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
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
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
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
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?
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
About your thumper vibrating the earth - never heard of this before.
The one's I used required a sensor attached to head phones.
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...
On Dec 14, 10:10 pm, Ignoramus31595
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.
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!
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 :)
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
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.
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
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.
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, 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'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)
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
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.
On Fri, 15 Dec 2006 04:10:09 +0000 (UTC), Ignoramus31595
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
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