Ground Voltage

[laketime]

Went to a house yesterday (built in '45) and they have voltage on their ground. A plug tester shows voltage to ground, a meter shows 3.7v from neutral to ground. The house is a mess electrically (new main panel though), wired with a mix of everything. I am thinking loose neutral somewhere. What are your opinions?

 

[Pierre C Belarge]

First I would look for grounded conductor TO equipment ground conductor bonding on the secondary side of the service. I would eliminate them and then see your results. Depending on the factors, there will be some voltage present.

 

[laketime]

what is "TO" equipment ground conductor? Another thing I found was the ground rod was connected to the meter section but I did not see an earth ground brought to the panel. The neutral/ground bus in the main panel was tied to cold water only.

 

[Pierre C Belarge]

Take a look at 250.24(A)(1), this describes where the GEC termination(s) are permitted.

 

[Pierre C Belarge]

what is "TO" equipment ground conductor?

Some call it "neutral to ground bond"; which is generally not permitted on the secondary side of the service disconnecting means.

 

[laketime]

So I read that to say that the GEC can be bonded in the meter, but needs to be pulled with the conductors to the panel. Is that correct?

 

[jaylectricity]

This is a house?

The GEC goes to the meter. The meter is bonded to the panel through the grounded conductor. The grounded conductor is bonded within the panel to the metal enclosure and the ground bars inside the panel. That is assuming the main breaker is in the panel.

If the main breaker is in the meter, then you would have four wires between the meter and circuit panel. Two ungrounded conductors (the hots) one grounded conductor (the neutral) and a ground wire that bonds the the meter socket with the metal enclosure of the panel and the ground bar located within the panel. In that case, the neutrals and the grounds in the panel should be separate.

Sometimes old houses like that don't even have two hots, so adjust accordingly.

 

[gar]

090721-1552 EST

laketime:

I believe you have a metallic water line coming into the house. Make a secure clamp connection to this pipe close to where it enters the house and before the water meter. Connect a long wire to said clamp connection. The EGC of an extension cord is a convenient way to get this wire. This wire is to make a long test lead so you can measure voltage relative to this ground reference anywhere in the house or yard.

Use a high impedance sensitive digital voltmeter for your test meter. A Fluke 27 or 87 is good.

All measurements are referenced to this one said point on the water line.

There should be less than 25 MV as a tentative criteria between the reference point and any water line or fixture in the house. Same for any EGC terminal in any outlet.

My following measurements are with a Fluke 27 and no bandwidth limiting.

I measure 0.9 to 1.4 MV from one end of my house to the other on the internal water line. Make sure you break thru any oxide layer of the pipes.

With my workbench powered off the voltage to the EGC pin is about 17 MV, but to the neutral wire 2 MV. The higher voltage on the EGC is because of other equipment that is powered and indirectly connects to the said workbench EGC. This probably results from noise filters at equipment inputs (computers).

In your case the first measurement should be across the water meter. I get 0.4 MV on mine.

Next measure to a screw on the main enclosure. I get 1.3 MV .

At my workbench with the bench on, means some computer equipment is on, the EGC voltage is 21 MV, the neutral is 1.9 V while the laser printer is warming up, and drops to 0.25 V with the printer on and heater off.

By making measurements at various points with loads on and off you can judge whether you have a problem and if so, then where.

.

 

[LarryFine]

So I read that to say that the GEC can be bonded in the meter, but needs to be pulled with the conductors to the panel. Is that correct?

Negatory. Everything upstream, from the enclosure that contains the main disconnect back toward the utility, is bonded to the service neutral.

The meter is looked at as a lump in the service cable. If there are only three conductors entering it, there only need be three conductors leaving it.

Everything downstream, from the enclosure that contains the main disconnect on, must have separate equipment grounding and neutral conductors.

 

[laketime]

OK, got that the grounded conductor is bonded to the gec at the meter than in the panel with a main breaker the grounded conductor is bonded to the can and the grounding conductors. do you also bond the grounded conductor to cold water at the panel? that is how it is at this house, and funny thing is it is bonded to copper but the stub out of the house is PVC.

 

[gar]

090722-1242 EST

laketime:

That the incoming water supply is PVC means that said water supply is not providing any grounding to the earth. There needs to be a ground rod or an equivalent.

Hopefully all water connections within the house are via metallic pipe, copper or galvanized.

Read the NEC for details on how to properly connect EGC, GEC, ground rod, gas pipes, and exposed metal conductive surfaces.

For example a refrigerator cabinet, it is conductive, must connect to an EGC and that to the neutral at the main panel.

.

 

[laketime]

thanks for the advice. What would cause a ground to be carrying voltage like that?

 

[don_resqcapt19]

thanks for the advice. What would cause a ground to be carrying voltage like that?

Assuming that you have a main bonding jumper and there is an EGC run with the circuit conductors, the only thing that will cause a voltage between the neutral and grounding conductor is a voltage drop on the neutral conductor.

 

[gar]

090722-1841 EST

If you have a resistance of some non-zero value, and a current flows thru that resistance, then there will be a voltage drop across that resistance.

Typical wires have resistance. Consider two wires connected to nothing else other than as described next. One end of each wire is connected to the negative terminal of a 1.5 V D cell. Call one wire neutral, and the other wire EGC. Connect the second end of the neutral wire to the positive terminal of the D cell. A current will flow thru the neutral and the voltage drop between the two ends of the neutral will be 1.5 V.

There is no current flowing thru the EGC wire. Measure the voltage from any point on the EGC relative to the D cell negative terminal and the voltage reading is 0 V.

Next measure the voltage of any point on the EGC to the positive terminal end of the neutral and the voltage will be 1.5 V.

Last pick any point on the EGC for one voltmeter connection and now gradually move the other meter lead along the neutral wire. The voltage will increase in a linear fashion from the negative end of the neutral to the positive end.

.

 

[LarryFine]

do you also bond the grounded conductor to cold water at the panel? that is how it is at this house, and funny thing is it is bonded to copper but the stub out of the house is PVC.

The water ground should be landed in whichever enclosure contains the main disco.

The water pipe is bonded to the service whether it qualifies an an electrode or not.