What neutral to earth voltage is acceptable?

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11bgrunt

11bgrunt

Pragmatist
Location
TEXAS
Occupation
Electric Utility Reliability Coordinator
I was involved in a recent complaint in a rural area where 6 volts was measured between the water well and the earth. I used an O-scope and saw a 60Hz sine wave. The voltage varied and many checks were made over time. The short story is I wound up at the POCO substation and measured 5 volts checking from the POCO pole ground wire to a screwdriver stuck in the ground. This pole check was ten feet from their fence. I was using a Fluke 345 so I saw the AC & DC content and could switch over to see the sine wave. All looked liked AC to me. I did get one measurement that was as high as 10 volts measuring from a bonded guy wire to the substation fence. Concerned that it was all make believe, I drug out my Sidekick analog telecom meter. The needle swung up to 10 volts AC.
I was thinking that this would be an easy sell, but I was wrong.
The POCO rep responded with this: "The voltage registered near the station would be expected (due to any imbalance and higher impedance of the earth), due to returning current through the pole grounds and neutral back to the sub. "

I do not agree. Please explain how this can be acceptable and what is a big number where someone should be concerned?


 
That high a voltage measured at the substation indicates to me that the metal connection of the primary neutral back to the substation has too high an impedance, forcing significant neutral current to flow through the earth. But that voltage offset at the substation will not by itself cause problems for customers.
The same 5-6V difference in potential between POCO ground and remote earth, despite multiple grounds along the route, on the other hand, is not normal and would not be acceptable at a farm or marina.
A knowledgeable POCO engineer, even if he did not try to reduce the offset, should be willing to install a neutral isolator at the customer's service transformer to prevent that offset from showing up at the service and causing problems.
Most often an offset of that magnitude at a customer site results from deteriorated distribution neutrals that the company is not currently willing to pay to repair or replace.

A big question for me would be what magnitude of GEC current is involved when the POCO neutral is bonded directly to the well casing (which it normally would be if the plumbing were metallic.) That bond should reduce or eliminate any symptoms seen by the customer except for those resulting from an offset between water pipe ground and the unbonded local ground of metal drain plumbing sitting in the earth.
 
I was with the POCO crew today as mechanical jumpers were run from the three overhead circuits' ACSR neutral to the copper wire connected to the substation transformer X0 bushing. The 11 volts that was present when measured between the bonded guy wire to the substation fence fell to .5 volts after jumper was connected. A test on a compression connector connecting that circuit neutral ACSR to the A frame copper conductor showed the same 11 volts suggesting that connector was an issue. The other stray voltage levels did not reduce significantly and the area at the end of the circuit where all of this started has not improved. That POCO MV circuit was opened briefly and the stray voltage at the end of the circuit fell from 5 volts to .1 volt. There are miles of overhead conductor involved after those circuits leave the substation. This stray voltage has been found in an area about one square mile and in the adjoining POCO. I don't know what will happen next.
 
11bgrunt said:
I was involved in a recent complaint in a rural area where 6 volts was measured between the water well and the earth. I used an O-scope and saw a 60Hz sine wave. The voltage varied and many checks were made over time. The short story is I wound up at the POCO substation and measured 5 volts checking from the POCO pole ground wire to a screwdriver stuck in the ground. This pole check was ten feet from their fence. I was using a Fluke 345 so I saw the AC & DC content and could switch over to see the sine wave. All looked liked AC to me. I did get one measurement that was as high as 10 volts measuring from a bonded guy wire to the substation fence. Concerned that it was all make believe, I drug out my Sidekick analog telecom meter. The needle swung up to 10 volts AC.
I was thinking that this would be an easy sell, but I was wrong.
The POCO rep responded with this: "The voltage registered near the station would be expected (due to any imbalance and higher impedance of the earth), due to returning current through the pole grounds and neutral back to the sub. "

I do not agree. Please explain how this can be acceptable and what is a big number where someone should be concerned?


Click to expand...
From code point of view 5% voltage drop for efficient operation. So for example for 120V supply
N-G voltage drop maximum 3V for equal size conductors. When electronic product specs require lower N-G voltage drop that should be met.
 
6 volts is very high, at least when compared to normal NEVs. But, you will be hard pressed to find anything official as to what is acceptable vs intolerable.
 
Sahib said:
From code point of view 5% voltage drop for efficient operation. So for example for 120V supply
N-G voltage drop maximum 3V for equal size conductors. When electronic product specs require lower N-G voltage drop that should be met.
Click to expand...
Ummm... we're talking about stray current here. Not voltage drop.
 
mbrooke said:
NEV is the byproduct of voltage drop. A 2 volt drop across a service neutral will cause 2 volts to show on the EGC system relative to remote earth.
Click to expand...
Uh-oh. Looks like we're gonna get technical here. :D

There would not be an electrical industry if we did not have voltage drop everywhere. What is of concern here is the particular branding we're attaching to this one.

The percent drop recommended by the NEC is only pertinent to the local system under its purview... not the substation miles away and the distribution system in between.
 
Without knowing anything about the sub
mva
kv
assume it's wye
hard to say but sounds normal

leakage, cap coupling, etc could easily be ~5 A
if the station bed is 2 Ohm there's 10 V

on our subs
10+ mva
12760/7200 wye
ngr 25 A 188 Ohm
we set protection at 10 A and see up to 2 A on long runs quiescent

what I find odd is that casing to earth on the isolated sec of the service xfmr is the same as the sub sec?

was power to the pump disconnected when measured?
 
Smart $ said:
Uh-oh. Looks like we're gonna get technical here. :D

There would not be an electrical industry if we did not have voltage drop everywhere. What is of concern here is the particular branding we're attaching to this one.

The percent drop recommended by the NEC is only pertinent to the local system under its purview... not the substation miles away and the distribution system in between.
Click to expand...


I fully agree, however, it does hold some water. Consider an older detached building, the NEC allowed for 3 wire feeds. High voltage drop on such would produce NEV. Also, it is possible to run an MGN feeder to a remote building where the drop across the noddle would present the same concern.
 
mbrooke said:
I fully agree, however, it does hold some water. Consider an older detached building, the NEC allowed for 3 wire feeds. High voltage drop on such would produce NEV. Also, it is possible to run an MGN feeder to a remote building where the drop across the noddle would present the same concern.
Click to expand...
You are correct. But I believe the voltage drop would have to be significantly higher than the NEC recommended maximum before becoming alarmed, don't you think?

OP is talking 10 volts and it is not coming from the henhouse on the back of the lot. Suffice it to say the neighborhood has gone to pots. (pun definitely intended :D)
 
In a premise, high NEV may be reduced by suitable grounding/bonding I.e. it is a grounding/bonding problem.
 
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