stray voltage

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090706-2152 EST

electricalperson:

In your post #14 you ask about measuring the neutral drop from the transformer center tap to the main panel neutral bus. Measuring from the ground rod at the transformer to the main panel neutral bus will give you this information. This voltage drop includes the neutral wire from the transformer and any parallel paths. If you measure the current on said neutral wire, then you can calculate the resistance of said neutral wire. Then from knowledge of the wire length, material, and size calculate the resistance and see if there is reasonable correlation.

If the transformer is too far away, then consider the method I have discussed in
http://forums.mikeholt.com/showthread.php?t=113160&highlight=neutral+voltage+drop+source+impedance
.

A Simpson 260 does not measure AC voltage below the 2.5 V range. Also note the 260 in the AC position will produce a reading from a DC voltage. To strip the DC component you need to use the output terminal which includes a series capacitor.

If you use 50 microamp terminals to measure DC voltage, then I believe the newer 260s have a voltage drop of 250 MV full scale.

For measuring voltages from stray currents in the earth I suggest that you use a high impedance meter. Fluke 27, 87, etc.

The input impedance of the 260 in DC is 20,000 ohms per volt. On newer 260s in AC it is 5000 ohms per volt, and older ones 1000 ohms per volt. On DC and the 2.5 V range the input impedance (resistance) = 2.5*20,000 = 50,000 ohms. On the DC 1000 V range it is 2 megohms. You can do the calculation for the AC ranges.

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gar said:
090706-2152 EST

electricalperson:

In your post #14 you ask about measuring the neutral drop from the transformer center tap to the main panel neutral bus. Measuring from the ground rod at the transformer to the main panel neutral bus will give you this information. This voltage drop includes the neutral wire from the transformer and any parallel paths. If you measure the current on said neutral wire, then you can calculate the resistance of said neutral wire. Then from knowledge of the wire length, material, and size calculate the resistance and see if there is reasonable correlation.
...
Click to expand...
In many cases the source if the "stray" voltage is the primary neutral votlage drop, and that will not show up in this test.
 
090707-1017 EST

don:

I am not sure what your comment meant.

However, I believe post #14 was not a stray voltage question, but one about a bad neutral path. Or how to determine if the neutral is good.


electricalperson:

To run experiments on neutral voltage drop you need to load one side of center tap. Further, it is best if you turn off all load in the house. Now the current thru the neutral should br near zero.

Some experiments this morning on my neutral.

The pole transformer and its pole are about 36 ft from my meter. There is a ground rod at the pole. I removed some dirt to get direct access to the GEC.

My service runs down the pole into the ground about 3 ft, goes across the yard, and comes up at the meter. Rigid conduit at both ends, but not across the yard. From the meter there is another rigid conduit down and into the main panel in the basement. The house grounding electrode is a very long copper water pipe. The resistance thru the earth path is very high compared to the pole to house neutral wire.

I used a Fluke 27 for measurements on the AC millivolt range. A long clip lead was connected to the pole GEC and the other clip lead to the conduit going to the main panel.

With whatever house load was present the voltage difference was 53 to 55 MV. Added 12 A resistive to one phase and this increased the voltage to 145, and to the other phase 74. The base house load was of an unknown phase and waveform. Averaging the two 12 A loads and the result is about 109 MV. Calculated resistance is Rneutral = 0.109/12 = 0.009 ohms.

Next I removed most of the house load so that total consumption was 170 W. This was mostly CFLs, clocks, and some small transformers. Now the voltage difference was 10 to 15 MV. Added 12 A to one phase and the voltage was 104 MV, changed to the other phase and it was 108 MV. The average is 106 MV. Note the close correlation of this with the average from above.

If I were to put an unbalanced load of 200 A on this supply the approximate drop would be 1.8 V .

Next I made measurements of the earth potential relative to the pole GEC in a straight line from the pole to meter. This was with the house base load plus 12 A on the phase that produced the largest voltage.

The results were
Feet Voltage Percent Distance
36 0.162 100
33 0.132 92
30 0.128 83
18 0.102 50
12 0.093 33
06 0.084 17
03 0.073 8
02 0.066 6
01 0.040 3

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if the neutral was completly dissolved underground would you get shocked if you touched the metal case of the meter socket?

if there was no paralell paths for neutral current do you guys think this stray voltage will rise to full line voltage? or would it just rise to phase voltage?

im sure i could measure voltage drop across the neutral using gars method i could quickly determine if that neutral is gone. just wondering if ill get a high voltage if i measure neutral to dirt
 
090707-2227 EST

electricalperson:

If there is no neutral wire, it is totally burned out, no grounding to a copper water pipe system, the main panel connects to an 8 ft vertical ground rod, and a similar rod is at the transformer, then only loading one phase with for example 1 ohm and assuming the path resistance from one ground rod to the other is 50 ohms the voltage of the main panel relative to the transformer center tap is about 50*120/51 = 118 V.

.
 
gar said:
090707-2227 EST

electricalperson:

If there is no neutral wire, it is totally burned out, no grounding to a copper water pipe system, the main panel connects to an 8 ft vertical ground rod, and a similar rod is at the transformer, then only loading one phase with for example 1 ohm and assuming the path resistance from one ground rod to the other is 50 ohms the voltage of the main panel relative to the transformer center tap is about 50*120/51 = 118 V.

.
Click to expand...

where is the 51 from?
 
don_resqcapt19 said:
In many cases the source if the "stray" voltage is the primary neutral votlage drop, and that will not show up in this test.
Click to expand...

There is much confusion and misapplication of the phrase "stray voltage/stray current".

As Don mentioned, and I want to emphasize, stray voltage is from the grounded/neutral conductor.

What some confuse this with is ground fault conditions.

It is generally easier to locate ground fault issues than it is to locate stray voltage issue.
 
Pierre C Belarge said:
There is much confusion and misapplication of the phrase "stray voltage/stray current".

As Don mentioned, and I want to emphasize, stray voltage is from the grounded/neutral conductor.

What some confuse this with is ground fault conditions.

It is generally easier to locate ground fault issues than it is to locate stray voltage issue.
Click to expand...

I don't ever remember reading anything that has made this point. Could be I just can't add. Not saying you aren't correct and it may help in discussions here, but not much when talking to customers.

Customer: "I have stray voltage"
Me: "No, you have ground faults, not stray voltage."
Customer: "But I am getting shocked when I milk the cows!"
Me: "Voltage schmoltage!! It's a ground fault!"
 
ptonsparky said:
I don't ever remember reading anything that has made this point. Could be I just can't add. Not saying you aren't correct and it may help in discussions here, but not much when talking to customers.

Customer: "I have stray voltage"
Me: "No, you have ground faults, not stray voltage."
Customer: "But I am getting shocked when I milk the cows!"
Me: "Voltage schmoltage!! It's a ground fault!"
Click to expand...


I am not stating that stray voltage does not exist, I am just stating that some confuse stray voltage with ground fault conditions.
 
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