ECN Forum Forums Technical Discussion Electrical Theory and Applications What Does a Ground Rod Do?

Here is a picture of a single line feed to a rural transformer in Saskatchewan.

I'm told the power company tries to get the ground rod resistance down to 10 ohms.

The lower voltage neutral is connected to the high voltage earth return, which also serves as a ground. In fact, the high voltage return is connected to the neutral in every light and plug.

Our new rules forbid grounding the neutral where it enters a barn, so I'm worried about what will happen if the ground system at the pole fails. Until now, grounding the neutral at the barn was required. The cable TV and the telephone are grounded in the building but we're going to quit grounding the high voltage return line.

I bet the cows can really dance there

Not that I've seen.

The neutrals on the secondary side of our transformers are separated by a resistance of about 20 ohms (about 10 ohms at each set of ground rods). How are your transformers isolated?

The neutral at the transformer is solidly grounded on the primary and secondary using the same ground electrode system. That is basically an auto transformer setup.
The only current going into the earth in normal operation is the voltage drop of the neutral conductors.
The neutral is grounded at the transformer and at the service entrance.

This is the typical US service

Doesn't this cause a problem where two buildings on separate transformers are serviced by a common metallic waterline?

With the common grounded conductor on the medium voltage side, on the above drawing, the neutrals are connected at the transformers and the neutrals at the buildings are connected through the common waterline where they are bonded. I assume you bond your waterlines.

This would put the neutrals from the two transformers in parallel. If a neutral to a large service on the large transformer failed, the entire neutral current would run through the neutral for the smaller service fed from the smaller transformer. The neutral and ground system might be too small for the load.

Here, with a resistance of even 10 ohms between ground systems (and neutrals) the current on the wrong system neutral would be limited to about 12 amps.

The assumption is the whole system will be tied to a common neutral conductor on the pole and no current will flow to earth but when I surveyed the 10 transformers on my street I found from a tenth of an amp or so to almost 3 measuring that solid wire coming down each pole. (all on the same MV phase)
I had close to 3 on my service neutral with the main breaker off and a similar amount on the GEC.

I checked my water line a few years ago and I could get an amp if I turned off all the breakers on one leg. 3 Amps on the ground sounds high. That would make me think that a neighbour has a broken neutral.

3 Amps on the ground would be a 60 ohm resistance assuming 120 volts (E = I x R). That sounds like a hot tied to a ground rod or maybe a light pole.

Do you use ground rods and bond the water line and jumper the water meter?

All of the plumbing is plastic here so the water line does not really figure into it.
My ground electrode is very robust tho. I have several rods and a lot of Ufer, not the least of which is a 15,000 gallon swimming pool.
Combined voltage drop in the neutrals has to go somewhere. I bet that if I strung a wire across the canal and checked the neutral at my neighbor's house there would be a very significant voltage difference and the "ground" would be different too.

Around here, a voltage difference is pretty much guaranteed.

I once measured more than 100 volts between an insulated ground wire and a buried ground wire over a couple hundred feet. It was close enough to 110 volts that I went back to the source to check the connections and found the power hadn't been turned on to that panel.

When the two grounds were connected, the spark was visible in the daylight, so there was some amperage with the voltage.