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Mr. Wizard:
Thinking more about your 240 V reading.
I believe you probably have more problems than just a single break in the overhead neutral.
1. Establish a ground (earth) voltage reference point. I would like this to be the copper water supply pipe going into the ground and on the ground side of the water meter. If plastic pipe is used for the supply, then outside drive a 10 or 12 inch screwdriver in the ground near the service entrance. To which ever one of these you use for a reference connect a long test lead to one side of your meter. I would suggest the EGC wire in an extension cord as a good test lead.
2. Use a high input impedance, 10 megohms, digital voltmeter, like a Fluke 27 for the following measurements.
3. The 240 V reading implies something wrong in the ground or neutral path from the transformer center tap all the way into the main panel including all required bonding and grounding. Or a very high resistance ground path from the pole center tap to the house ground bus.
4. We know that the overhead neutral is open because you can see this. There should be a parallel path from the transformer center tap to a ground rod at the pole thru the earth to a ground and/or water pipe or other grounding electrode at the house near the service entrance.
This path thru the earth, if it exists, might not be a very low resistance. In my yard this whole path is probably less than 10 to 20 ohms. My water line to ground is probably less than 1 ohm, but the pole ground rod is likely more in the range of 10 to 20 ohms. In other locations this total ground path resistance might be 100 ohms or higher. Suppose it was 100 ohms, then 1.2 A would produce a drop of 120 V. Equivalent to a 150 W bulb.
5. If you have to create a ground reference in the yard, then use insulated boots and use only one hand or insulated gloves. There may be some large potentials somewhere. What do I mean by one hand? Do not do anything that would allow current to flow thru your body.
6. The first procedure is to determine if all the proper bonding of neutral, EGC, ground bus, and grounding electrode exists.
7. If you have copper or iron water pipe inside going to the earth, then measure the voltage from the entry water pipe to the main panel enclosure. You must make good contact to the water pipe and the main enclosure with the test leads or you may read capacitively coupled voltage. Sanding may be necessary to break thru to base metal. In my house this morning it is 2.1 millivolts. I expect you should read a low voltage, but it might be as high 240 V for some unlikely reason.
If this voltage is above 1 V, then there is a very poor connection between the entry water pipe and the main panel.
8. Assuming the voltage of (5) is low, then check the voltage to a neutral in an outlet. If there is no load on the circuit to the outlet, then this voltage should not differ from the measurement to the main enclosure by very much. I suggest that a fraction of a volt would be high. Maybe 0.1 to 0.2 V would be OK. It depends on how the neutral gets to the outlet.
If the voltage seems high, then open the main panel and check to the neutral bus.
9. If any of these voltages measurements are high, then you need to find out why.
If the neutral bus is not bonded to the ground bus in the main panel and in turn to the entry water pipe, then you could have a reason for your previously stated 240 V reading.
10. If you get low voltages between the earth, the ground bus, and the neutral bus, then from the earth reference point measure the voltage to each hot bus in the main panel. These each should be around 120 V. But your 240 V measurement implies that this won't be the case.
11. If you did not have the water pipe as a reference, then you have to use a probe in the ground near the service entrance for a ground reference point. Measurements made to this reference might be somewhat higher than to a water line in the ground.
12. Note: as you make these various measurements it may become clear where the problem is.
13. If relative to earth at the entry water pipe all the voltages are low to EGCs and neutral, then I believe that the transformer center tap is not grounded at the pole for you to get a 240 V reading from a hot to neutral. This could also be possible if the neutral that you used as a reference when you read 240 V does not have a low resistance path back to your earth reference point.
Suppose the pole center tap does connect to a pole ground rod and the pole to house resistance is 100 ohms, that there is no load on one phase, and the load resistance on the other phase is 10 ohms, then the voltage drop on the 10 ohm load is about 120*10/110 = 11 V and the drop from pole center tap to your house neutral is 120 - 11 = 109 V, and the voltage drop from your other phase to neutral is about 240 -11 = 229 V.
14. If copper or iron pipe is used throughout the house, then between the entry water pipe reference point and any conductive plumbing fixture the voltage should be very small, millivolts.
15. If there is a ground rod at the service entrance, then its voltage should be low compared to the ECG and neutral in the panel.
16. If you make voltage measurements in the yard between the house neutral and the pole ground rod and there is no load on one phase and a large load on the other phase (maybe 10 ohms --- a 1500 W electric heater is about 10 ohms), then you should see an increasing voltage as you approach the pole ground rod. The largest changes will be close to the house grounding electrode, and then near the pole ground rod. So across most of the yard the voltage may not change much.
Depending upon the resistance of the pole ground rod to earth in relation to the resistance of the house grounding electrode and the bulk resistivity of the earth will determine what the earth voltage will be at a mid point between the house and pole. Suppose the house to ground resistance is 20 ohms, the earth 10 ohms, and the pole to earth is 70 ohms, then assuming 120 V across this path the drop from the house neutral to mid point is 120*25/100 = 30 V. There are not these discrete steps in resistance, but the assumption makes an easy calculation.
Be aware that you will see a very rapid rise in voltage close to the pole ground rod.
17. Assume that the pole center tap is not connected to a pole ground rod, then you should not see much voltage in the yard relative to the house neutral.
If I remember correctly the maximum voltage in my yard is around 100 MV and that is between my yard and my neighbor, and he is on a different transformer. We have a primary 3 phase delta supply and no primary neutral. The substation secondary may be a Y, but from our perspective it is a delta source.
18. Any troubleshooting of this sort will have the path that you take change as you make measurements that provide new information as to what to check next. From a remote point without first hand information it is hard to conjecture what steps to take.
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