DC voltage & well casing.

[ptonsparky]

A board member went to a meeting...so here we go

Local village wants to check for DC voltage in reference to the damage to their well(s) metal casing. My Fluke 43B was a good meter close to 20 years ago but I don't think it was good enough even then.

Next what, where, would I attach the leads to?

How would I know that it is not the casing and water/soil contact producing the voltage vs any other equipment in the area? Is there a polarity that would tell me? Any suggestions on reading?

I appreciate that they come to me first for a lot of things but this might be a bit of a stretch.

 

[gar]

130212-1712 EST

Try putting a sensitive clamp-on DC current probe around the well casing and measure the DC current. This may be difficult. Current is really what you want to measure. You may not be able to measure the very low current levels existing.

To measure DC voltage possibly try one lead on the casing and the other lead on a screwdriver at the end of possibly a 100 ft test lead. Pick various locations to probe the earth.

An experiment:

I have an isolated copper clad ground rod in the backyard for experiments. This means nothing is connected to the rod except for experiments. To this I connected a Fluke clip lead plus one extension to the plus meter terminal. High impedance sensitive meter. Another comparable lead from the minus terminal to unplated steel screwdriver. Distance between probes about 20 ft. Should not be a factor unless some other voltage source was present.

Copper to steel read +0.56 V.
Chromed screwdriver to first steel one about 0.2 V. Forgot the sign.

From an electromotive-force series table I get:
Copper to iron +0.345 - 0.44 = 0.78 V.
Iron to chrome -0.44 - (-0.71) = 0.27 V.

The screwdrivers won't be pure iron and chrome.

I did not measure any DC current.

 

[mike_kilroy]

Do they have a limit to not exceed? a spec of max current? max voltage? Did the board member come back with any tangible specs to measure against? If you do measure ANYTHING then what will it mean?

In my ground testing I have found I must use AC to make significant current flow. Even voltages upto 100vdc do not seem to cause current flow from one ground rod to another a few feet away, even when 120vac will pass full 100watt light bulb current over same path.... there is a good chemical reason for this but no sense talking about it here now. Just seems maybe you should have a better definition for what they are after.

 

[GoldDigger]

In my ground testing I have found I must use AC to make significant current flow. Even voltages upto 100vdc do not seem to cause current flow from one ground rod to another a few feet away, even when 120vac will pass full 100watt light bulb current over same path.... there is a good chemical reason for this but no sense talking about it here now. Just seems maybe you should have a better definition for what they are after.

And polarity really does matter, since for one direction of current assorted stuff will be plated onto the outside of the well casing (not harmful), while for the opposite polarity metal will be dissolved from the well casing (not so good). If the steady current is low voltage AC, then metal will be moving around from one place to another on the exterior of the well casing, which is not as bad but can still lead to thin spots or pinholes.

There are some very good references about electro-galvanic currents as related to boats. And the reason that TELCO chose to use -48 volts polarity on their ungrounded wires (positive ground) was because it would not cause their wires to dissolve/corrode when small leakage currents flowed from wet wires.

Basically, you do not want the connection of dissimilar metals or ground currents from faulty equipment to be driving the well casing positive with respect to earth. In some cases a battery or similar galvanic protector is inserted in series between the current source and the well casing to force the well casing to go negative, or at least to drive current in the safe direction.

 

[iceworm]

A board member went to a meeting...so here we go

Local village wants to check for DC voltage in reference to the damage to their well(s) metal casing. My Fluke 43B was a good meter close to 20 years ago but I don't think it was good enough even then.

Next what, where, would I attach the leads to?

How would I know that it is not the casing and water/soil contact producing the voltage vs any other equipment in the area? Is there a polarity that would tell me? Any suggestions on reading?

I appreciate that they come to me first for a lot of things but this might be a bit of a stretch.

minor disclaimer: Last time I did this was maybe 20 years ago

You need a copper/coppersulfate half cell. That's the standard reference for cathodic protection measurements.

The half cell goes in wet ground - negative meter lead there. The positive meter lead on the metalic structure you are checking.

Most any high impedance DMV will work

I'm remembering about -.85V is where steel starts corroding. (more positive = corrosion, more negative = less corrosion) But don't quote me - please look it up.

Another hazy memory; If there is an impressed current system, you will need to make two measurements. One with the cathodic protection on and one after the protection has been turned off for an hour.

ice

 

[hurk27]

In almost all cases I have been asked to find what is damaging well casings and or underground water pipes it was cause in the neutral voltage drop or a voltage drop on the primary MGN, some cases was the result of a lost primary neutral feeding a pad mounted transformer causing the transformer to seek its return path back to the substation through Earth and the grounding electrodes or anything that was connected to the buildings grounding system.

I have seen it several times and all resulted in pipes getting pin holes and ground rods dissolving and severe corrosion of anything grounded and in contact with Earth such as post lights and A/C condenser unit bases.

The first thing I would check is the AC voltage from the service equipment to Earth, just stick a screw driver or rod into the earth a few feet away and measure the voltage, if there is a pad mounted transformer check it here also from the case of the transformer to earth, if you have 10 volts or above then its is time to get the utility out there as if all the grounding gets dissolved you can have the primary voltage on all the grounding connected to this service.

Where utilities have used primary HV cable with an exposed concentric neutral it is most likely this CN has burned off because of a lost connection back at the MGN where it is fed from.