Rick (W-A-one-R-K-T) wrote:

I went down to the electrical supply house and bought 3 8-ft ground
rods and started driving one in at about a 45 degree angle under the
window of the shack.

As expected, I am having one hell of a time getting it into the ground.

Every little while I stop and put a pencil mark on the rod down near
the ground, and beat the end of the rod 10 or 15 more times, then look
to see if the pencil mark has moved. So far it is moving but not
much... but at least it's moving so presumably I haven't hit immovable
rock yet.

Someone said something about an electric hammer and that I could drive
the rod into the ground in "minutes". If I go to try to rent one of
those, what should I be looking for?

There's a potential language gap here, but "electric hammers" fall into
three broad groups. Working downward in size:

3. Electric hammers that are the size and weight of a small road drill
- too big for this job.

2. Smaller "hammer drills" that are a two-handed heft but weigh less
than 10 pounds (sometimes known as the "4 kilogram" class). These are
combination rotary/hammer drills which can also be used for pure rotary
drilling, pure impact hammering. The drill bits have a splined end that
snaps into the chuck, and the whole system is usually known as "SDS
Plus" (not the same as the "SDS Max" which is used in some of the
larger drills).

1. One-handed SDS+ drills that look like a stretched version of an
ordinary electric hand drill (sometimes known as the "2 kilogram"
class).

And below those, I should add:

0. Ordinary electric drills with a hammer option - useless for this
particular job.

As well as having the splined chuck, the SDS+ system is like a miniature
air hammer that delivers a very sharp impact. Forget your old percussion
electric drill - this is in a totally different league. In rotary hammer
drilling, the drill bits go through hard masonry and stone like cheese!
The 2kg size is the best option for general home improvement (you
quickly get tired of holding a 4kg drill) and it's truly wonderful to
hold all that drilling power in one hand.

(Sorry, all of us SDS owners get carried away like this... but it
genuinely *is* that good.)

SDS+ drills and accessories don't seem to be as common in the USA as
they are in Europe, and maybe the "SDS" label isn't so prominent; but
they are easy enough to find, once you're aware that they exist.

Any SDS+ drill with the "rotary stop" option will make a very effective
hammer for earth rods. The dead weight of the drill makes very little
difference - what does the driving is the sharpness of the hammer-blows.
All SDS drills have that feature, so a 2kg-class drill will do fine.

You can buy/rent a special ground rod driving attachment. This is simply
a hollow tube that drops loosely over the end of the ground rod, and on
the other end a splined stub that snaps into the SDS+ chuck. However, a
better and cheaper option is an SDS-to-0.5in square socket drive
adaptor, because you can snap on a different sized socket to match the
diameter of the rod.

In normal soil, a "2kg-class" drill on its hammer-only setting will sink
a 4ft ground rod in minutes. But now we come to "ledge"...

The good news is that with the rotary-hammer action you can drill 3ft
pilot holes into just about anything. A 10mm diameter makes a good pilot
hole for a 3/8in rod, and SDS+ drill bits are readily available in
lengths as long as 1.0m. The hammer action still works, even at such a
long distance away from the drill chuck (though you need a shorter drill
to get started). Having drilled your pilot hole, and cleared it very
thoroughly of small pieces of rock and grit, you can then hammer in your
rod. That means you can place a guaranteed 3ft of earthing rod wherever
you want it.

The bad news is that, in hard rock, anything beyond the 3ft depth of
your pilot hole may be completely impossible - you may just have to cut
off the extra length and settle for what you've got.

The even worse news is that rods sunk into hard rock may be totally
useless as earth electrodes.

For example, the hill that we live on looks like lush green
pasture-land, but it's actually huge heap of glacial boulders of all
sizes, with a very thin covering of soil. Any vertical rod is almost
sure to hit something hard in the first few inches below the surface. It
may be a small pebble, or it may be the hill itself. The first 3ft rod
that I placed measured 600 ohms, which was a total waste of time. After
two years of steady rainfall and bedding-in, the resistance had improved
to... 500 ohms!

The earth connection that really works here (1.2 ohms) is the water
pipe, which runs a long way horizontally through the very wettest level
of soil, just on top of the rock. It isn't strictly "to code" in this
country either, but would I disconnect it? - not on my life!



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek