Stray Voltage in Pool / EGC

[al38]

I work for an electric co and we are investigating a report of stray voltage/shocks in an outdoor inground swimming pool. We measure 2.5 ? 3.0v from a metal ladder in the pool to a temp ground in the grass about 2 ft beyond the concrete walkway around the pool. Shutting off the power to the customer?s transformer, as well as the transformers feeding the neighbors on both sides, does not cause the voltage to disappear.

A neutral isolator/blocker is not an option, because when we cut the service neutral from the transformer, the stray voltage remained, apparently coming thru on the UG phone and/or tv cable system as well as thru our neutral.

Measurements from different ladders in the pool varied, with some as low as 0.5 or less to the same reference ground 2 ft off the concrete.

In the pool shed there is a sub-panel fed by 4 wires HHNG from the customer?s main panel at the house. If we disconnect the EGC that comes from the main panel from the grounding bus in the sub panel, which includes pool equipment grounds and the grounding grid around the pool, the stray voltage disappears completely.

What are the safety concerns if the EGC from the main panel is disconnected in the pool shed, separating the house grounds and the pool grounds? There is a possibility that the ground potentials between the house and the pool area could be different, but what scenarios other than lightning could introduce enough fault current into the ground to make this difference a danger to people? There are no high voltage electric lines either OH or UG anywhere near the pool area or closer than 200 ft from the house.

I understand there are exceptions allowed by the NEC where the EGC to a pool out-building is not required if the pool is built later. Is disconnecting the EGC in order to remove a stray voltage problem in a pool a valid exception? Any thoughts on this would be appreciated.

 

[360Youth]

I don't think you are going to find much willingness to disconnect and EGC. 2 things I am curious about is does the stray voltage remain on the EGC once it is disconnected, or is it completely gone, as you said. And also, you said the voltage remains when power is disconnected from utility, I assume that is what you mean by "transformer." If that is the case, you've got a stored power somewhere, maybe battery back-up on a low-voltage lighting. Pump motor has capacitors, but I've never known a motor capacitor to release its stored voltage in such a manor. Maybe I am misunderstanding something.

 

[winnie]

Thanks for providing your background, because this question would otherwise raise people's 'DIY' flags.

Grounding and bonding from the NEC perspective is rather different from grounding and bonding from a utility perspective.

In particular, the made metallic bonds (the EGC) are _required_ by the NEC in order to provide a fault current path. Without the made metallic bonds usually provided by the EGC, circuit breakers will generally not open in the event of a ground fault. This is in contrast to ground faults in distribution systems, where voltages are higher and circuit currents are lower, so that a fault to soil will actually cause a breaker or fuse to open.

There are essentially no exceptions in the NEC that would permit you to eliminate the metallic bonded path to earth between the two structures. The only exception that permits the elimination of the EGC requires the neutral to be used to provide the metallic bonds.

Removing the EGC would be a violation and would be extremely dangerous. In the event of a ground fault, exposed metallic items would be energized, but the breaker would not open.

I can imagine several possible solutions to your difficulties, however all of them would need to be confirmed by a suitable engineer and the local AHJ.
1) Provide a separate service to the pool shed, with its own transformer, with a neutral isolator/blocker on that separate service.
2) Add additional equipotential bonding around the pool so that step potentials are minimized even though the stray current is not eliminated
3) Check the local area and find and repair the source of the neutral current.

Good Luck
Jon

 

[don_resqcapt19]

Al,
As you work with the utility, I have some questions. Is the primary power in the area underground? If so does it have an unjacketed concentric neutral? If the primary is overhead, has the primary neutral back to the substation been checked out?
Don

 

[don_resqcapt19]

As far as the pool goes, I think the code needs to move in a different direction. The pool pump should be double insulated and not have any metallic parts in contact with the water. No electrically powered equipment of any kind should be permitted within 10' of the pool. The equal potential bonding at the pool should be isolated from the electrical system. Note, I am not suggesting that this be done with an existing pool...it could be very dangerous.
It is my opinion that the current cold rules actually require us to energize the pool bonding system with any voltage that is on the service neutral. This voltage is often the result of the voltage drop on the primary grounded conductor.
Don

 

[dbuckley]

2) Add additional equipotential bonding around the pool so that step potentials are minimized even though the stray current is not eliminated

Thats the one. The bonded metalwork of the pool is equipotential with the soil that the ground rod(s) are in by the service entrance (and other bonded stuff), whereas the soil potential near the pool is different due to some potential gradient across the soil.

A "ring of copper" ground around the pool area connected to the sub-panel ground bar will eliminate the problem.

 

[don_resqcapt19]

A "ring of copper" ground around the pool area connected to the sub-panel ground bar will eliminate the problem.

It won't eliminate the problem, but it may mask it.
Don

 

[peter d]

Just so I understand this: The source of this stray voltage is due to current from a utility source (normal current flow from an MGN system, faulty neutral, faulty cable shield, etc) flowing in the earth?

Can a premises wiring system cause it? Like a faulty service neutral which causes current to flow on a metal water supply system (assuming there is one)?

 

[al38]

We measure 3v from the disconnected EGC as well as from the neutral in the shed to the temp ground. Reconnecting the EGC brings the 3v back into the pool. Hadn't thought of the stored power possibility.

The primary power is OH 3-ph 13.2kv, the house has an UG svc from a pole mounted transformer with jacketed cables including the neutral. Our neutral back to the substation appears to be in order.

The suggestions to add bonding around the pool and connect to the sub-panel grounding seem to be the first step until the problem is found. In the meantime, we are still investigating the source of the voltage. It may be from induction onto the communication cables, and hopefully additional bonding by us and the other utilities along the main line will help alleviate most of it.

Peter, the source appears to be current from a utility. A faulty svc neutral is a possibility.

Thanks everyone for the advice and suggestions!

 

[dbuckley]

Anything that causes current to flow across the soil causes a potential gradient. All your listed causes are possible culprits.

Almost always, potential gradients are harmless and unnoticeable, becuase the rate of change of voltage in the distance between your bare feet is insufficient to be noticable. Until you get wet and have a cut

In my opinion, its pretty much impossible to prevent potential gradients across the soil, so the only hope one has of preventing shock in a dodgy environment like a pool is to create an equipotential bonded zone, so the currents that cause the potential gradient choose to use the copper paths you provide, rather than the soil. Thus within your protected zone there should not be any possibility of a potential difference between things people come in contact with.

Don calls this masking - I don't necessarily disagree with his terminology, but I don't know of any other solution that always works.

 

[don_resqcapt19]

al,

The primary power is OH 3-ph 13.2kv, the house has an UG svc from a pole mounted transformer with jacketed cables including the neutral. Our neutral back to the substation appears to be in order.

What about the primary neutral? That is the source of many problems like this.
Don

 

[don_resqcapt19]

Peter,

Can a premises wiring system cause it? Like a faulty service neutral which causes current to flow on a metal water supply system (assuming there is one)?

Anything that causes the neutral of the service to have a voltage to remote ground can cause this problem. When we apply the code rules we are required to bond the pool to the electrical system and in cases where there is any voltage between the service grounded conductor and "remote" earth, we are being required to energize the pool bonding system.
Don

 

[ghostbuster]

Anything that causes current to flow across the soil causes a potential gradient. All your listed causes are possible culprits.

Almost always, potential gradients are harmless and unnoticeable, becuase the rate of change of voltage in the distance between your bare feet is insufficient to be noticable. Until you get wet and have a cut

.

Cattle are very sensitive to this potential difference between their front and rear legs.They can refuse to milk or drink.The following link has some great info on stray voltage (and lawsuits) http://www.strayvoltage.org/

 

[Pierre C Belarge]

This situation can sometimes be very "tricky" in searching for the source.
There are many issues that we here cannot see, as we are not there. Some other questions:

What is the proximity of the other homes to the house in question and to the pool.
Is it possible that the utility source (primary) is on one side of the house and the neighbors home(s) - in other words the house with the pool is in between the nieghbors and the utility primary.
Is the voltage measured on 1 side of the pool, or all around the pool? Which side of the pool if only one side. This information can help in determing the source.

I would definitely not disconnect the equipment ground path from the house to the pool house!!!

Has anyone checked the neutral connections from the house in question? both at the house and the utility connection?
Has anyone also checked the neutral connections from the other houses in the neighborhood that are connected to the same transformer as the house in question?



If those connections are intact (I would be surprised to see if they are),then
my suggestion for now is to increase the size of the Equipotential ground grid around the pool. This way the step potential when one is wet from the pool will be larger around the pool.


Something I helped with on a utility issue a few years ago helped a restuarant.

Take the water supply to the house and dig outside around 10-15 feet from the house, exposing the water supply. Have the water company cut the water supply and insert a plastic junction (this may take some coercing).
If there is primary neutral current in the earth running through the water supply, this will isolate it in the home of question.

Good Luck
Please keep us appriased of the situation.

 

[dnem]

post #4
Al,
As you work with the utility, I have some questions. Is the primary power in the area underground? If so does it have an unjacketed concentric neutral? If the primary is overhead, has the primary neutral back to the substation been checked out?
Don

post #9
The primary power is OH 3-ph 13.2kv, the house has an UG svc from a pole mounted transformer with jacketed cables including the neutral. Our neutral back to the substation appears to be in order.

One quick question that's off the subject at hand. . I need to get "up to speed" about a power company setup. . I thought all transmission was delta.

Why would the secondary of the substation transformer be supplying the local transformers thru a wye configuration ? . Why is there a neutral between the substation and the roadside transformer ?

David

 

[don_resqcapt19]

David,
In this area most of the distribution voltages are fed from a delta/wye transformer with the wye grounded on the secondary side. On the one line that a local utility filed as part of a hearing for a new 138 kV transmission line, even the 138 kV is a grounded wye. There would be no voltage drop on the 138 kV grounded conductor as there are no single phase loads, but on the distribution system, there are often single phase line to grounded conductor loads and there will be a voltage drop. The one line does not go into enough detail to show those loads.
Don

 

[al38]

Don, we checked the neutral from house to transformer and visually inspected the neutral back to the sub and everything seems to be in order.

Pierre, the primary electric lines run along the road in the front of the house, which is about 200 ft off the road. This customer is the only one served off this transformer. The pool is in the back of the house, and only an open field adjoins the property in the back. We checked the neighbors on each side of the house, no problems found.

The highest voltage readings in the pool were on the side opposite the house but near the shed with the subpanel. It's interesting that some ladders in the pool have voltage and others have none.

David, as Don mentioned our transmission lines are typically delta, and our distribution feeders are mostly grounded wye from delta-wye sub transformers, although we still have some older vintage delta distribution lines that we haven't yet converted to wye.

 

[dbuckley]

I thought all transmission was delta. ... Why would the secondary of the substation transformer be supplying the local transformers thru a wye configuration ? . Why is there a neutral between the substation and the roadside transformer ?

Delta - you'd have thought, wouldn't you - well, check out this recent thread where I obtained enlightenment on this very issue, in particular follow the link to a PDF I subsequently found which was most interesting...

 

[vayidaho]

Mutiple Grounding and Unbalanced Utility loads?

Mutiple Grounding and Unbalanced Utility loads?

Not aimed at any particular posting, but I have always wondered about the distributed grounded neutral used by the utility in my area. The 12.4 KV lines are accompanied by a steel reinforced aluminum neutral on insulators about 6 feet below the power lines. Then the utility, at each pole, connects a #8 solid bare copper wire to the neutral and staples it down the pole and coils it up on the bottom of the pole so that when the pole is set, it provides a ground to the neutral at every pole base. Unless the neutral is absolutely free of current, there might be a small voltage drop between pole neutral-ground points.

I wonder if this could be a source of stray earth gradients seen near the pools?

 

[dbuckley]

I wonder if this could be a source of stray earth gradients seen near the pools?

How can it not be?

I was only introduced to this facet of (uniquely American) MV distribution recently. It's hard enough to keep "stray voltages" out of dwellings and businesses without parallel current paths through the soil being almost mandated by law.

Here in New Zealand on a business program this past Saturday there was a section about electrical problems in rural New Zealand, and mentioned stray voltages issues in cow sheds, and the research done that illustrates how stray voltages stress the cows, it how that impacts the rural sector in terms of cow production volumes. Its a big issue.