1.7 Volts Between Utility Neutral and Grounding Electrode System

[Ravenvalor]

Hello,

I have a customer who has 1.7 volts between the utility's neutral and the grounding electrode conductor. This reading is taken with the meter pulled and the grounding electrode conductor completely removed from the meterbase. Is this something that he should be concerned about? About a year ago he had me put an Intermatic A/C TVSS on his condensing unit because it was damaged by lightning or surges twice. This condensing unit is about 5' away from his service. Since I installed the TVSS he hasn't fried his A/C but is still concerned about this small about of voltage.

Thanks for the help with this one.

 

[Sunny_92]

Unless he is feeling a slight shock when standing barefoot on the ground and touching a grounded metal object, it shouldn't be an issue.

Typically when measuring stray voltage, best practice is to insert a 500 Ohm resistor across the meter to be sure you aren't measuring capacitive voltage.

 

[ptonsparky]

Not unusual at all. It should be expected.

 

[Little Bill]

Hello,

I have a customer who has 1.7 volts between the utility's neutral and the grounding electrode conductor. This reading is taken with the meter pulled and the grounding electrode conductor completely removed from the meterbase. Is this something that he should be concerned about? About a year ago he had me put an Intermatic A/C TVSS on his condensing unit because it was damaged by lightning or surges twice. This condensing unit is about 5' away from his service. Since I installed the TVSS he hasn't fried his A/C but is still concerned about this small about of voltage.

Thanks for the help with this one.

Put the GEC back and you have just removed that potential difference of 1.7V.

 

[don_resqcapt19]

Assuming that the transformer that feeds this service feeds other services and that the neutral from this service does not run directly from the utility transformer, you are reading the voltage drop on the neutral that is carrying load for the other services. You can also be reading voltage drop on the utility primary neutral as they bond their primary and secondary neutrals together in most cases.

 

[infinity]

Since you disconnected the GEC are you reading the voltage between the neutral and the earth via the electrode?

 

[__dan]

In the old days more than 1.5 volts between N and G would be a suspect cause of damaged electronics. Today most electronics float from the ground so it is less of an issue. I always check the N to G voltage. More than 1.5 volts I trace upstream

If the GEC is attached and you read that voltage at the service, I would keep looking at it especially to look at the placement of the system bonding jumper. Make sure there is one where it is supposed to be.

I followed one to the main switch at a small strip mall and inside the main, the system bonding jumper busbar was in the factory open position. Probably had another at the utility transformer, but the difference at the outlet caught my eye and I followed it all the way to the main.

Once you get far enough downstream, normal voltage drop on the neutral should show a small raise above the EGC. Usually less than 1.5 V.

 

[hbiss]

I'm surprised it's only 1.7V.

-Hal

 

[victor.cherkashi]

the further your home from utility transformer a higher voltage you will see between neutral and ground.

Sent from my Nexus 5X using Tapatalk

 

[GoldDigger]

the further your home from utility transformer a higher voltage you will see between neutral and ground.

Sent from my Nexus 5X using Tapatalk

Although it will also vary with the degree of unbalance in the other loads on the secondary (and perhaps primary). If the primary distribution line is single phase, an offset between POCO's MGN (Multiply Grounded Neutral) and local or remote earth is much more likely.

If the offset gets large enough and POCO is unwilling or unable to address the root cause they may install a neutral isolator on the supply transformer. (I believe that a neutral isolator interrupts the direct wire connection between secondary neutral and POCO MGN and inserts a circuit which will break down at higher voltages (>10V?) to provide a fault current path with a lower impedance that just the earth connection in case of a primary fault.

 

[Adamjamma]

the further your home from utility transformer a higher voltage you will see between neutral and ground.

Sent from my Nexus 5X using Tapatalk

Ok..got my answer..even though my response was deleted as being off topic, this answer shows me I was correct in my own thought... sorry that someone thinks I am just asking stupid stuff here.. but.. the voltage he saw is what I am being taught here is Ze.. which goes up the further from the transformer on is...

 

[ActionDave]

Ok..got my answer..even though my response was deleted as being off topic, this answer shows me I was correct in my own thought... sorry that someone thinks I am just asking stupid stuff here.. but.. the voltage he saw is what I am being taught here is Ze.. which goes up the further from the transformer on is...

If you have a question that is not directed to the op start your own thread.

 

[Ravenvalor]

Since you disconnected the GEC are you reading the voltage between the neutral and the earth via the electrode?

Yes sir. Here in NC it is actually 2 - ground rods.

The distance between the service and the transformer pad is probably 200'.

I believe that the line from the transformer pad to the service is not also feeding another service. These look like approximately 2+ acre lots.

Thanks for the excellent explanations. The 1.7v may just be the great distance between the transformer pad and the service.

It has been an interesting topic.

 

[Ravenvalor]

the further your home from utility transformer a higher voltage you will see between neutral and ground.

Sent from my Nexus 5X using Tapatalk

Sorry for my ignorance. It seems like a lot of trouble for the neutral voltage to travel 200' to enter the earth when there is a ground rod at the utility transformer.

 

[GoldDigger]

Sorry for my ignorance. It seems like a lot of trouble for the neutral voltage to travel 200' to enter the earth when there is a ground rod at the utility transformer.

Which is why it travels over the neutral wire instead, with only a relatively small current actually traveling in parallel with the neutral from the building GES to the POCO ground at the transformer secondary.
The earth current will be, in effect, 1.7V divided by the series resistance of the two ground systems. The latter can easily be 50 ohms or more. So the earth current is very low compared to the neutral current.

 

[ptonsparky]

Voltage is a product of current flow through a resistance. If the meter was pulled with no load on the secondary of that transformer, and the rods are isolated from the grounded conductor you are measuring the voltage produced by the current flow of other sources through the earth.
You just happened to place your test leads approximately 200’ apart, assuming your grounded conductor has no other path to earth at the house.

 

[winnie]

IMHO there are several factors here, and understanding their source will help determine if this is something that the OP needs to be concerned about.

Currents are flowing through the earth from many sources, including utility sources. Whenever current flows through the earth you have voltage differences at different points along current path.

Power distribution generally uses a 'multi-earth-neutral' conductor which is a circuit conductor that is regularly connected to earth electrodes. This makes the earth a parallel path for the neutral current. Sometimes faults in neutral conductors of high voltage systems can make the earth a major path for neutral currents.

Stray current in the earth can be a serious problem if structures or features in contact with the earth concentrate voltage differences. For example a metallic plumbing systems grounded at one side of a house shocking someone standing barefoot on the other side of the house.

If you stick two electrodes into the ground and measure the voltage between them, you will be measuring the voltage caused by current flowing in the soil. As someone else mentioned, making this measurement with different loads (high impedance volt meter, and also say 500 ohms between the electrodes) will give you an idea of just how much current this measured voltage can actually supply.

On top of this in the US the utility feed includes a grounded conductor which is used both as a circuit conductor and as the bonding conductor between the utility grounding electrodes and the house grounding electrodes.

As others have noted, current flow on this conductor will create a voltage drop on the conductor. If you try to use this grounded conductor as your connection between electrodes, then voltage drop on the conductor will interfere with your measurement. This is the Ve value that was mentioned.

If other users share the same utility transformer, then voltage drop on the shared portion of the grounded conductor would interfere with your electrode to electrode measurement.

-Jon

 

[Ravenvalor]

Voltage is a product of current flow through a resistance. If the meter was pulled with no load on the secondary of that transformer, and the rods are isolated from the grounded conductor you are measuring the voltage produced by the current flow of other sources through the earth.
You just happened to place your test leads approximately 200’ apart, assuming your grounded conductor has no other path to earth at the house.

Thanks for all of the great info from all of these replies.

Just to clarify, the meter is pulled and the ground rods are isolated from the grounded conductor.
If the customer is still nervous I will isolate the utility ground from the meterbase as well.
I will also isolate the low voltage services that are bonded to the ground rods.

Best regards,

 

[ptonsparky]

Thanks for all of the great info from all of these replies.

Just to clarify, the meter is pulled and the ground rods are isolated from the grounded conductor.
If the customer is still nervous I will isolate the utility ground from the meterbase as well.
I will also isolate the low voltage services that are bonded to the ground rods.

Best regards,

As fun and interesting as this is, I believe you are chasing a non-issue. Especially in context with the AC unit. 1.7V means nothing to a unit that operates at 240v.

Your TVSS is looking for Transient Voltages to suppress, not a steady state that you are seeing. You would need to read the data sheets on the TVSS to know what it is designed to do.

That said, I like to run tests and learn when someone else is paying the bill.

 

[kwired]

Yes sir. Here in NC it is actually 2 - ground rods.

The distance between the service and the transformer pad is probably 200'.

I believe that the line from the transformer pad to the service is not also feeding another service. These look like approximately 2+ acre lots.

Thanks for the excellent explanations. The 1.7v may just be the great distance between the transformer pad and the service.

It has been an interesting topic.

In OP you said you had the meter pulled - which we must assume means you had no load on your service at all. The voltage you were seeing is coming from somewhere on POCO side, you can't have voltage drop on neutral between the transformer and your service disconnect if you have no neutral load. You can have voltage drop on primary neutral and because it is bonded to the secondary that drop will show up on everything bonded to it.

That 1.7 volts is only .023% drop on 7200 volts - POCO will not notice a thing at that level.