Panel ground showing voltage?

[jimbob843]

Ok so recently I have started a job doing some hurricane repairs on a dock and the first step was to repair fallen pvc conduits under this dock. These conduits contain the 480v feeders to transformers on each branch dock of the marina. Soooo in this process I discovered the panels earth ground (to ground rod) had been separated from the rod. I was curious and wanted to verify this as a ground or at least a deenergized wire. Stuck my meter on it and it is reading 106V between it and the rod. What would cause this to happen? Would this cause damage if it were attached back to the rod? Most of this is going to get replaced completely but I’m just curious as to what would cause it to read voltage. Sorry if it’s a newb question

 

[junkhound]

"Stray' Capacitance and high Z meter.

Have the journeymen you are apprenticing with been unable to answer that common question?

Good to ask them if you have not already, they could demonstrate the effect first hand to you and write out the appropriate equation of capacitive reactance and meter impedance . AKA 'phantom' voltage.

 

[gar]

180421-0759 EDT

jimbob843:

I assume it is salt water. Is it?

Whether salt or fresh water find an earth shore point put a short rod, possibly just a large screwdriver, in this earth point. Run any size insulated wire, #20 to #12, extension cords will work, from the earth reference point to the ground rod location at your panel. Initially while running out the wire use insulated gloves, and do not connect at either end.

Connect a high impedance voltmeter between the water located ground rod and your long test lead. Then connect that long test lead to the earthing test rod. What is the voltage?

While everything is still connected place a 15 to 100 W incandescent bulb across the high impedance voltmeter input. What is the voltage, and does the bulb glow at all? If the bulb glows, or voltage is greater than 1 V, then also measure the current in the long test lead.

Do the same test to your dock panel instead of the ground rod at the panel.

In part these tests will give some indication if there are substantial currents in the earth.

Report back with your results.

Be very careful to make sure that no current can flow thru you. You need to be insulated.

.

 

[ptonsparky]

180421-0759 EDT

jimbob843:

I assume it is salt water. Is it?

Whether salt or fresh water find an earth shore point put a short rod, possibly just a large screwdriver, in this earth point. Run any size insulated wire, #20 to #12, extension cords will work, from the earth reference point to the ground rod location at your panel. Initially while running out the wire use insulated gloves, and do not connect at either end.

Connect a high impedance voltmeter between the water located ground rod and your long test lead. Then connect that long test lead to the earthing test rod. What is the voltage?

While everything is still connected place a 15 to 100 W incandescent bulb across the high impedance voltmeter input. What is the voltage, and does the bulb glow at all? If the bulb glows, or voltage is greater than 1 V, then also measure the current in the long test lead.

Do the same test to your dock panel instead of the ground rod at the panel.

In part these tests will give some indication if there are substantial currents in the earth.

Report back with your results.

Be very careful to make sure that no current can flow thru you. You need to be insulated.

.

The most difficult part of this experiment is getting it done safely under the guise of required work and getting paid for it.

Maybe you can talk your journeyman into helping you at lunch.

 

[junkhound]

BTW Jimbob:

Welcome to the forum.
There are NO insignificant questions for someone starting out, we all had to learn the very basics at some time in our life.

Do not be afraid to ask seemingly 'dumb' questions, there are no 'dumb' questions, only the best way to gain knowledge.

 

[gar]

180421-1357 EDT

My power company pole transformer is at one corner of my yard, and has a ground rod driven adjacent to the pole. Pole is wood.

Inserted a screwdriver adjacent to the ground rod. It may or may not touch the rod. Then measured the voltage with a Fluke 27, high impedance, to various points in the yard. One reading was 250 millivolts, but later on rereading it was 150 millivolts. Most readings were around 150 millivolts.

I don't see a gradient thru the yard. Why? Don't know.

Touching the city water valve or points close to it my reading was 30 millivolts, or thereabouts. This valve is 150 ft or so from the pole transformer. There is a 1" copper line from said valve to my meter in the basement where the main panel is grounded to this water line. This 150 ft water line is my ground rod, and whatever city water lines actually electrically connect to it.

Now over a considerable time period I made more measurements.

A redo of the water value location the reading was 10 to 30 mV relative to transformer rod. A redo of 250 mV location and it jumped around from 50 t0 250 mV settling to 150 mV at the end of the measuring time.

Pole rod to main panel neutral voltage was 30 mV. This will depend upon neutral current.

I think we have very little local ground current causing our earth voltages vs some more remote deeper earth currents. Also I do not have a narrowband 60 Hz filter to get rid of other noise sources.

Other locations than mine can have much more of the voltage from local currents, and there can be higher voltages.

I don't believe I had much induced voltage error from my test leads.

My power company primary line is a 3 phase delta with no neutral anywhere near us. A neutral probably exists 3/4 of a mile away at the substation.

.