Problem creating a real NEV scenario

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George Stolz

George Stolz

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Hospital Master Electrician
I'm trying to accurately recreate a Neutral-Earth-Voltage scenario, using Ohm's Law to demonstrate.

I have a problem. I can't realistically expect to find 62V between an EGC and remote earth, right? I thought NEV was usually 2v to 8v.

ebonding.jpg
 
George Stolz said:
I'm trying to accurately recreate a Neutral-Earth-Voltage scenario, using Ohm's Law to demonstrate.

I have a problem. I can't realistically expect to find 62V between an EGC and remote earth, right? I thought NEV was usually 2v to 8v.
Click to expand...

I have not run your numbers, but for the case of an open primary neutral, I would not discount the possibility of 62 volts. The effect is smaller when you are just looking at voltage drop in the primary neutral rather than an open neutral, but the voltage buildup is insidious.
Are you using the parallel resistance of the ground and the pond as well as the other houses to calculate the resistance to ground and the amount of primary current flowing through that combined resistance? Are you looking at the primary current from a fault or during normal operation?
 
I am trying to demonstrate the voltage between a swimming pool and remote earth under normal operating conditions.

I threw in neighboring houses' electrodes that would be physically connected via the primary neutral, which is the 1Ω towards the left, in the field of power poles. It helped, but didn't bring the numbers down to reality. :happyno:
 
George Stolz said:
I'm trying to accurately recreate a Neutral-Earth-Voltage scenario, using Ohm's Law to demonstrate.

I have a problem. I can't realistically expect to find 62V between an EGC and remote earth, right? I thought NEV was usually 2v to 8v.

View attachment 8322
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Most definitely. I am assuming that is a pool you are getting the 62 volts from.
Two years ago we had a situation much like the one you describe. Underground subdivision, CN cable. Trouble call on a bright Saturday afternoon. Kids were getting shocked getting out of the pool. The primary neutral was corroded to the point of being almost non existent. Had to string a neutral out on the ground to the next PMT. We read 41 volts at the pool. It made for miserable swimming...

PS. There is no primary wire in the drawing. :p
 
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Right, but I'm trying to work the problem with an intact neutral between the house and the utility. There will still be a voltage present, just not anything near 40V under normal operating conditions.
 
So you have zero Ohms from the pool bonding to the XF neutral?

I may be missing something here. The 346 volts is coming from where? Shouldn't it be a heat loss in the line? I2R
 
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I'm thinking it's one or several of several potential errors:

1. 20A over 10 miles is unrealistic, creating an insane amount of voltage drop.
2. The MGN resistance contributions (neighboring houses) between the substation and our house are underestimated.
3. I've got some basic concept missing that would fix the difference.

Edit to add:

4. 13.8 kV not being common for this application?
 
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George Stolz said:
I'm thinking it's one or several of several potential errors:

1. 20A over 10 miles is unrealistic, creating an insane amount of voltage drop.
2. The MGN resistance contributions (neighboring houses) between the substation and our house are underestimated.
3. I've got some basic concept missing that would fix the difference.

Edit to add:

4. 13.8 kV not being common for this application?
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1) The 20 amps would be spread out over 10 miles and is low. Realistically it could be upwards of 100 amps. That is still only 1400 kW. Also there would probably be a regulator somewhere on the line.

2)The Zero at the house is probably too low, so are the ground resistances. I realize 25 ohms is recommended, but 25 ohms today may be 90 ohms next week.

The 346 volts isn't returning. It is a power loss (I2R)
 
Hv&Lv said:
1) The 20 amps would be spread out over 10 miles and is low. Realistically it could be upwards of 100 amps. That is still only 1400 kW. Also there would probably be a regulator somewhere on the line.
Click to expand...

Increasing the amperage would drive the theoretical NEV up, not down.

2)The Zero at the house is probably too low, so are the ground resistances. I realize 25 ohms is recommended, but 25 ohms today may be 90 ohms next week.
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I'd beg to differ on that. A Ufer should hang in at around 5Ω resistance to earth, and a pool should be close to the same value.

If the pool is connected to the service with the equivalent of 200' of #8 stranded (which would be reasonable, if not an excessive length) then the resistance would be .5102Ω. Close enough to zero to disregard.

The 346 volts isn't returning. It is a power loss (I2R)
Click to expand...

Okay, agreed. I burned 6920W getting to the house, and 6920W from the house back to the substation. So where does my NEV number come from then?
 
George Stolz said:
I am trying to demonstrate the voltage between a swimming pool and remote earth under normal operating conditions.

I threw in neighboring houses' electrodes that would be physically connected via the primary neutral, which is the 1Ω towards the left, in the field of power poles. It helped, but didn't bring the numbers down to reality. :happyno:
Click to expand...

Possibly your primary current, and therefore wired neutral voltage drop, is too high? Or the primary resistance value is too high. Will take another look.
 
GoldDigger said:
Possibly your primary current, and therefore wired neutral voltage drop, is too high? Or the primary resistance value is too high. Will take another look.
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Not sure if it will help or not, but you cannot assume that the voltage drop across the 10 mile line is acting equally at all of the intermediate grounds that are contributing to your estimated 1 ohm to ground.
Have you looked at the similar calculation example in Mike Holt's video on stray currents to see what he may be doing differently?
How did you choose the wire size for the primary?
 
The problem appears to be the drop you are assuming is in the feeder from the station. The current does not flow on the neutral only and the circuit impedance is not just the conductor.

I modeled a quick substation and added a 10 mile feeder on a 25 kV 3-phase line with #336 ACSR phase conductors and a #4/0 ACSR neutral. I then dropped a single-phase resistive load from phase A to neutral at the end (~ 20 A). I used 100 ohm-meter soil, 0.05 mile spans, and 25 ohm grounds at each pole (but no additional secondary grounds). I put no ground at the load and got:

0.0 miles out (at substation):
Ia = 19.79A<151.44?
Vn = 2.211V<157.01?
In = 9.79A<-25.44?
Ie = 10.13A<-36.70?

2.5 miles out:
Ia = 19.80A<150.77?
Vn = 174.5mV<125.50?
In = 8.293A<-20.80?
Ie = 11.77A<-38.48?

5.0 miles out:
Ia = 19.82A<150.09?
Vn = 111.7mV<58.62?
In = 8.276A<-19.63?
Ie = 11.85A<-39.26?

7.5 miles out:
Ia = 19.84A<149.43?
Vn = 1.714V<-97.12?
In = 8.164A<-27.55?
Ie = 11.72A<-33.78?

10.0 miles out (at load):
Ia = 19.87A<148.76?
Vn = 16.6V<-3.97?
In = 19.87A<-31.24?
Ie = 2.398pA<96.98?

Load:
Ia = 19.87A<-31.24?
In = 19.87A<148.76?



I added a 5 ohm ground at the load and got:

0.0 miles out (at substation):
Ia = 19.79A<151.44?
Vn = 2.211V<157.02?
In = 9.79A<-25.43?
Ie = 10.13A<-36.70?

2.5 miles out:
Ia = 19.81A<150.77?
Vn = 170.6mV<125.99?
In = 8.294A<-20.82?
Ie = 11.77A<-38.46?

5.0 miles out:
Ia = 19.82A<150.10?
Vn = 143.5mV<48.09?
In = 8.305A<-19.62?
Ie = 11.83A<-39.30?

7.5 miles out:
Ia = 19.85A<149.43?
Vn = 1.381V<-104.60?
In = 8.111A<-25.67?
Ie = 11.81A<-35.02?

10.0 miles out (at load):
Ia = 19.87A<148.77?
Vn = 13.34V<-11.65?
In = 17.38A<-34.18?
Ie = 2.668A<-11.65?

Load:
Ia = 19.87A<-31.23?
In = 19.87A<148.77?

5 ohm ground:
Vn = 13.34V<-11.65?
In = 2.668A<-11.65?
Ve = 0
Ie = 2.668<168.35?



I then changed the spans to 0.125 miles (less grounds/mile) and got:

0.0 miles out (at substation):
Ia = 19.78A<151.44?
Vn = 2.336V<155.77?
In = 9.279A<-23.35?
Ie = 10.70A<-37.93?

2.5 miles out:
Ia = 19.80A<150.77?
Vn = 748.9mV<145.77?
In = 8.278A<-21.44?
Ie = 11.75A<-38.02?

5.0 miles out:
Ia = 19.82A<150.10?
Vn = 694.3mV<-120.78?
In = 7.965A<-22.28?
Ie = 12.04A<-37.09?

7.5 miles out:
Ia = 19.84A<149.43?
Vn = 4.666V<-72.22?
In = 8.784A<-32.93?
Ie = 11.08A<-29.86?

10.0 miles out (at load):
Ia = 19.86A<148.77?
Vn = 19.05V<-15.08?
In = 16.24A<-34.98?
Ie = 3.809A<-15.08?

Load:
Ia = 19.86A<-31.23?
In = 19.86A<148.77?

5 ohm ground:
Vn = 19.05V<-15.08?
In = 3.809A<-15.08?
Ve = 0
Ie = 3.809<164.92?



I then removed the ground at the load and got:

0.0 miles out (at substation):
Ia = 19.78A<151.43?
Vn = 2.340V<155.70?
In = 9.259A<-23.27?
Ie = 10.72A<-38.00?

2.5 miles out:
Ia = 19.80A<150.76?
Vn = 813.0mV<153.79?
In = 8.224A<-21.52?
Ie = 11.80A<-37.92?

5.0 miles out:
Ia = 19.81A<150.09?
Vn = 1.174V<-110.45?
In = 7.894A<-23.82?
Ie = 12.05A<-36.05?

7.5 miles out:
Ia = 19.84A<149.42?
Vn = 6.516V<-61.65?
In = 9.500A<-36.53?
Ie = 10.43A<-26.40?

10.0 miles out (at load):
Ia = 19.86A<148.75?
Vn = 26.66V<-4.08?
In = 19.86A<-31.25?
Ie = 3.038pA<33.75?

Load:
Ia = 19.86A<-31.25?
In = 19.86A<148.75?


So you can see the NEV depends on where you are located on the feeder. At the end of the feeder, adding the ground dropped the NEV (works up to a point). Less pole grounds can increase the NEV at the end. It also matters where the loads are at in the system.

With many parallel grounds on the system, and with the loads reasonably balanced, the 2V to 8V you wondered about is reasonable. If it is way higher than that, there might be a problem on the system.
 
George Stolz said:
Increasing the amperage would drive the theoretical NEV up, not down.
Click to expand...
I was suggesting real life amperages, regardless what it does to the numbers.



George Stolz said:
I'd beg to differ on that. A Ufer should hang in at around 5Ω resistance to earth, and a pool should be close to the same value.
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I was referring to the Ohm values used on the poles for the 25 ohms, and around here ufers aren't really used much. The two rods are used, and the values could be 25 one wet day, and 90 ohms in the summer.
 
George Stolz said:
I'm not worthy! :D

Thanks Mivey. :cool:
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You are certainly welcome. I did run another scenario similar to your graphic and get around 18-20 volts for an end-of-the-feeder scenario of 20 amps of unbalanced feeder current. While this is probably a fringe case, it is important to remember what we are doing at the pool. We are trying to create an equipotential area.

I used the 20-amp scenario and modeled a slab with rebar with a ground ring 5 foot outside. While the pool area voltage to remote earth was 18-20 volts, the voltage drop across the pool area was only a few volts so stayed pretty close to the service neutral voltage.

Think about what would happen if we brought an insulated conductor from remote earth into the pool area. Things might get ugly. You could imagine things like extension cords from a neighboring facility, etc.

There are also transient conditions that could cause significant NEV. Suppose you have a long extension cord bringing a NEV with you as you work at a remote earth location? The drill housing is at the elevated NEV voltage but when you put your knee in the puddle of water, you become a conductor between the elevated neutral and remote earth. Might not be an issue when the NEV is less than 30 volts but during a system fault the NEV might reach a level that can easily overcome body and contact resistance. Something to think about if you are working near a high fault current area.
 
Thinking about transporting a NEV got me to wondering. Take an end-of-feeder case with 20 amps of primary unbalance. Assume a body resistance of 1,000 ohms and zero contact resistance.

The service where you are working has a NEV of 15.18 volts. Someone making contact between the neutral and remote earth will experience about 15 mA. The pool at the worksite has an earth voltage varying from 12.9 to 14.7 volts over the grounded area and drops to about 9 volts within a few feet of the grounded area. The earth voltage at the house ground is about 7.9 volts. The earth voltage at the service pole ground is about 7.4 volts.

The NEV at a nearby service pole is 15.23 volts. If you get between the two service neutrals (like an extension cord scenario) you will experience about 0.3 mA (negligible extension cord resistance). The earth voltage at this service pole ground is about 6.8 volts.

Earth voltage about 50 feet away from everything is about 2-3 volts.


Now let there be a 600 amp primary line-ground fault at the working location's service pole.

The service where you are working now has a NEV of 359 volts (the primary was dragged down from 7.5 kV to 0.36 kV). Someone making contact between the neutral and remote earth will experience about 360 mA. The pool at the worksite has an earth voltage varying from 305 to 350 volts over the grounded area and drops to about 230 volts within a few feet of the grounded area. The earth voltage at the house ground is about 180 volts. The earth voltage at the service pole ground is about 175 volts.

The NEV at a nearby service pole is 155 volts. If you get between the two service neutrals (like an extension cord scenario) you will experience about 15.1 mA. The earth voltage at this service pole ground is about 349 volts.

Earth voltage about 50 feet away from everything is about 60-80 volts. Earth voltage about 50 feet away from everything is about 30-50 volts.


The same setup about 3 miles from the substation instead of 10 miles from the station yields an extension cord scenario with 42 mA between the service neutrals. 1 Mile away gets you 100 mA.
 
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mivey said:
Earth voltage about 50 feet away from everything is about 60-80 volts. Earth voltage about 50 feet away from everything is about 30-50 volts.
Click to expand...
Should have read:
Earth voltage about 50 feet away from everything is about 60-80 volts. Earth voltage about 100 feet away from everything is about 30-50 volts.
 
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