Grounding problems...

[CoopContractor]

I work at a cooperative in a very rocky area that is having some grounding problems. The area is too rocky to drive multiple ground rods, sometimes not even 1 eight foot rod will get half way in. The coop engineers need some advice on the way to improve the impedance of their system. This problem is occuring on both overhead and underground distribution systems (at poles and underground transformers).

Ideas?

Thx

[NJ glove]

you can put the ground rod next to the pole once the hole is dug out. If you can't get it in next to the pole you can try to put a rod further away from the pole and just run the ground wire to it. The ground wire should be underground at least a ft or so. We've done that where I work and took reading and it work great. Resistance reading came back in most cases less than have of the original reading. In some cases we drove in multiple rods in 3ft apart and just connected them to one another that also work. It all depends on how much space you have around the pole to work with. Let me know how it works out.

[500 KVA]

Grounds must be placed in undisturbed earth. Placing them inside of the pole hole before you backfill is easier for the man driving the ground rod, but is not the correct way of doing it.

We are looking for a minimum of 25 ohms or less in resistance from the placement of a ground rod. Depending on soil conditions, one rod may not be sufficient to gain this 25 ohms or less. Multiple rods must be driven.

Placing multiple rods a few feet apart is incorrect as well. The minimum distance a rod must be driven from another rod should be the total length of the ground rod.

Think about it as if you have thrown a pebble into the water. The ripples that radiate out from the center are like the zones of the ground rods ability to carry away the over voltages that it may be called upon to perform. The further away from the center, the less effective.

It would actually look like an ice cream cone if you are trying to visualize it.

If you put multiple ground rods too close together you will cancel out the ability of the ground rods to perform their function, not help them.

Ground rods need to be driven as straight down as possible. You are trying to find as much moisture in the soil as you possibly can. Dry soil reduces the ground rods effectiveness dramatically.

Extremely hot or even frozen soil greatly reduces the ground rods effectiveness, as does sandy and rocky soils.

My advice would be to purchase an electric or pneumatic ground rod driver. They look like mini jack hammers but are for driving ground rods. They work well! then there would be no excuse for not getting the ground rods in that are necessary to protect the system and the public.

[thrasher]

As 500kva said if there is any way to drive properly spaced rods do that first. However two other alternatives, that do not work as well, are to build a "counterpoise" or to hire a rock drill. You build a counterpoise by burying at least 18 inches deep a long loop or grid of bare copper wire. This is usually 1/0 or larger and it may need a couple hundred feet in really bad soil. The other way is hire a rock drill, drill a couple of deep holes screw together the rods then backfill with fine ground carbon. You can buy the carbon from a company called Loresco or any other company that sells anode bed supplies for pipeline companies. If you backfill the carbon use water and laundry soap to get the carbon to go down the hole and pack in the hole.

[LEAFMAN]

just out of curiousity, a ground gradient mat works for a air break. Would a larger size ground plate work in that area?

[wudwlkr]

Counterpoise

[Squizzy]

You could always get them drilled, had an "incident" a while ago where a contractor was using a large highdraulic driver to hammer in earth rods and couldn't get a good reading. Can't recall how many rods he'd used but it was a lot. Some one then notice about 20 meters away that the rods were coming back up out of the rocky ground..

[CoopContractor]

Hey Guys,
Thanks for the replies. Regarding getting the rods down, we've been trying the a pneumatic driver for a while, and although it works okay, we still have a good number that will not drive further than 3-5 feet. The driver keeps beating, but to no avail. Could you cut the rods in half and drill a 3-4 of them correctly spaced out and attach my some #6 copper?

The suggestion about burying a length of 1/0 copper (or similar) for a length and then attaching it to the ground rod may work - does length of conductor matter? OF course, my only worry with that is the rogue that finds out and begins driving around pulling it up to sell.

What does everyone think of these two options?

Thanks in advance for your time.

CoopContractor

[500 KVA]

I think the depth of the rod and the size and length of the conductor definitely matters a great deal.

You are looking at getting as much surface area as you possibly can to reduce resistance. The more rod or wire surface area you have buried, the lower the resistance will be. Theoretically speaking.

Cutting off rods = Bad policy. Someones life could be depending on a system that is full of basically half assed workmanship. It's too hard.

Don't leave a trap for the next guy.

If you cannot get rods in, then trench copper wire into the ground. The proper size and length. You'll need to keep meggaring until you get enough wire trenched in to get a minimum of 25 ohms of resistance. Even if it takes 500 feet.

Add the carbon like someone suggested to enhance the surface area of what you can get into the ground.

Do it right. Don't half ass it! Linework isn't easy work. I'm not trying to be insulting. That's just my opinion.

[Swollen Tongue]

The air driver works pretty well here. On the mountains you do have to drive at an angle. Seems to deflect off the rocks better that way. You got to pay attention when putting several rods together though, you may drive the rod and see it coming out of the ground 20 feet away.

[Stinger]

When i worked in VA and NC some of the coops we contracted too had us put but plates then wrap the ground wire around the pole butt for the length of the setting depth. Thraser, if your read this post does Rapphohanick coop do this practice. We were alway pretty lucky to obtain the 25 ohms or less.

[billfoster67]

The story is since it was frozen permafrost and sand. And didn't have a good ground, they welded the ground grid with old big boilers and threw them in the sea for the generation station.

I have seen a lot of crazy things up there, because of lack of resources this man that past away did alot of ingenious things. Brass radiators buried with a backhoe.

The best ground I have done, when the ground was poor get a 40 pound jackhammer tie it up with rope and put it in the hole, It will go a foot below the polehole. Pull the hammer out with the rope. When we did it conventionally we were getting readings of 30 to 50 ohms. We kept paralleling and it would only improve five ohms and the inspectors wanted 15 ohms. I said enoughs enough when you do 5 or 6 ground rods, the inspectors came up with this idea. We buried it in the polehole, H frame hole 10 ft. rod (12 ft) driven 11 ft. down, we were getting readings of 5 to 15 ohms. If its sloppy sand or caving ground I would just use a hammer hilti with a small honda generator. Because the air exhaust will destroy your hole with a compressor. They have a cadweld mold for ground rods, put 20 feet of copper on it. Just have the warehouseman prebuild them, do it inside- condensation or any type of moisture will ruin the weld and it will break off. You'll have a real good ground!!! Bug the tail to the pole ground.

With the hammer hilti you don't have to man it, tape the handle and tie the rope to it, and the end to something else or you will have to pull it out with a grip all- thats no fun. you can frame while the hilti drives the rod. Get a long shaft ground driver for the end of the hilti. Trust me you'll get the best ground in the worst conditions.

If you have buried utilities in the right of way, and you need to drive rods and you have a backhoe. put a grip and a shackle put your teeth on the shackle and drive it really slow once you know your in the safe zone drive it, put your hand on it if you feel bumps- your scraping pipe or a rock. If you use a hammer you coud bust sewer, irrigation, gas, or whatever else is down there.

[Squizzy]

We are currently building lines for a mining project and they are getting the earth rods drilled and installed to a depth of 90 meters- (3 and third feet to the meter) through a mojority of solid rock.

[LINETRASH]

the best ground is your system neutral.

A butt wrap spiral ground or ground plate on the pole will help.

I dont know what distance of line we are talking, but there are miles of ground rods driven at a given sub.

I had a cap bank pole in trouble a while back, when I arrived, the pole bond was arcing rather spectacularly with the ground around the pole.

Having had experience with cap banks, I shot a look up at the neutral, and lo and behold, the pole bond to neutral had come apart.

The substantial amperage of the cap bank was overwhelming the pole bond.

I deduced an insufficient grounding of the pole bond and had the dispatcher drop the feeder, at which point I opened the fuses, and had him throw it back in.

In this wye system, our subs are numerous as could be expected in Miami, and well grounded, we hit coral at about 3 to 5 feet here, while not as hard as you are talking, it does create pole bond grounding issues from time to time.

This may or may not address your problem, I have no idea of the circumstances of your system.

I hope this helps.

[fleaman]

have you tried using steel poles?