Radio World Engineering Extra's Aug. 9 issue featured an article
on grounding by David Brender and was a good start for understanding
the right approach to system grounding at a facility. Let me expand on
it for radio stations.
Most studios and all radio transmitter sites have a tower in close
proximity to a building. Each tower is a skyhook to bring lightning into
the facility. We should think in terms of the path that lightning will
take when a strike occurs.
The Nautel transmitter people have thought this through.
Courtesy of Nautel, Fig. 1 shows an example of bad grounding technique.
A lightning bolt can come from the tower and go through the transmitter
on its way to the electrical service ground and the power line. The
result is the transmitter acts as a "fuse" that can easily break open,
so to speak, under high surge currents from lightning.
Fig. 2 is a schematic that shows how wiring should be routed. Nautel and
I concur that a reference ground, which is a single point ground, should
be employed at a site to work around a possible 20,000 amperes of peak
current from 1 million peak volts of lightning. The goal is to have
lightning bypass expensive equipment on its way to ground. Some energy
might go to the power company.
Yes, I said "the power company." They might not take kindly to this, but
they are a path for lightning-induced energy, too.
Conversely, lightning from a power line could use a transmitter facility
as a path to ground. A ground ring around the building is especially
popular with cellular providers. They find it works, and we should
follow their example. Note that the ring, with ground rods, connects to
the reference ground at one point and nowhere else.
Having the transmitter and other expensive equipment on a non-lightning
path is the right plan. You do that by putting electronic gear on a
"stub," away from the reference ground. Lighting will come in from the
tower and go to ground, not bothering to go to the stub because there is
no path to ground at the equipment. Lightning should go to the reference
ground instead; it will usually seek the shortest path. A stub uses that
DESIGN WITH GROUNDING IN MIND
Ideally, the facility will be designed and built with all of this in
mind. The best plan puts the incoming electrical service panel on the
same side of the building as the tower. But that may not be the case in
a facility you are working on. The task then is to route cables and
grounds to produce the same results electrically. Easier said than done
sometimes, but worth the effort, if it prevents just one equipment
Fig. 3 from Nautel shows the mechanical layout of a transmitter plant
with grounding designed to route lightning away from expensive
equipment, using the stub idea.
Most transmitter manufacturers send ferrite cores with their equipment
for isolating power cables, coaxial cable lines, control cables,
monitoring samples and audio feeds from lightning induced surges. You
should look into purchasing some if your equipment does not have them.
Ferrite cores tend to block lighting from traveling down a conductor.
They are easy to install and are worth every penny.
Surge protection to reference ground at the power feed and other outside
lines is important, too. There are many surge arresters on the market
from brands like Eaton, EP2000, LEA, Transtector and others. The idea is
to clamp excess voltage to safe levels.
Fig. 4, from a facility I built, shows a reference ground at the
electrical service entrance panel on the right. The National Electrical
Code prohibited this at one time but now it is OK. Some local
electricians and electrical inspectors may not be up to speed on the
Note a 3-inch copper strap was laid down so protection devices could be
attached easily. Best to use stainless steel hardware to prevent
corrosion by dissimilar metals.
One trick to prevent or reduce lightning surges on coaxial lines is to
coil five or six turns of line to ?choke? lightning, thus preventing it
from continuing down a line into the building. That is because lightning
does not follow curves well. It wants to go straight. Each cable then
goes to a surge protector, which is bolted to the strap. You can see
that on the right side of the photo.
PolyPhaser and CommScope are well-known brands. These devices limit the
voltage between the center and outer conductors of a coaxial cable. They
need to be attached firmly to the reference ground to do their work
Fig. 5 is almost as good. Again, all coaxial cables go to reference
ground as they enter a building.
Chris Kreger at RF Specialties Group tells me the company does a brisk
business selling lightning-protection products during storm season. The
best approach is to install protection before lightning hits.
On the other hand, we should look at ways to prevent lightning strikes
or reduce the intensity of strikes when they occur. One such technology
is static dissipaters. The most well-known brand is Nott Ltd. They
manufacture stainless steel needles that bleed static charges from the
sky to ground. The goal is to reduce the voltage between the sky and
ground so lightning will strike elsewhere or be greatly reduced in
amplitude so a strike is less damaging.
Fig. 6 shows a Nott EN-1 Eagle's
Nest high on a tower. One dissipater typically is placed at the top with
smaller dissipaters every 100 feet or so down the tower.
Here's a real-world example: There is a three-tower AM transmitter site
that suffered lightning damage every summer without fail. I was the
contract engineer for the station and my request for static dissipaters
was ignored. Another of my clients purchased the station and bit on the
proposal to add static dissipaters to all three towers. The hardware
cost was about $2,500. Tower climbers put them on during normal
inspection work. The investment paid off. Not one lightning-related
problem has occurred in the 18 years since. Even when a tornado took
down one of the towers, the rest of the facility survived without
To be truthful, there is no sure-fire method of protecting a facility
from lightning, but thinking it through and working accordingly will go
a long way to preventing problems.