dropped Neutral question

[strawboss]

Last week I got a call that the water tank was shocking the horse's. My initial thought was the neutral and ground were bonded together at the water tank. They were not. In addition to the egc there was a bare wire ran under the slab, to what I don't know. The panel is a sub panel fed with 3 wire from another building. I figured the wire was either going to a ground rod or tied into the other water tank that is fed from the other building. Either way I don't need/want it so I determ it. Before I do I shoved some #12 bare in the mud and took a volt reading from the egc to ground. I read 14.7v after I determ it I read 30v not what I expected to see.
I string the #12 wire that has one end stuck in the mud inside for further measurements.
There is a metal water pipe coming into the building that I assume connects between the other buildings onsite since I don't want any other paths back to the other buildings I determ the wire from the water pipe. I plug in my shop vac to a recep and measure from the #12 in mud to the egc in the circuit, 57v. My thinking is there are parallel paths back to the source. The only other path I could think of is the #6 leaving the panel. I assume this goes to a ground rod but in case it was ran all the way back i determ it. This time I turn on a light and measure from the #12 in the mud to the egc in the circuit 120v.

After I quickly shut off all the power to the building I began tracing the neutral all the way back and found I dropped it at the weather head of the other building. After crimping the neutral back on I still have .5v between the neutral and egc? I also temporally re-termed the water pipe and mysterious bare wire at the water tank, this made no difference. I left it and decided .5v is acceptable. Would you agree?

I realize a 3 wire system is no longer acceptable, but it was for a long time. This is the way I understand it. It was okay to feed a separate building with 3 wire as long as there were no other connections to the main panel. You bond the neutral and ground again and all fault current then must go back on neutral. You still must drive a ground rod, so if you drop a neutral as I did, electricity is going to find a path back to the source and directly though the earth is the only one left. Is this correct?

Was the path I took to find the problem logical or were there cues I should have picked up on sooner? My background is not in troubleshooting but I enjoy and hope to become more efficient.

sorry for such a long post. feedback is welcome

 

[gar]

111221-2341 EST

strawboss:

I think you did a good job of troubleshooting. Was there a quicker way? Maybe. But it is not always easy a priori to know where to start. Your 0.5 V is probably pretty good and may not be easy to lower.

I will provide some more comments later.

See dictionary.com for "a priori" and vs "a posteriori".

.

 

[hurk27]

I too agree that your trouble shooting was good, but your understanding of the circuit can use a little help, with a 3-wire circuit, you must know that the unbalanced neutral current will cause a difference of potential between this neutral and earth dependent on the resistance of the neutral and the amount of current flowing on it, this voltage drop will exist on everything bonded to this neutral at the load end, this is why it is not a good method for places where there are animals or people, and can be a problem with swimming pools even when fed from the main service panel, the fact you found the lost neutral was great because if a high load or a fault happened while someone or animal was in contact with something bonded to this neutral there could have been a death, now after fixing the neutral you got a reading of 5 volts, now this could be the voltage drop of this feeder neutral or the voltage drop of the main service neutral or both, to go one more farther, it can be the voltage drop of even the main primary neutral out on the power poles, or a combination of all three, by measuring this voltage without a load at this panel will say it is from the next two thing ahead of it, then you could go to the main service and if you have this voltage from the service grounding to earth then turn off its main, if you have still a voltage then it is ahead of this point.

But you did good and at least found the main problem, I would think that the extra grounding wire you found might be part of an equal potential bonding or an concrete encased electrode that is embedded into the concrete pad, and is why it had such an effect on this voltage.

 

[ptonsparky]

Sounds like you did well...unless it took you a week.

 

[Rewire]

the mystery ground could be to a equipotential ground grid.

 

[480sparky]

I too agree that your trouble shooting was good, but your understanding of the circuit can use a little help, with a 3-wire circuit, you must know that the unbalanced neutral current will cause a difference of potential between this neutral and earth dependent on the resistance of the neutral and the amount of current flowing on it, ........

Maybe this will help.

 

[SG-1]

The voltage difference between the water in the trough & the ground where the horses must stand while drinking will determine if they will drink or not. Quadrupeds tend to be very sensitive to small amounts of voltage. Use a metal plate or some tin foil to get a good foot (hoof) print for the measurement. Measure back away from the trough several feet to approximate the distance to the back hoofs.

 

[ptonsparky]

There are a heck of a lot of tests you can do, with so many different ideas on what is proper it is mind boggleing. Simplest way is to watch the animals. Are they nervous when approaching the tank? Do they shy away? You are not trying to get milk as a dairy would but reproduction and general health can be affected. Your measured .5 volts will depend on your meters accuracy and what type of load it adds to the circuit while taking measurments. For cattle, if I remember correctly, we added a 500 ohm resister in parallel with the leads. You want to get rid of the "ghost voltage". Some of the newer meters have that ability built into them.

If the customer is happy and the horses aren't nervous (give them a couple days to settle down), send your bill for a job well done.