stray voltage

[jinglis]

Twenty some years in the business ( mostly service ) and I still can't answer all the questions. Here's the scenario. Seven wires in a pvc conduit in a concrete slab feeding two 600 volt three phase motors with a ground. 40' from the MCC in a pool filter room. A few years back I had a bad wire and replaced all three with new. I should have replaced both sets at the time but Friday afternoon at 4:00 made one set seem the right answer. Now they are both new. I had a mechanical maitenance person call to say that when he shut down the motor and pulled the fuses he still had 200 plus volts at the motor. I doubted this fact until I tested and verified this as true. After shuting down the second motor in the conduit the stray voltage went away. The megger showed the older set as having poor insulation. The newer set tested good. I pulled all conductors out and replaced them with new. I found one conductor in the older set with a pinhole in it. All the other conductors looked to be in good shape with no visible signs of deterioration. Any thoughts?

 

[George Stolz]

To prune this down a bit, here's the question I think you are asking, but I'm not entirely sure:

How could the old set of conductors with the pinhole put 200V onto the new set of conductors that had no damage?

Is that the question?

What type of meter did you test for voltage with?

 

[Pierre C Belarge]

What kind of tester was being used to perform the voltage test?

You may not have had voltage leakage, but just 'GHOSTING'.

 

[jinglis]

stray voltage

I used a VOM and read above 200 volts to ground.

 

[iwire]

Check this from 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.

 

[roger]

As the NEMA article Bob posted points out, a low impedance meter should be used.

I use an older analogue Amprobe and a wiggy for back up to my DMM's.

Roger

 

[George Stolz]

What was described above is most accurately called "induced voltage."

"Stray voltage" is the name more accurately attached to the following phenomenon:

Consider the picture above a cross section of a pole-supplied building with a grounding electrode system installed. The pole is supplied with a ground rod, as would be required by the NEC and POCO/NESC standards.

The brown dirt has no detectable voltage. The pink-shaded earth is affected by stray voltage, and will have a certain amount of potential on it. I imagine the affected area as a large watermelon-shaped volume of earth, with a grounding electrode (system) on either end of it.

As the neutral path overhead begins to deteriorate, current begins to make more use of the grounding electrodes to move the unbalanced neutral current. Since electricity seeks any path back to it's source, as the resistance of the service-conductor neutral rises, then the grounding electrodes and earth see more and more duty as a stand-in for the true intended neutral conductor.

Bear in mind that all this current is trying to get back to the transformers on the pole, the source.

As more current travels through the earth than the intended conductor overhead, it gets easier to detect, especially for barefooted critters.

This is stray voltage, as I understand it. Very different than induced voltage among conductors in a raceway, IMO.

 

[ibew441dc]

Is your 600v nominal system feeding the mcc delta or wye? Solidly grounded or ungrounded?

If it is wye , solidly grounded, is the system bonded via a main bonding jumper or system bonding jumper. If the system is not properly bonded the unusual voltage to ground would make sense.

There are many factors that come to play in your problem.

This might not be the problem in your case but worth looking at.


Ibew441dc

 

[jinglis]

stray voltage

It is a wye system with the neutral bonded at the main service entrance. Once all of the conductors were replaced with new the induced voltage read at less than 10 volts to ground.

 

[drg]

George your picture shows a path for the current to flow and probaly the single most reason it would take this path is because the bonding of the grounded conductor and EGC downstream of the service panel . Thats going to cause stray voltage problems more than anything else wouldn't you say.

 

[George Stolz]

George your picture shows a path for the current to flow and probaly the single most reason it would take this path is because the bonding of the grounded conductor and EGC downstream of the service panel . Thats going to cause stray voltage problems more than anything else wouldn't you say.

John, I don't follow.

If the EGC and neutral make contact downstream of the main bonding jumper, neutral current will return on both the neutral and the EGC of the branch circuit back to the service. Once at the service, if the service neutral is in good shape the current will funnel from the EGC and neutral onto the service neutral, back to the transformer. If the service neutral is bad, the "funnel" will be half-plugged, and voltage will "spill" into the earth via the grounding electrode system.

I don't see how a bootleg ground after the main bonding jumper would affect the outcome in regards to stray voltage. That is downstream from the earth contact we've provided by installing the grounding electrode system.

I'm not saying you're wrong, I'm just saying I'm not following your train of thought.

 

[drg]

I'm not saying you're wrong, I'm just saying I'm not following your train of thought.

I'll explain what I believe then George .

Lets take a typical old farm with all kinds of old buildings adjacent to the service ok .

More than likely they are going to have the neutral and grounds all mixed up
and connected together in these other buildings ,there will be no seperation of the two conductors and the neutral current will flow back through the ground and anything metal trying to get back to its source .

When animals are getting shocked and will not go near food or water in a metal trough , don't you think there is neutral current passing thru the metal
parts ? I do , and believe this happens because the two conductors are not isolated from each other , metal parts have become part of the grounded conductor circuit because they were never installed right in the first place.

Thats what I am thinking and believe what stray voltage is ,or at least on a
farm this is something I would refer to as such.
Now, if you think I am wrong go ahead and say so , Iv'e been pretty consistent with being wrong on other things so hammer away i'll listen.

 

[George Stolz]

Not at all, you've got an excellent point - I'm gonna be thinking about that while I'm proofreading my wife's homework.

 

[ptonsparky]

I'll explain what I believe then George .

Lets take a typical old farm with all kinds of old buildings adjacent to the service ok .

More than likely they are going to have the neutral and grounds all mixed up
and connected together in these other buildings ,there will be no seperation of the two conductors and the neutral current will flow back through the ground and anything metal trying to get back to its source .

When animals are getting shocked and will not go near food or water in a metal trough , don't you think there is neutral current passing thru the metal
parts ? I do , and believe this happens because the two conductors are not isolated from each other , metal parts have become part of the grounded conductor circuit because they were never installed right in the first place.

Thats what I am thinking and believe what stray voltage is ,or at least on a
farm this is something I would refer to as such.
Now, if you think I am wrong go ahead and say so , Iv'e been pretty consistent with being wrong on other things so hammer away i'll listen.

You describe a typical farm very well, but I disagree that they were never installed right in the first place. A farm is no different then the block you live on in town. Each building has it's own service disconnect and own GEC. Several of the homes are probably fed by the same transformer. If your housing area is older it may vary well have a common copper water line feeding all the homes.The farm, or at least those in this area, does not have one single service disconnect nor do all the homes in your neighborhood. They were wired correctly at the time and may very well be now, even the new installations. The milk parlor of my Dads had one light bulb. Not much neutral voltage drop on that and consequently not much stray voltage.

 

[George Stolz]

I revised my doodle a touch.

When animals are getting shocked and will not go near food or water in a metal trough , don't you think there is neutral current passing thru the metal parts ?

I think you're right, current could be travelling either way - either

• earth, cow, water/metal tank, EGC, service, transformer, or
• load, metal tank, cow, earth, transformer

Edit to fix code and add:

Now, the question is, under either scenario, wouldn't a GFCI trip?

 

[acrwc10]

Now, the question is, under either scenario, wouldn't a GFCI trip?

If it is a local gfi outlet the fault could be on the wiring right before or after the outlet causing a neutral ground conection and the outlet would still be functional. Or some one could have connected the neutral to the ground on an old two wire system to make it appear to be grounded, I've seen that trick. Or an unknowing person could think they are making it safer by connecting the bonding jumper in a sub panel neutral to ground. there was a post ,in grounding, about horses getting shocked at a heated watering trough a month back. This supports your drawing.

 

[drg]

Now, the question is, under either scenario, wouldn't a GFCI trip?

Perhaps the GFI would trip but what does that solve !! You will have a frozen block of ice in the morning and bigger problems with the animals.

On the older farms a lot of the equipment beyond the service panel is screwed up and its going take rewiring to fix the mistakes .
I have been on newer farms and of coures these places are installed right, but you take a 100 or 150 year old farm and for sure beyond the service most the added equipment is going to have its share of problems , a lot of hands have been in their through the generations and there is some scary wiring out there gfi's serve a purpose but they will not solve stray voltage problems.

 

[ptonsparky]

Nice Doodles.

Now that you have a damaged, to small or overloaded, neutral conductor, you will have stray current flow at the tank even without improper neutral/EG bonds.

 

[George Stolz]

Nice Doodles.

Now that you have a damaged, to small or overloaded, neutral conductor, you will have stray current flow at the tank even without improper neutral/EG bonds.

It was presented as an "either/or" type picture, sorry for the confusion.

 

[George Stolz]

Jinglis, I just looked in on this thread again and realized I had completely hijacked it. I'm sorry about that!

Here's the last relevant post:

It is a wye system with the neutral bonded at the main service entrance. Once all of the conductors were replaced with new the induced voltage read at less than 10 volts to ground.

Can anyone think of a reason why the induced voltage would drop so dramatically with the old set getting swapped out? That seems kind of odd to me.