Ufer and Tower Grounding

   
Concrete is a fair conductor and can be used safely
and effectively to augment your tower grounding system.
The characteristic which makes this practical is the
concrete's ability to quickly absorb moisture and
release it slowly over a long period of time. The pH of
the released moisture in turn enhances the conductivity
of the surrounding soil.

It is a common misconception to think that a lightning
strike will blow up a concrete pad. However, consider
first, a myth-perpetuating case of an improperly
designed system where the tower leg "J"-bolts are
imbedded directly into the concrete pad. In this case,
due to the poor nature of the tower ground system, each
of these J-bolts will actually share a significant
amount of strike current which in turn will flow
through the concrete. Since the surface area interface
between the J-bolts and the concrete is small, the
surge current density is very large. The corresponding
heat generated by the energy transfer can turn the
concrete moisture into steam and possibly crack the
pad. We have only seen this happen once on a mountain
top in the Nevada desert. However, a few poorly
implemented occurrences can give a valuable technique a
bad reputation.

Now, consider a proper implementation of a Ufer ground
system as shown in the accompanying sketches. If,
during construction, you incorporate all of the rebar
in the concrete pad as an integral part of your ground
system the overall surge current density will be
several orders of magnitude lower than the
myth-perpetuating case above. With the surge current
distributed over all of the rebar there will be little
to no opportunity to develop the temperatures necessary
to vaporize the imbedded moisture. Thus, the pad will
not crack.

To successfully implement a Ufer ground system it is
necessary to bond each of the independent pieces of
rebar together. This is best accomplished using an
exothermic process. Failure to bond all elements of
rebar could allow for a spark gap between the
unconnected elements and thus an opportunity for
localized heating of the imbedded moisture. The
electrically unified rebar is connected to the tower
leg with a pigtail as shown in the sketches. The
subsurface radial lines, used with ground rods to
further dissipate the strike energy, are also bonded to
the rebar as shown. The Ufer ground is enhanced by the
fact that the local earth resistance will be lower due
to the leaching of the concrete pH into the earth which
in turn lowers its impedance. The better the ground
system, the more current flows through the tower leg
into the Ufer ground. Also, since the strike charge is
all of the same polarity, it naturally wants to spread
out. With the large surface area of the rebar closer to
the earth surface than the tower J-bolts, the current
passes easily through the concrete to get to earth
where it continues to spread out even further.

As well as the Uferground works, it should not be used
alone. We always recommend that radials or radials with
ground rods be used as the main ground system and that
the Ufer ground be used to further reduce the ground
resistance of your system. Many tests have been done,
dating back to 1968, which prove that the Ufer is a
safe and very effective way of augmenting a ground
system. For more information on Ufer grounding, see
Chapter 2 of the newly updated and soon to be published
"The Grounds for Lightning & EMP Protection."
In 1980, the National Electric Code incorporated the
use of a Ufer ground (called encapsulated ground) as
part of the code requirements for a residential house
safety ground. It is primarily used for ground fault
protection. In this usage there is actually more energy
than in a lightning strike due to the comparatively
long delay involved in waiting for the circuit breaker
to trip.