while checking for 120V. It's not that safe either.

And it doesn't matter how well you understand the physics, you never know if a voltage is real or a phantom unless you add some small amount of load and find out if the reading goes to zero.
 
rattus said:
(remember "Nixie tubes?)
Click to expand...
I do! I've been into electronics since Popular Electronics was a small-format magazine.

Of course, the classic Tripletts and Simpsons used a d'Arsonval meter movement for sensing and display, analog that is.
Click to expand...
The moving-coil 'motor' makes them true-RMS-reading, too.
 
steved said:
Fluke makes a Stray Voltage Adaptor that plugs in between a DMM and the test leads and puts a 3K load on the circuit under test.
Click to expand...
F-sv225_02a_200p.jpg


Very cool. Nice link.
 
General Radio made a double banana plug on 3/4" centers. Screws were provided for mounting external components. The plug was inserted into the meter or whatever, and the test leads were inserted into the plug.
 
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