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What Does a Ground Rod Do?

Posted By: twh

What Does a Ground Rod Do? - 06/08/16 01:59 PM

The people who write the Canadian Electrical Code seem to think that if ground rods are installed on a service, all the electricity will take that path and none will follow a metallic water line that joins adjacent properties.

The result is that the common metallic water line that connects houses in a city will no longer be a path for electricity and only needs to be bonded near the panel with a #6 wire. CEC Rule 10-406(2)

The requirement to put a jumper across the water meter is gone.

If the neutral is grounded with rods, the wire to the water line is renamed from "ground" to "bond". The bond to the water line is still connected to the neutral at some point in the service at the house.

Here is an example of how difficult it is to get 10 ohm resistance on ground rods. Grounding - Ground Resistance Measurement

What do you think will happen if the power company neutral breaks? Will the electricity all follow the ground rods or will the piping become the ground path through the neighbours property, just like before?

Posted By: ghost307

Re: What Does a Ground Rod Do? - 06/08/16 03:22 PM

The electrons behave exactly the same way...no matter what country they're in or what the local 'rules' say.

Physics trumps all.
Posted By: gfretwell

Re: What Does a Ground Rod Do? - 06/09/16 03:57 AM

If this is truly a metallic water system, that is the best electrode but there is no guarantee these days that there is any metal as soon as it leaves the house. I agree electricity doesn't take the path of least resistance, it takes all paths. You just get an inverse proportional current flow to each.

Ground rods tend to be the highest resistance ground electrode of the approved types. There is no requirement to even what that resistance is after you drive the second one.
Posted By: twh

Re: What Does a Ground Rod Do? - 06/09/16 04:37 PM

We don't need ground rods. We can bury a galvanized plate two feet below grade. The other problem I see is that in Saskatchewan the ground freezes to a depth of more than two feet for several months of the year.

Has anyone tested the resistance of a ground encased in frozen earth?
Posted By: gfretwell

Re: What Does a Ground Rod Do? - 06/10/16 03:33 AM

It is probably too cold to get an engineer out there to test it wink
Posted By: renosteinke

Re: What Does a Ground Rod Do? - 06/11/16 01:45 PM

What does a ground rod do?

A ground rod simply lets static electricity return to planet Earth. All other electricity could care less - it just wants to go back to the transformer that 'made' it.

A metal water main is sure to be an excellent ground. The only problem is that we use plastic so often, we can't be sure it's all there anymore.

So, we make a ground electrode - be it a plate, a rod, or a Ufer.

What if the utility neutral is lost? Then electricity might very well travel through your water pipes, cross over to your neighbor's house, and return using his utility neutral.

That's why, if there's a shock hazard in a house, you turn the power off at the main, and TEST to see if power is still present. If things are still hot, then power is coming from elsewhere.
Posted By: gfretwell

Re: What Does a Ground Rod Do? - 06/11/16 05:36 PM

There is also a significant amount of circuit current going through the dirt. (an amp or more at my house)
This is just an inconvenient truth whenever wye distribution is used and they run a single phase down the street.
There is current in my ground electrode system with the main breaker off and there is current in every one of the ground wires going down the poles. It is just impossible to avoid as long as there is voltage drop in the neutral.
Posted By: twh

Re: What Does a Ground Rod Do? - 06/11/16 06:19 PM

Originally Posted by gfretwell
there is current in every one of the ground wires going down the poles
The current in the ground wire going down the pole is the second line to the high side of the transformer. Doesn't everyone feed single phase transformers with just one line and use the earth as the return path?
Posted By: renosteinke

Re: What Does a Ground Rod Do? - 06/12/16 01:44 AM

In a word: NO.

Most single phase transformers are fed by two "hots" on the high side, much as we power a water heater.

On the low side, there are three connections. Two are the 'hots' to your house. The third (or middle) one is your neutral. It's the bare wire that supports the other wires on their way to your house. This wire is also grounded at the pole. Once it gets to the first overcurrent device at your house, it is grounded again (through the ground rod).

Look at the power lines. Typically, you will see two wires WAY up high, one lower down .... then, at transformer level, you will see two more wires. Finally, there will be some wires even lower down.

The top wires are the high-voltage feeds. That single wire below them is the neutral for the high voltage side (if there is one). The wires at transformer level are the hots from the low voltage side. Next down is the neutral from the low side. Finally, lowest of all, are the phone and cable wires sharing the poles.

What does the ground wire do? Who really knows? Dirt, even moist, "conductive" earth, is a rotten conductor ... especially when compared to a real wire. Some modern countries get along just fine without earth grounding.

The ground rod certainly does NOT help breakers trip under fault loads. Ground rods are allowed 25 ohms of resistance ..... A dead short to earth would draw less than 5 amps (120/25).

Older / obsolete distribution systems tried to use the earth as a conductor. Most of these utilities are locally owned. These are the places where, because of the resistance of the dirt, we have "stray voltage" problems.

A second cause of 'stray voltage' problems as ungrounded systems, where there is a fault between one leg and ground. When there are multiple high resistance faults (say, through lighting ballasts), there will be current flowing through anything conductive.
Posted By: gfretwell

Re: What Does a Ground Rod Do? - 06/12/16 02:00 AM

In spite of what happens up there in blue collar country, they serve the street here with a single hot lead (MV) and a neutral. 3p wye with a line to neutral load wired as an auto transformer (neutral shared from primary to secondary)
I have heard of ground return but I doubt it would work in Florida.

[Linked Image]

Posted By: twh

Re: What Does a Ground Rod Do? - 06/14/16 04:54 AM

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.

[Linked Image]
Posted By: gfretwell

Re: What Does a Ground Rod Do? - 06/14/16 04:38 PM

I bet the cows can really dance there
Posted By: twh

Re: What Does a Ground Rod Do? - 06/15/16 05:32 PM

Originally Posted by gfretwell
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?
Posted By: gfretwell

Re: What Does a Ground Rod Do? - 06/15/16 08:06 PM

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.
Posted By: gfretwell

Re: What Does a Ground Rod Do? - 06/15/16 08:45 PM

This is the typical US service

[Linked Image]
Posted By: twh

Re: What Does a Ground Rod Do? - 06/16/16 01:18 AM

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.
Posted By: gfretwell

Re: What Does a Ground Rod Do? - 06/16/16 05:43 AM

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.
Posted By: twh

Re: What Does a Ground Rod Do? - 06/16/16 04:26 PM

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?
Posted By: gfretwell

Re: What Does a Ground Rod Do? - 06/16/16 05:16 PM

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.
Posted By: twh

Re: What Does a Ground Rod Do? - 06/18/16 01:35 AM

Originally Posted by gfretwell
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.
Posted By: gfretwell

Re: What Does a Ground Rod Do? - 06/18/16 05:06 PM

That probably worked great until we started connecting data lines between buildings.
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