Getting Grounded
By Ernest M. Duckworth
Jr., P.E.
Mobile Radio Technology, Apr 1, 2002
With more than 150 lightning strikes per second, damage to
equipment brings losses exceeding $26 billion annually in
North America
and nearly three times that amount worldwide. Insurance payments for
lightning damage claims total 6.5% of all property and casualty
claims. Ironically, damage to equipment can be prevented.
Methods that prevent lightning damage are simple, reliable and
inexpensive when compared to the cost of repair — not to mention
possible injury or death. Remarkably, methods for lightning protection
cannot be found in building codes such as the National Electrical Code
or the National Electrical Safety Code. Yet, builders rely on these
codes for practically all construction in the
United States
.
Electrical equipment damage from lightning usually can be blamed
on one or two reasons: 1) improper or insufficient grounding and 2) no
special protection from a ground potential rise (GPR). Improper or
insufficient grounding allows equipment to be stressed or damaged by a
difference in electrical potential from nearby equipment and metal
objects as current flow is misdirected. A lack of special protection
from a GPR allows equipment to be stressed because of its attachment
to a remote ground through communications wire lines or power-supply
wiring.
Standard protection for the termination of communications
wire-line services is the gas tube. Gas tubes are shunting devices
that can be found on virtually every telephone pair terminated in
homes and buildings. They are designed to shunt (connect to ground)
“incoming energy,” and thereby protect equipment and people from
harm.
However, no shunting device protects electronic equipment from a
GPR, or “outgoing energy.” High outgoing current flow from a
lightning strike effectively lifts electrical ground. During a GPR,
the shunting devices are connected to an elevated ground and merely
offer an additional current path from the site to a remote ground. In
this unfortunate way, gas tubes guarantee a connection of the
communications path in the reverse direction from which they were
intended to operate when there is a GPR. (See Figure 1.)
Dangers from outgoing energy
The
outgoing energy from a GPR places most telephone and power
installations at risk for equipment damage and the people near them at
risk for harm. One of the most dangerous locations for people is a
9-1-1 public safety answering point. The typical PSAP is a small
building beneath a large radio tower. The tall tower supports radio
communications antennas and forms a lightning target. Workers taking
emergency calls must be at the PSAP phones constantly. They do not
have the luxury of staying off the phone during lightning storms,
which is recommended in virtually every telephone book.
The only effective method of protecting equipment connected to
wireline communications from a GPR is through isolation — using
optical isolators or isolation transformers. These devices prevent
current flow. With no metallic path, there will not be outgoing
current flow, equipment damage or risk of injury to workers. (See
Figure 2.)
To control lightning-strike energy as it dissipates requires
division. There is no substitute for division for successfully
dissipating the energy because of the magnitude of the current and the
resulting surge impedance of any single dissipation path. Using 10
radials connected to a ground ring that is bonded to an antenna
divides lightning current into 10 smaller segments. This division
helps to ensure that the lightning will follow the grounding
conductors for dissipation into the earth. The improved dissipation
also serves to reduce the resulting GPR to the adjacent
equipment-building grounding system.
The copper wire grounding system can be greatly improved by
placing conducting cement around the radials at the time of
installation. The cement hardens into concrete, protecting the
grounding system and giving it many more years of life. It also gives
the system a lower ground resistance.
Tower location
Equipment
buildings must be at least 30 feet from antenna towers. This is done
to avoid damage caused to electronic circuits by the magnetic field
associated with lightning. Magnetic field strength drops off as the
square of the distance. If the real estate configuration prevents
separating the building at least 30 feet from its antenna tower,
consideration must be given to engineering a Faraday cage (wire mesh)
around the interior of the building. Without a Faraday cage, equipment
damage cannot be prevented no matter how well the equipment is
grounded or isolated from remote ground.
Another reason to place equipment buildings or shelters at least
30 feet from antenna towers is to keep the lightning GPR at the tower
base from saturating the building grounding system before most of the
energy is dissipated in the earth. The grounding systems for the
building and its tower must be bonded together at one single point,
but a bond of 30 feet or more will significantly reduce the resulting
GPR at the shelter because of the impedance of the lengthy bond. This
is a rare exception in which a lengthy bond is an advantage in
supporting a robust grounding system.
Single-point grounding (a “ground window”) is absolutely
necessary to prevent equipment damage because the GPR from lightning
strikes is a wave of rising voltage or an energy surge that passes
through a grounding system. The nature of the wave demands that all
equipment should be bonded to the grounding system at one location to
ensure that the electrical potential of every metallic object
connected to it rises and falls together. (See Figure 3.)
Anyone using equipment susceptible to GPR must be protected by a
single-point grounding system to guarantee that they won't be injured
by touching different pieces of equipment that otherwise might have
different and harmful electrical potentials when lightning strikes.
This phenomenon is also known as “touch potential.”
The use of a ground window (also called a bulkhead panel or
waveguide hatch) where coax cables, waveguide and antenna wires
penetrate the wall of the equipment building is indispensable. The
bulkhead is made of solid copper. Its proper engineering design and
installation will ensure that lightning energy does not enter the
equipment building on cables from the antenna tower.
The bulkhead must be bonded to the building grounding system at
the single-point grounding location — the same single point ground
where the tower grounding system is bonded to the building grounding
system.
Isolate wireline comms
A lightning
strike to a grounding system produces an elevated ground or GPR. Any
equipment bonded to the grounding system and also connected to
wireline communications is likely to be damaged by outgoing current
seeking remote ground. Individuals who may be working on equipment
connected to the wire lines could be harmed because they would be in
the path of the outgoing current.
An engineering design that protects this equipment isolates the
wireline communications from the remote ground. Isolation is
accomplished using optical isolators, isolation transformers or both.
The isolation equipment, called the high-voltage interface, is housed
together and mounted on a non-conducting surface in a non-conducting
cabinet.
The HVI isolates the communications equipment during a GPR and
prevents a current from flowing from the grounding system with higher
electrical potential to a grounding system with a lower potential. The
isolation completely protects equipment from damage and people from
harm.
Simple lightning protection
Simple,
reliable and inexpensive protection methods can prevent lightning
damage to equipment. The three most important concepts are:
divide and control the lightning strike energy.
design a
true single-point ground system.
isolate
wire-line facilities from a remote ground.
A reliable,
well engineered grounding design will all but completely eliminate
lightning threats to equipment and people.
© 2002,
PRIMEDIA Business Magazines & Media Inc. All rights reserved. This
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