Utility Creating High Service Ground vs. Earth Ground potential

[cmdr_striker]

I have recently had issues with my utility creating a high ground potential on my neutral and thus ground circuits. Service is fed from an overhead 2-phase 15 kva transformer directly to the main which is properly bonded and grounded. There is a sustained 20-25 vac potential between any grounds on the property and earth even with the utility main breaker open. This can also carry a few milliamp current if connected to a independent grounding rod.

Utility has replaced the transformer and tested the wire run from the transformer to the service meter. L1 and L2 to neutral are balanced voltage even under load testing. It is simply that the service neutral/ground is at an elevated potential compared to earth ground. This can under extreme conditions result in very small shocks even with all breakers open which seems dangerous in the extreme.

With the main breaker open if the utility neutral is removed this potential and any current flow disappears. This is obviously a utility issue right? How can I convince them of this? What is a plausible explanation?

Overhead service is two-wire, both hot and this house is at the very end of a long line.

Thanks all.

 

[GoldDigger]

It is a utility issue. Specifically the Multiply Grounded Neutral (MGN) which is run the length of the high voltage line to carry the high voltage secondary current is either broken or has a high resistance link in it. The result is that the primary return current (which will flow from hot to neutral even when the load on the secondary is balanced) is instead flowing through the Earth connection of the utility pole ground rod. Even when there is no load on the secondary, there will be magnetizing current in the primary. Because of the resitance of the utility Earth electrode, the transformer end of the ground wire will be elevated above distant earth ground. Because the primary neutral and the secondary neutral are connected to the same ground, this will put an offset from distant earth potential on the utility neutral to your house.
The responsibility for fixing it lies squarely with POCO. If for some reason it is not feasible to repair the real problem, a device called a neutral isolator can be placed at any point along the service neutral to allow the service ground potential to be set by the local ground electrode system instead of the POCO neutral. If there is a fault that applies full line voltage to the customer EGC the neutral isolator will break down and conduct, allowing the fault current to return to the POCO transformer over a wire path, so that a breaker can trip.
Neutral isolators are typically used for an offset on the order of 10 volts or less. The 20-25 volt offset really must be fixed by POCO. The value of the offset may fluctuate over time, as the primary current changes from just the magnetizing current to the full transformer rating current.

 

[GoldDigger]

Mod Note: I am allowing this post as it does not refer to DIY action by the OP, but asks rather for an explanation of the nature of the problem, to be corrected by POCO, with support from a local licensed electrician if POCO will not pay any attention to the customer directly.
This 20-25 volt offset reported is a potential life safety hazard, which requires urgent action.

 

[winnie]

I have several questions.

1) what is the symptom that triggered these measurements to be made? Electronics not working properly or people getting shocked?

2) by 'two phase' transformer, what is meant exactly? A common single phase transformer with split phase secondary, or a less common type of single phase transformer with two primary bushings, connected to two primary 'hot' conductors? (A transformer can be connected hot to grounded neutral or hot to hot. The former is cheaper, the latter is done to reduce neutral current problems. )

3) if it is a two bushing transformer, are both supply conductors 'phase' (hot) or is one actually a primary neutral?

4) what sort of grounding electrode system does the customer have? Is it properly connected?

5) how are the measurements from 'grounds on the property and earth' made? What sort of meter, what sort of soil electrode, and how far apart are they?

6) Are there any electric fence chargers in use near the property?

7) Are there any high power radio transmitters near the property?

Jon

 

[cmdr_striker]

All replies are much appreciated. As noted by GoldDigger this is NOT to conduct DIY action but for me to understand the problem and get the proper people involved. There is a shortage of troublemen in our area and while they do their best they don't always have the time or bandwidth to properly address unique problems. Their typical solution is to replace the transformer and walk away. I have had a local electrician involved but I think this is a bit beyond his comfort zone. To respond to Winnie's questions:

1) what is the symptom that triggered these measurements to be made? Electronics not working properly or people getting shocked?

SHOCKS. Very minor shocks when making contact with ground and a grounded metal.

2) by 'two phase' transformer, what is meant exactly? A common single phase transformer with split phase secondary, or a less common type of single phase transformer with two primary bushings, connected to two primary 'hot' conductors? (A transformer can be connected hot to grounded neutral or hot to hot. The former is cheaper, the latter is done to reduce neutral current problems.

I have a single-phase transformer with two primary hot conductors. Which makes me think it is not a multiple ground neutral issue as mentioned by gold digger.

3) if it is a two bushing transformer, are both supply conductors 'phase' (hot) or is one actually a primary neutral?

According to troublemen BOTH are hot. No primary neutral. I will note that the transformer grounding lugs are unused and I don't see how the case is grounded or bonded to neutral which is seems like it should be but it is the same in the entire area.

4) what sort of grounding electrode system does the customer have? Is it properly connected?

Grounding system is 3 - 8' 5/8" rods at 6' intervals that appear to be properly connected. This ground system is elevated 20v above earth ground. Neighbor fed from same transformer is elevated as well15-18 v.

5) how are the measurements from 'grounds on the property and earth' made? What sort of meter, what sort of soil electrode, and how far apart are they?

Measurements from property 'grounds' are made from either the grounding system itself or a bare copper ground wire to a standard stand-alone 5/8" ground rods pounded in 4-6' away. The area is sandy soil and very simple. Measurements made with a Fluke 117. It should be noted in most cases there is also measurable current flow at times as high as 30 milliamps.

6) Are there any electric fence chargers in use near the property?

No. None within at least a mile.

7) Are there any high power radio transmitters near the property?

No. None within several miles. I will add that there is a ground solar array professionally installed and permitted. It can be physically disconnected for troubleshooting with no effect.

Thanks all for the help...again this is not a DIY effort both an electrician and POCO troublemen are involved but they seem to have stagnated and I'm left with a fairly dangerous situation. I'm trying to nudge them in the correct directions.

 

[winnie]

It sounds like your local transformer is pretty reliably not connected to the utility primary neutral.

Are you sure that the utility primary neutral isn't present? Usually this is a wire lower on the poles than the primary phase conductors, and usually this _conductor_ also serves as the messenger cable carrying low voltage wires, so you might not be seeing it as a conductor.

Current from the utility neutral may still be present, even if it isn't from your local transformer.

There should be a 'grounding electrode conductor ' from your service to your ground rods. Can you have an electrician measure current on this conductor with an 'amp clamp'?

There approach of measuring ground system to earth voltages that you used is sound. You measured both voltage and current, eliminating the possibility that this is some sort of static charge.

You might try adding additional measurements where your probe electrode is placed at different distances and directions from you home grounding electrodes. Since you mention the PV array, it wouldn't hurt to measure both DC and AC voltage. A significant DC component would suggest the PV system is involved. (I don't think this is likely, but if you are making the measurements then switching the meter to DC is a trivial addition. )

It is possible that the source of this voltage is not from the local utility infrastructure, but from more distant equipment. The practical fix may not be to eliminate the true source of the problem, but local equipotential bonding to eliminate the symptom.

Jon

 

[ptonsparky]

How close are your neighbors? This can come from miles away.
Back when dairies were common 'stray voltage' was an issue we investigated on occasion. You may need to load up your meters to get a more accurate voltage check. It may be somewhat lower than the 25 volts you are measuring. Low impedance meter vs high.

 

[Rcsroper]

It seems there is confusion regarding HV supply. Your two bushing pole mounted Xfmr is receiving phase to phase from a remote substation and there is apparently no neutral conductor on the distribution HV line.

There is no doubt a capacitive charge from the Xfmr element to the tank, which may not be grounded to earth. The tank is not bonded to the neutral from the tank lug beneath the neutral. The utility’s intent may be to “float” the tank. However, if a path opens to ground from the tank, your neutral will be elevated by this capacitive voltage, and this creates a dangerous situation.

How do the forum members feel about this? If this is a common utility connection throughout the area …

 

[winnie]

How close are your neighbors? This can come from miles away.
Back when dairies were common 'stray voltage' was an issue we investigated on occasion. You may need to load up your meters to get a more accurate voltage check. It may be somewhat lower than the 25 volts you are measuring. Low impedance meter vs high.

Good point about using the low impedance meter. I think the OP's current measurement covers this. In current mode the meter has very low impedance. By combining the high impedance voltage measurement with the low impedance current measurement you can calculate the impedance of the source of the voltage.

-Jon

 

[cmdr_striker]

It sounds like your local transformer is pretty reliably not connected to the utility primary neutral.

Are you sure that the utility primary neutral isn't present? Usually this is a wire lower on the poles than the primary phase conductors, and usually this _conductor_ also serves as the messenger cable carrying low voltage wires, so you might not be seeing it as a conductor.

Current from the utility neutral may still be present, even if it isn't from your local transformer.

There should be a 'grounding electrode conductor ' from your service to your ground rods. Can you have an electrician measure current on this conductor with an 'amp clamp'?

There approach of measuring ground system to earth voltages that you used is sound. You measured both voltage and current, eliminating the possibility that this is some sort of static charge.

You might try adding additional measurements where your probe electrode is placed at different distances and directions from you home grounding electrodes. Since you mention the PV array, it wouldn't hurt to measure both DC and AC voltage. A significant DC component would suggest the PV system is involved. (I don't think this is likely, but if you are making the measurements then switching the meter to DC is a trivial addition. )

It is possible that the source of this voltage is not from the local utility infrastructure, but from more distant equipment. The practical fix may not be to eliminate the true source of the problem, but local equipotential bonding to eliminate the symptom.

Jon

Winnie thanks for all your continued input,

I am certain that a utility neutral isn't present. We are in a rural area and the poles are pretty bare. Just the two HV lines up high and no lower wires, not even phone or cable utility lines.

I will measure current in the grounding electrodes and get back to the group.

 

[gar]

230505-1238 EDT

cmdr_striker:

If you have two primary wires going to two insulated bushings on the pole transformer, and no other primary wires appear to go to the transformer, then I assume the transformer is an isolation transformer fed from two hot phase wires relative to ground ( earth ). Therefore, I would expect the primary source has no major effect on earth currents.

This is exactly what I have at my home. No electrical connection from primary to secondary except capacitive in the transformer, and the intended magnetic coupling from primary to secondary.

At my home I measure between two 12" screwdrivers inserted in the earth about 12 ft apart a voltage from less than 0.1 V to several tenths of a volt. This is direction sensitive, and varies with time. I suggest you make some similar measurements at various locations.

If you are in a non rock area then it is unlikely you will see an impedance as low as 1 ohm ( this might occur in a salt water area ), 20 ohms is more typical, and higher resistances would occur in very rocky areas. Once you interface to a much larger area of earth the impedance drops to near 0 ohms.

The neutral wire of a center tapped transformer secondary should be grounded with a rod at the pole the transformer is mounted on. From the pole transformer there are three wires to your home. One wire is the neutral from the transformer. This neutral wire is again grounded with one or more ground rods at the service entrance at your home. The voltage drop on this ground wire from the pole transformer to you main panel should be possibly less than 1 V at a current of 12 A in that wire. A 1500 W heater at 120 V is a means to get a current of about 12 A.

Do some two probe measurements at various locations, and directions in your yard and report back. Use a high impedance meter ( a Fluke 27 or 87 ).

. .

 

[Rcsroper]

230505-1238 EDT

cmdr_striker:

If you have two primary wires going to two insulated bushings on the pole transformer, and no other primary wires appear to go to the transformer, then I assume the transformer is an isolation transformer fed from two hot phase wires relative to ground ( earth ). Therefore, I would expect the primary source has no major effect on earth currents.

This is exactly what I have at my home. No electrical connection from primary to secondary except capacitive in the transformer, and the intended magnetic coupling from primary to secondary.

At my home I measure between two 12" screwdrivers inserted in the earth about 12 ft apart a voltage from less than 0.1 V to several tenths of a volt. This is direction sensitive, and varies with time. I suggest you make some similar measurements at various locations.

If you are in a non rock area then it is unlikely you will see an impedance as low as 1 ohm ( this might occur in a salt water area ), 20 ohms is more typical, and higher resistances would occur in very rocky areas. Once you interface to a much larger area of earth the impedance drops to near 0 ohms.

The neutral wire of a center tapped transformer secondary should be grounded with a rod at the pole the transformer is mounted on. From the pole transformer there are three wires to your home. One wire is the neutral from the transformer. This neutral wire is again grounded with one or more ground rods at the service entrance at your home. The voltage drop on this ground wire from the pole transformer to you main panel should be possibly less than 1 V at a current of 12 A in that wire. A 1500 W heater at 120 V is a means to get a current of about 12 A.

Do some two probe measurements at various locations, and directions in your yard and report back. Use a high impedance meter ( a Fluke 27 or 87 ).

. .

Gar:
Is there a connection between your Xfmr tank ground lug, located beneath the neutral bushing, to the neutral bushing? Without this connection, the tank is grounded to earth at the pole, and the neutral service conductor is grounded to earth at the service. Doesn't this effectively raise the neutral, with respect to earth, by the capacitive voltage coupling the element to the tank ? If so, it seems odd that this would be a common connection in the utility area.

 

[mtnelect]

I have recently had issues with my utility creating a high ground potential on my neutral and thus ground circuits. Service is fed from an overhead 2-phase 15 kva transformer directly to the main which is properly bonded and grounded. There is a sustained 20-25 vac potential between any grounds on the property and earth even with the utility main breaker open. This can also carry a few milliamp current if connected to a independent grounding rod.

Utility has replaced the transformer and tested the wire run from the transformer to the service meter. L1 and L2 to neutral are balanced voltage even under load testing. It is simply that the service neutral/ground is at an elevated potential compared to earth ground. This can under extreme conditions result in very small shocks even with all breakers open which seems dangerous in the extreme.

With the main breaker open if the utility neutral is removed this potential and any current flow disappears. This is obviously a utility issue right? How can I convince them of this? What is a plausible explanation?

Overhead service is two-wire, both hot and this house is at the very end of a long line.

Thanks all.

This is the problem.

 

[gar]

230505-1442 EDT

mtnelec:

My pole transformer has a secondary neutral on the transformer on its supporting wood pole. There is no primary neutral only a set of 3 three phase delta primary wires. Two of these wires supply my transformer.No associated neutral wire on the primary side. At the pole the transformer is mounted on there is a grounding wire connected to the transformer secondary neutral that comes down the pole to a ground rod right next to the pole. The service drop consists of three wires, one is the already grounded neutral. This neutral wire is again grounded at the main panel by 150 ft of 1.25" copper water pipe out to the water system at the street. Basically I have a well grounded system.

.

 

[cmdr_striker]

mtnelec: I have a very similar setup to gar. No primary neutral and 2 wires (from a three-phase delta primary back at the sub-station). At the pole the transformer has a secondary neutral and that is connected to a local ground at the pole. Service drop is three-wires (two hot and one neutral) runs to my main where it is grounded again by 3 - 5/8" and 8' ground rods at 6' intervals.

As there is no primary neutral, I'm not sure that the multi-grounded neutral situation is the issue, although I do agree that is becoming an increasing problem. I will do some voltage measurements per gar and a ground current measurement when I get home later today.

 

[mtnelect]

230505-1442 EDT

mtnelec:

My pole transformer has a secondary neutral on the transformer on its supporting wood pole. There is no primary neutral only a set of 3 three phase delta primary wires. Two of these wires supply my transformer.No associated neutral wire on the primary side. At the pole the transformer is mounted on there is a grounding wire connected to the transformer secondary neutral that comes down the pole to a ground rod right next to the pole. The service drop consists of three wires, one is the already grounded neutral. This neutral wire is again grounded at the main panel by 150 ft of 1.25" copper water pipe out to the water system at the street. Basically I have a well grounded system.

A poll of electrical engineers NOT associated with utilities
were asked if Figure 12 was the electrical wiring diagram for
the transformer located on the pole outside their home, or the
wiring diagram for the pad mounted transformer located on
the ground near their home or if they had an underground
electric transformer vault, the correct representation for the
transformer?

 

[cmdr_striker]

mtnelec: Neither Gar nor I (OP) have a phase-neutral primary. We have phase-phase primaries (two primary bushings). I have been assured this by the troubleman, linemen, and it can be seen in the transformer bushing type. So in this case figures 12 and 13 aren't really relevant.

 

[gar]

230505-1706 EDT

An experiment I would like to make, but probably will never do so is:

In my backyard drive nine 10 ft rods in the earth with one in the center of a circular path of equally distributed rods. With a scanning monitor look at the rod voltages.

Questions to be looked at are:
1. Is there a consistent dominate angle of largest voltage?
2. Range of magnitudes of voltage?
And other characteristics that might show up.

.

 

[Rcsroper]

230505-1442 EDT

mtnelec:

My pole transformer has a secondary neutral on the transformer on its supporting wood pole. There is no primary neutral only a set of 3 three phase delta primary wires. Two of these wires supply my transformer.No associated neutral wire on the primary side. At the pole the transformer is mounted on there is a grounding wire connected to the transformer secondary neutral that comes down the pole to a ground rod right next to the pole. The service drop consists of three wires, one is the already grounded neutral. This neutral wire is again grounded at the main panel by 150 ft of 1.25" copper water pipe out to the water system at the street. Basically I have a well grounded system.

.

Gar:
Your description is clear except for grounding of the actual tank. Is the tank ground lug tied to neutral at the tank? If so the capacitive charged tank goes to ground. If not, the tank elevates the neutral above ground, doesn’t it?

 

[synchro]

Grounding system is 3 - 8' 5/8" rods at 6' intervals that appear to be properly connected. This ground system is elevated 20v above earth ground. Neighbor fed from same transformer is elevated as well15-18 v.

Measurements from property 'grounds' are made from either the grounding system itself or a bare copper ground wire to a standard stand-alone 5/8" ground rods pounded in 4-6' away. The area is sandy soil and very simple. Measurements made with a Fluke 117. It should be noted in most cases there is also measurable current flow at times as high as 30 milliamps.

It sounds like the resistivity of your soil is quite high, because I'd expect significantly higher currents than 30mA when the voltage between the grounding electrodes and earth some distance away is around 20V. There are companies that make conductive cement/concrete that can be used to achieve a lower resistance with less conductive soils, but I have no direct experience with those products.