There is so much mis-information flying around that is sometimes becomes
Some facts: These can all be verified by reading information that
(1) All metal within the concrete tower base should be bonded together. You
do not want an arc of any significance to occur inside the concrete.
Wire ties are sufficient for the rebar bonding in the concrete.
(2) If you mount a tower on top of the concrete base the J bolts are
extending into the concrete so there is no way that you are going to keep
any lightning energy isolated from the concrete. Same is true of a tower
planted in the concrete.
(3) The concrete base with rebar bonded makes a good UFER ground as concrete
absorbs and maintains moisture from the surrounding earth so its
conductivity is rather good compared to most soil. The concrete is many
times more conductive than the soil so lightning isn't going to jump through
the concrete from the rebar to the soil around the concrete. The charge will
move through it just like it does through the soil.
(4) The concrete UFER ground MUST NOT be the only ground for the tower. An
auxiliary ground system of several (many) ground rods and radials are needed
for an adequate ground.
(5) No metal should exit the concrete base below grade because of corrosion
problems with the metal that will work its way back into the concrete.
(6) Besides the corrosion problems it is best not to have ground leads etc
exiting the concrete below grade because you are inviting more lightning
energy to travel through the concrete on its way to the ground rods system.
(7) Concrete can indeed crack from lightning. I have personally seen it in a
couple of cases. However the ones that I saw were mounted with J bolts into
the concrete and did NOT have any additional grounding.
(8) Making a ring around the tower by tying the ground rods together is a
waste of copper. Use that copper to make another radial.
Lightning energy will try to spread out away from the tower. Assuming that
you have your ground rods and radials placed around the tower like the
spokes in a wheel, all the radials are going to share the energy so the
voltage is going to be relatively equal on all the radials. The voltage
difference between any of them will essentially be zero so no current will
flow on any connection between them if the ring is attached the same
distance from the tower on each one.
Additionally you would be asking the lightning energy to make a sharp right
angle turn to travel on the ring. It isn't going to happen.
(9) Square footage or area is not as important as length for ground rods.
Very long ground rods become less effective because their inductance becomes
too large. Several shorter rods are more effective. Extra long rods are
sometimes used when it is necessary to get to moist soil.
Buried plates are not as effective as rods driven into the ground. However
sometimes plates are used when the soil is so poor or rocky and there is no
other way to get any kind of ground.
(10) If a rather sharp bend is required in a ground lead coming off a tower
leg it is better to make the lead longer and attach it higher up on the leg
so a longer radius bend can be make. This lowers the inductance of the bend.
(11) Use solid conductor ground wires or strap. Multi-strand wire works ok
but the problem comes when it is time to renew connections some time down
the road. It is all but impossible to properly clean multi-strand wire once
it has been exposed to the elements.
(12) A ground system should be constructed so that it can be serviced.
Connections get broken or corrode and need to be renewed.
> -----Original Message-----
> From: firstname.lastname@example.org [mailto:towertalk-
> email@example.com] On Behalf Of Richards
> Sent: Thursday, May 29, 2008 8:30 AM
> To: Skip K3CC
> Cc: firstname.lastname@example.org
> Subject: Re: [TowerTalk] Tower grounding
> So... Is the rebar in the concrete to be kept separate
> from the tower metal? It merely reinforces the concrete
> base, right?
> Thanks for the clarification. // Richards - K8JHR //
> Skip K3CC wrote:
> The water boiled in the concrete from the heat and exploded the
> > concrete bases.
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