2 "ground rod" questions (Thumper aka cable fault locator)

[Ignoramus31595]

I have a particular device that I would like to test, which is called a "thumper". It was made by associated research (model 8613) and is comprised of two under-desk-refrigerator-sized pieces, about 600 lbs total weight.

For those who do not know, a thumper is a device that delivers pulses of high voltage (up to 25 kV DC in my case) and huge currents, usually above 1000 joules energy, to a buried high voltage cable where an insulation fault needs to be located to find out where to dig to splice it. The lineman walks along the cable path until he feels "thumps" under his feet from electrical discharges in the faulty insulation.

There are warnings on my thumper that say in big letters that it must be grounded to a ground rod. For obvious usual reasons. I do not want to ground it to my house ground rod for safety reasons. So I went to Home Depot and bought their 5/8" 10' "copper clad" ground rod.

What I would like is to achieve two goals with this:

First is to ground the thumper to test it.

The second is to use this rod later for lightning protection. My house is on top of a modest hill and was already hit by lightning. See

So later on I could save some $$ on needing to install a ground rod for that.

FIRST QUESTION

So, my thinking is, the second ground rod would go in the ground close to the first rod, maybe 2-4 feet or so distance. It would be close to where other electricals are located and would be in the area that would be convenient for connecting to some lighting rod to be installed in the future.

Is that a sensible plan in light of wanting to use that second rod for the thumper?

SECOND QUESTION

How to drive this ground rod. I have a decent compressor and a cheap "medium" air hammer. Would I be able to drive it in my clay soil? I read somewhere a suggestion to dig a small hole and fill it with water, which would then liquefy the soil under air hammer's pounding. Is that really helpful? How to actually angage an air hammer to a ground rod?

thanks

i

[Jerry]

On the first question: Your existng ground rod is already provides lightning protection to your electrical supply. If you are going to add a ground rod it should be the length of the ground rod or 10 feet in your case from the existing ground rod and they should be bonded together. If you are going to install a lightning protection system this quite another story. The NEC does not cover lightning protection systems but has this to say:

250.106 Lightning Protection Systems. The lightning protection system ground terminals shall be bonded to the building or structure grounding electrode system. FPN No. 1: See 250.60 for use of air terminals. For further information, see NFPA 780-2004, Standard for the Installation of Lightning Protection Systems, which contains detailed information on grounding, bonding, and spacing from lightning protection systems. FPN No. 2: Metal raceways, enclosures, frames, and other non-current-carrying metal parts of electric equipment installed on a building equipped with a lightning protection system may require bonding or spacing from the lightning protection conductors in accordance with NFPA 780-2004, Standard for the Installation of Lightning Protection Systems. Separation from lightning protection conductors is typically 1.8 m (6 ft) through air or 900 mm (3 ft) through dense materials such as concrete, brick, or wood.

On your second question about driving the ground rod. I have driven many without pouring the water. I have used a sledge hammer, a pipe driver, and an electric jack hammer equipped with a special hollow bit that goes over the top of the ground rod supplied by the local rental company.

About your thumper vibrating the earth - never heard of this before. The one's I used required a sensor attached to head phones.

[DAC]

Don't know about the first...the second...

if you use the water method...you don't need anything other than a sledge to drive it home the last foot or so. Just put the rod in the depression and start move it up and down...add water as needed. The rod will push it's own way though the soil.

best of luck...

DAC

[John Gilmer]

Sorry, but I'm having trouble picturing what you are doing.

Are you saying you have a fault in a HIGH VOLTAGE (more than 1 kV) cable that is on your property? Why isn't this the problem of the utility?

Underground high voltage cables are usually shielded with the shied having a good portion of the conductivity as the center conductor. If you can connect your "thumper" then you are at a place with the cable comes above the ground surface. In my observation, there is ALWAYS a ground rod at this point.

For a few years I was having so many problems with the 440' of low voltage cable service the house that the repairman and I became casual friends and we still chat when I take my kids to activities at his church. Anyway, they found faults by connecting your run of the mill digital voltmeter (AC volt range) to two sharp pins at the end of two poles. The guy would stick the two pins into the ground and see if he detected any voltage drop. He keeps moving the pins in the direction which gives the largest reading. He can find the break within a fraction of a foot.

But this would not work with a shielded cable.

[EXT]

Don't know what type of soil you are used to working with, but the heavy hard-pan type of clay we have in this area would have no effect on easing a ground rod into the soil. I installed 2 10 foot rods with a 10 pound sledge and at the half way point I was getting 1/16 of an inch per hit, that meant the last 5 feet required 16 hits per inch or 960 hits or more to get the rod into the soil, no wonder I was exhausted after driving only one rod.

[Ian Malcolm]

Hey Iggy,

I would personally want the test ground rod an order of magnitude of its length away from the house, under ground utilities, neibouring buildings etc. I also reccomend you wear rubber boots while testing as under high voltage fault conditions significant potentials can be developed over a fairly short distance at the soil surface for some soil types and moisture levels. Unless the bulk resistivity of your soil is extremely low, you will get significant coupling between rods seperated by one rods length. Try 50 to 200 feet away from any structures. If you must drive the rod near the house, disconnect all electronic appliances in the house from all signal and power cables before you start testing. From the specs you have given, if you are too close to it, the thumper is easily capable of causing ground transients comparable to a nearby lightning strike.

[Ignoramus22371]

Ian, thanks. You gave me some good reasons to pound the rod away from other rod. Also, I will control the thumper with something insulating, like a dry wooden board, and will use minimal voltages (like 5 kV instead of 25 kV). I may also stand on a plastic bucket just in case.

i

[Steve Smith]

Wood is not a reliable insulator at these voltages. Plastic is your friend.

Steve

[JerryN]

The IEEE Green Book recommends one ground rod length. That is where I got it.

[Leon Fisk]

Hi Iggy,

If you are really interested in lightning protection for your house you should look-up the "Motorola R56 manual" communications site grounding specification.

will give you some info to start with.

Several equipment sites I used to work at were built to this spec. Lightning would still damage equipment there, but it was a whole lot less than not having it. You can't stop damage from a direct hit short of having everything disconnected. No power hooked up, no phone cables, no ground connections... just a box with no connections what-so-ever. IMO the telephone connections are a bigger problem or more likely point of entry rather than the power connection.

Keep in mind that willy-nilly adding ground connections could possibly make matters even worse than they already are. R56 is almost insane grounding procedures and damage still occurs...

[Ian Malcolm]

Well Iggy is planning on testing a device intended to generate extremely high current with enough voltage to overcome high resistance connections etc. If under fault conditions, the return path is via the soil to the test setup ground rod, it will couple into any long metallic objects such as utility cables either by induction or even direct conduction if they dont have excellent insulation. It would not be unreasonable to expect an induced current of tens or even hundreds of amps.

If Iggy has effictive lightning protection with multiple ground rods round his building perimiter each with a resistance to ground of less than a couple of ohms linked by a heavy bonding conductor and all incoming services are bonded at a single common point to it, with spark gaps and transorbs for all comms ccables, then all should be well, but if he has a single reletively short ground rod that may have had a resistance to ground of up to 25 ohms at the time of installation, well he's going to suffer the mother of all ground bounces.

Worst case, what do YOU reckon the chances of his PC and phones surviving a 10 to 20 KV high current transient between the mains supply and the phone lines are? I reckon we'd not see him online again till sometime near the end of next month at the soonest (assuming he's got cash to spare for a new PC, or a laptop on the shelf or whatever) and he might be offline for longer if he fries enough kit at the exchange to piss off the telco.

Mr Muller and a few others might think Christmas has come early :-) (not intending any slur on Nick), but I for one admire Iggy's persistance and willingness to expand his knowlege with *TOUGH* projects and would regret it if he dropped off the net.

Now if he just wanted the extra ground rod for lightning protection, I suspect your advice up thread is right on the money especially as you state its based on IEEE reccomendations. I'd have to research the relevent electrical code for Iggy's jurisdiction to confirm your advice and as I am *NOT* qualified to practice as a professional engineer in his jurisdiction, my approval would be of no real value.

Life's too short to tilt at EVERY windmill :-)

I dont think the IEEE anticipatied Iggy!

[Ian Malcolm]

Interesting links. Thanks.

[Ignoramus23437]

Ian, thanks for your friendly advice.

See another post (outcome post) that I posted today. I tested the unit more today. This thing seems to have a ground fault (manifested by blowing GFCI), and thus it should be considered broken. I will part it out and will try to learn something as I do so, it has a lot of interesting HV parts that I mentioned in the outcome post.

i

[Ian Malcolm]

I've seen some phantom GFCI tripping on the assymetric inrush current on larger high voltage transformers especially if one side of the secondary is grounded. I wouldn't write it off before it had failed a Hi-Pot test between the commoned live and neutral and chassis. Some stuff just cant be run off a consumer GFCI.

Before parting it out, it might be worth seeing if there is any interest in it from the Tesla coiler and coin shrinking communities.

If you do part it out make sure you have worked out a safe procedure to confirm the capacitors are fully discharged and keep their terminals shorted in storage. (I seem to remember this issue coming up with a previous purchase you made)

[Ignoramus32160]

Well, I just tried running it on a regular outlet, it still does not work. Which may be because I am not doing it quite right, but I do not have a manual, and neither does Associated Research. I connected things the way that makes sense.

Well, you see, I am a little leery of selling a unit that may not be functioning right, to people who may not be very good at never making any mistakes.

Remember that the energy stored in this unit is roughly equal to kinetic energy of a high power rifle bullet (1500 joules, roughly like a bullet shot from a Kalashnikov).

I do not want some chump to die just because they were a little too stupid and did not quite figure everything that was wrong, even if legally I would not be at fault I still do not want it. It is not the same if it is sold by components. Many if not most of those coilers and such, are wannabes with not too much clue.

The parts there, are very fun parts and will likely arouse a lot of interest by themselves.

Yes, sure, I will do that -- though there are internal shorting devices built into this associated research -- but I will double check it safely and will wire it shorted for storage.

i

[DAC]

Oh Man, I feel for ya!

I was really fortunate. This last pair the about 6 feet down the clay had an unusual amount of sand, the rod basically jumped into the ground...couldn't believe it. 2 rods in about 5 minutes.

Hang in there!

DAC

EXT wrote:

[blueman]

I had the same only my soil I think is rocky so I somehow felt like I was pounding it through rock -- for all I know the brute force sledgehammer blows ended up just bending the end of the rod into circles. But in any case after a good 45 minutes of blows, the rod was all the way in and I was a sweaty mess. (Unfortunately, I only had a

4lb sledge, so I probably needed more like 30 blows per inch :)

[blueman]

If things go terribly wrong, he could become a candidate for next year's Darwin Awards...

[Mark]

I don't know how you got a hold of one of these, but thay are LETHAL.

I would not even think of turning it on without someone around who knows how to use it...

Mark