First, it is important
to understand that there is a difference between minimum design
standards and good engineering practice, as related to lightning
strike energy and power line faults. Minimum addressed
requirements do not always follow good engineering practice,
particularly in the design of facilities that may be subjected
to Ground Potential Rise (GPR) from lightning strike or power
line fault energy.
IEEE Std. 487-2000 states that you do not have to use isolation
on wire-line communications if the GPR is less than 1000V-Peak
Asymmetrical, because not doing so will not result in a fire.
However, a GPR under 1000V will still damage cable and
equipment, and possibly harm personnel. Also, this standard does
not directly address the need for isolation of wire-line
communications in locations other than in power-type
high-voltage environments, because of its mandate to only
address those environments. Thus, there is presently no standard
that addresses the dangers of GPR resulting from lightning
strike energy in non-power environments.
To protect personnel
and equipment from the possible effects of a GPR, you should
consider using isolation equipment on all wire-line
communication services where the calculated GPR will be over
300V-Peak. If you have any highly reliable Class-A services that
you want to continue to function at all times, even during a GPR,
then you must use isolation no matter what the calculated GPR.
Any tower, antenna, or other object that is higher than
surrounding objects, is at risk of being struck by lightning.
Thus, these locations are susceptible to the resulting GPR from
lightning strike energy passing down through the impedance of
that grounding system, and isolation must be considered if
equipment damage, personnel safety or communications reliability
is a concern.
As you may be aware, standard grounding devices, i.e., gas
tubes, MOVs, SADs, etc., that shunt energy on wire-line services
to ground, will not protect those services from a GPR. In fact,
these very fast acting devices will actually fire backwards
placing the current from a GPR back on the line.
Obviously, in cases related to power line faults, the better the
cell site grounding system, the lower the GPR. This is basic
"Ohms Law" --Voltage is equal to current multiplied by
(x) resistance, in cases related to power line faults. However,
in the case of lightning strike energy, the equation is: V=Ldi/dt.
Very conservatively, an average 30kA lightning strike will
produce a 7.5kV GPR. A 100kA lightning strike will produce an
approximate 25kV GPR. Although rare, super strokes of 200kA or
more are known to exist.
However, don’t be fooled in believing that good grounding
systems can be had easily. No matter how hard you attempt to
provide a good grounding system for the cell site equipment that
matches the grounding system of the object that it is using for
an antenna, you will probably not be successful. Consider this:
for every foot of 4/0 ground conductor, there is approximately
1.5kV of voltage drop at lightning frequencies. Thus, as an
example; a cell site grounding system could be grounded to a
water tower grounding system (very good ground) located just ten
feet away with a 4/0 ground conductor, and still have a 15kV
difference in potential at lightning strike frequencies.
Maintaining reliable, safe and maintenance free wire-line
service, year after year, requires the use of isolation
equipment in GPR locations and very good grounding systems.
Don't be lulled into a false sense of safety just because your
grounding system is tied into another very good grounding
system. Remember, at lightning frequencies, just a few feet of
heavy ground conductor (4/0) can represent a significant
difference in potential between two grounding systems.
If you require your cell site to function through a lightning
strike to the tower, call LPGI & Affiliates at 303-688-5800.
We can engineer a design that meets IEEE Std. 487-2000.