ECN Forum Forums Technical Discussion Electrical Theory and Applications Does ground rod work??

Iwire stated ""In that case the voltage might be 7000 volts or more to ground.

A 25 ohm ground rod with a 7000 volt fault to it will result in 280 amps of current flow, (forgetting about the POCOs grounding resistance) well above the primary overcurrent protection device.""

the rod etc is more for surge and lightning,,,,someone do that math!

Grounding can be very confusing but it really is not. A ground rod is not for clearing faults on under 600 volt systems. even a 480 sustem (480/25=19.2) is not likely to clear a fault very quickly.
The grounding electrodes larger than #6 almost always go to the water service entrance and/of building steel.
The ground rod also is for lightning disapation not disapation of an electrical circuit's electron flow. While electricity takes ALL availble paths, the most current will flow on the path of least resistance. Some current will flow from a ground rod back thru the earth to the next closest grounding means ( a water pipe , another ground rod of the POCO ground on a pole). The amount of this current flow will be small in relation to the current back thru the service neutral.
SO the question does a ground rod work? Yes for what it is designed for. No for clearing faults.

Remember that the 25 ohms to ground is just that 'to ground'. What the resistance back to the grounding electrode at the transformer may be is anyones guess.

And yes the ground rod and grounding electrode will 'dissipate' lightning current, as it does not need to complete a circuit. It will also function as a ground reference.

The original question was what will it really do?
In my opinion - remember my opinion!! it is a waste of time and money in a standard service installation. For years and years, the cold water or steel sufficed. I know that with the advent of plastic pipe, the rods supplement the cold water, but the concrete encased electrode should be made to supplement the cold water, if they really want to accomplish any real purpose.

Pierre

By cs409

A rod all by itself can open the overcurrent protection at the voltage levels that the power company uses for local distribution.

It also can help dissipate a lightning strike.

But what kind of surge can it help with?

Only a surge on the neutral, it will do nothing for a surge on either of the ungrounded conductors.

I have a mind set like Pierre's, a ground rod is of little use for the services I generally deal with.

Bob

If you didn't ground the service at the building, wouldn't you risk the earth around the building, and the cement foundation, being at a different potential than the service ground?

It is my view that the purpose of the service ground is to bring the earth to the same potential as the building's bonded systems, and the ground conductor is sized so that it can withstand fault currents that may occur.

twh, I was only talking about a ground rod, most of the buildings I deal with have other much better electrodes that we are already connected to.

Bob

Okay, I handled this poorly. Let me explain:

Where I am, we don't use a ground rod when there is a water line. Last week, I grounded a service to a water line that was 100 feet away from the service.

I think that is a bit far because I once bonded two buildings together that were 200 feet apart and the voltage between the grounds was over 100 volts. That was an unusual circumstance, but once bit, twice shy.

The topic comes up here, and I am interested in the comments. I don't want to take sides, because I don't know.

nesparky said:

In my clumsy way, I'm saying I agree with him.

Then, nesparky gave an opinion:

And iwire gave a somewhat different opinion:

From the context, I didn't think iwire meant that it was of little use "for what it is designed". Perhaps a direct question would help me understand.

Bob, how can I tell when a ground rod, in addition to the water line, would be of use?

tim

In a standard service, there is usually a cold water copper pipe that grounds the service. Generally it is located fairly close to the service disconnect.
I will agree that if the coldwater pipe connection is not close to the service disconnect, that ground rods will help to keep the surrounding area in the same potential as the disconnect/system.

But as a supplement to the system in general it is not altogether very effective. That is why I would like to see that a concrete encased (footing) electrode become mandatory for all new installations. I believe that there are some jurisdictions requiring this.

Pierre

Pierre,
Not really. About 80% of the total voltage drop associated with a ground rod occurs in the first 24 to 30" around the rod. This small area would be the only area that is at the same potential as the system.
Don

No disrespect intended, because sometimes I will speak "matter of factly" too.
But you all state your "opinions" freely, which, again I have no problem, but where is the physics proof in the statements here?