PS Magazine - TB 43-PS-629
PS, The Preventative Maintenance Monthly
ISSUE 629
APRIL 2005
Not attaching the grounding strap correctly to the ground rod.
The generator set’s three-section ground rod, NSN 5975-00-878-3791, comes
with a clamp for the ground strap. The shelter’s 8-ft ground rod has a thumbscrew
to do the job. Too often, the clamp is lost and the thumbscrew is broken. When this
is the case, the ground strap is often tied or loosely wrapped around the rod. This
does not provide the good connection that’s needed to conduct the current down the
rod and into the earth.
So, check your ground rods. Order replacements for missing clamps with NSN
5975-01-034-8882. For missing thumbscrews, a nut, bolt and washer should do the
job, but a replacement clamp can be ordered with NSN 5999-00-496-5834. The
clamp for the three-section rod is too narrow to use on the 8-ft rod.
If you’re missing a clamp or thumbscrew and no replacement is handy, tie the
ground strap to the rod with at least 24 tightly wound turns of stripped telephone
wire or other bare wire. Use this as a temporary fix until a clamp or screw can
be found.
Mobile Subscriber Equipment…
Are You
Making
These
Grounding
Mistakes?
Not paying attention to path
impedance—resistance to current
flow—when you lay out and attach
the grounding strap.
Keep the ground strap as straight and as
short as possible. Make sure there are no
loops, kinks, knots or sharp bends.
Run the strap under or around obsta-
cles, not over them. If an obstacle is in the
way, remove it, or pick another spot for
your ground rod.
The key is to make sure nothing
increases the ground strap’s impedance
and causes failures.
Not driving the 8-ft ground rod, NSN 5975-00-296-5324,
beneath the soil surface.
Before you drive the rod into the ground, dig a hole about 18 inches square and 8
inches deep. Then drive the top of the rod to about 3 inches above the bottom of the
hole. Keeping the top of the rod below the surface of the ground reduces dangerous
voltages near the rod during a storm. And also keeps you from tripping over it.
After you attach the ground strap to the rod, fill the hole with water and let it soak
in. Then fill the hole with dirt. Add water as often as needed to keep the soil moist
around the rod. A good constant source of water is your air-conditioning unit. Run
a tube from the air-conditioner drain to the rod area to keep the soil wet.
Just because the connection between the strap and the rod is out of sight doesn’t
mean it should be out of mind. Check it every day to make sure it stays connected
and tight. People walking around the ground can unintentionally pull the strap loose.
Unless you check it regularly, you won’t know if you’re still grounded.
If rocky or frozen ground stops you from driving a ground rod deep enough, con-
sider other forms of grounding or multiple ground rods. See TC 11-6 or FM 5-424
to learn how.
Take a walk
around your
MSE shelter
site…
PS 629
APR 05
45
Not wearing safety goggles when driving ground rods.
Safety goggles protect
your eyes from flying
metal chips. Don’t think
for a minute that a piece of
metal won’t chip and fly
off while you’re doing the
hammering. Lucky Larson
thought that, and now he’s
called One-Eyed Willie.
Not using the right type of
grounding strap.
The ground strap must be as large
as possible—at least 6 AWG. A flat-
surface strap is much better than a
round one since there is more sur-
face area and current passes over the
surface and not through the strap.
The strap should be copper or cop-
per-clad aluminum. The best strap is
made of braided copper. If steel or
stainless steel must be used, it should
be only temporary and inspected
often for corrosion.
William
“Lucky”
Larson is now
“One-Eyed”
Willie
…and see if
you can spot
any of these
earth-
grounding
mistakes…
629.44-45(C)
2/21/05
5:40 PM
Page 1
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