high voltage on the neutral

[magoo2]

For 4-wire multigrounded distribution circuits, what types of faults on a distribution feeder can produce high voltages on the neutral?

By high, I mean something more than 150 V. I'm just looking at what faults can possibly result in high voltages at a residence. I know open neutrals can exceed this, but I'm focusing on just primary faults.

 

[waross]

I know of an instance where too many multiple protective earth connections were missing or in very bad condition. A primary distribution conductor came off the insulator and lay on the cross arm. Current then tracked from the line conductor to the neutral conductor. The elevated voltage on the neutral, in the absence of proper grounding broke down the insulation of some building wiring and started at least two fires in nearby buildings.
The circuit was eventually cleared at the substation. I don't have any documentation. The investigating engineer was a personal friend of mine and we discussed his findings at length.

Bill
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"Why not the best?"
Jimmy Carter

 

[magoo2]

In that instance, suppose we have 4 grounds per mile per the NESC. If each ground is 50 or even 25 ohms, with phase to neutral contact in the first mile or two, I don't see those grounds carry much current so the situation resembles a phase to neutral contact without any grounds closer to the source. To my way of thinking, this gives about half primary voltage on the neutral if they are the same size conductor,and a little bit more if the neutral is a smaller size.

 

[waross]

With a common neutral conductor serving but primary and secondaries, that may put more than enough voltage on customer's neutrals to cause insulation breakdown.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[magoo2]

Bill,

In rereading what I posted, I mispoke about the half voltage. With a phase to neutral fault, a sizeable part of the drop will be through the substation transformer and the source impedance behind it. What I meant to say is the remainder of the voltage drop will divide about equally if the phase and neutral wires are the same.

 

[Mbrooke]

Open neutral on a lateral or on an imbalanced 3 phase line with no/few customer water bonds and high soil resistance. Seen it before were the neutral splice broke on a single phase tap and all the customers were on well water. The voltage between the broken neutral and the intact one was estimated somewhere at 9kv, where the phase to ground voltage was 13.2. Soil around the ground rods got crispy too.