In an overhead service drop for a residential service, where does the source transformer get its ground besides the actual center tap on the windings. The water pipe is in the ground and there has to be that connection through the actual dirt ground.

The utility will drive a ground rod at the pole that supports the transformer, as well as additional ground rods at every third pole or so. Different utilities will have different rules based on local ground conditions and the likelihood of lightning strikes.

I am not sure what you mean when you say.

iwire, I mean the water pipe runs through the ground and has to be an extra path back to the Utility ground.
Along with the ground rods.

Not exactly, the grounding electrodes installed at a service will not help clear a fault at 120 volts.

They may help with lighting and a power company fault that sends high voltage onto the neutral.


The neutral conductor through the bonding jumper is the Effective Ground-Fault Current Path back to the source for 120 volt ground faults.

Actually the utility ground is the total of all the service ground rods and water service grounds tied together by the secondary neutral on the street. Everybody contributes.

-Hal

Hal I agree they do all contribute but at 120 volts not to much.

The Code uses 25 ohms as a reference for a minimum for one rod or you need to toss in another one.

Lets say all the electrodes together get you down to 8 ohms of resistance.

At that level of resistance a fault (not including the neutral) will draw 15 amps.

This will take a long while to trip a 15 amp breaker if ever.

The electrodes contribution will be a very small percentage of the total fault current once you include the neutral.

Bob

The vast majority of UG water lines installed within the past 30-40 years are PVC. Not much of a ground, but a friend was red-tagged because he didn't bond to the water line )PVC).
Rowdy

Bob I agree also. I think aldav53 is a bit unclear as to the purpose of grounding hence the confusing question.

The "ground" is not current carrying. The unbalanced and fault current is carried by the neutral conductor that connects to the transformer center taps and runs from pole to pole and into every service.

This neutral is grounded at each service (through ground rods and bonding to the water line) as well as at each transformer with a ground rod. The purpose of grounding is to offer some protection from lightning and primary power crosses as Bob points out.

If the area has a continuous metallic water system, yes the pipe is, in effect, in parallel with the neutral on the poles and also in contact with the earth providing a ground.

-Hal

Hal, I'm just wondering where the transformer ground is that is linked (has a path to) to the connection to the water pipes and ground rods at the home.

The secondary neutral conductor on the street. It connects to all the center taps and all the service grounds as well as having ground rods of its own at each transformer.

-Hal