Towers : Lightning Risks

Direct lightning strikes and the transients following them can create major
equipment failures resulting in downtime at communication sites when little or
no purpose-designed protection is provided. On the other hand, installation of
lightning protection costs only a fraction of the cost of transmission
equipment, which will be more than compensated through increase in revenues
received as a result of the superior services provided.

The Problem

The earth is constantly loosing electrons. Even though this current is
extremely small, (2µA per sq km) globally, it sums up to around 1.5 kA
continuously. Mother Nature's way of balancing this current is through a bolt of
lightning striking the earth 150 times every second. At any given time, around
3,000 thunderstorms are in progress. They occur most frequently in the tropical
and subtropical belts surrounding the earth, where the temperatures and air
humidity are somewhat on the higher side.

In order to evaluate lightning hit risk at a site, a number of factors such
as-the annual number of days that thunder is heard; altitude of the area; height
of the building; surrounding profile; buildings in the neighborhood; the
distance to water; earth material, etc-have to be considered. Some parts of
India are subjected to frequent lightning strikes, resulting in hazards to human
beings, high-rise buildings, and electrical and communication installations.

Depending upon the average number of thunderstorm days (ATD) in a year, the
country has been classified into five categories according to the severities of
hazards involved.

Hazards

There can be serious hazards to communication installations due to
interference of direct lightning current injections and high surge voltages
induced by the electromagnetic field of nearby lightning channels down
conductors.The high energy and high voltage transients generated by the
lightning are called Lightning Electromagnetic Pulses (LEMPs), which cause
flashovers and in turn damage electronic components, leading to the failure of
the system itself. The resulting damages to telecommunication, mobile radio and
other communication installations are due to LEMPs produced at the site by
direct lightning strikes to antenna towers-I overhead power lines and induction
on telecommunication-I power cables, transients / surges, originating mostly
from near strokes via capacitive-I inductive coupling and communication stations
at the end of long power distribution lines. These lines are prone to suffering
equipment damages due to transients caused by switching surges.

Threats

LEMPs have a typical rise time of 10 µs, with a duration of 350 µs (half value
time) as per IEC 61312-1, which is a good match to the first stroke of
lightning. LEMPs are accompanied by an electric field intensity of up to 500
kV/m, before the lightning strikes and can reach magnetic field intensity of up
to 32 kA/m (200 kA strike, 1 m distance). This combination of high electric and
magnetic fields can induce very high voltage pulses (in the range of kilovolts)
in unprotected communication equipment are far above the damage threshold of
most electronic components used in the communication equipment and hence the
problem.

Targets

Telecommunication, mobile radio, broadcasting, television and military
communication sites are particularly vulnerable to strikes of LEMPs and
transients. This is because they are, for the sake of the best propagation,
sited in normally exposed and elevated locations-on top of mountains-I near tall
towers-I on top of tall buildings-just where the lightning is looking for the
path of least resistance to ground.

Damages

LEMPs and transients striking on any of the above-mentioned sites could bring in
death or injury to personnel; damage to equipment, buildings, or structures;
loss of services leading to inadequate availability of the network; and loss of
stored data and even corruption of stored data. The end result is the loss of
revenue due to operational downtime and loss of image in the competitive
operator environment.

The Solution

No single technology or protection device could possibly provide immunity
from lightning and transients. The whole concept of lightning protection is to
control and direct the lightning surge energy safely to ground via a separate
and preferred safe path, so that it does the least amount of damage to the
installation, thereby protecting people as well as the installation and reducing
the electromagnetic field. No one can predict if the installation will survive
or not, as the intensity of a hit is a variable factor. The main idea is to
protect equipment and people against lightning by conducting the lightning
current to ground via a separate preferential solid path and reducing the
electromagnetic field using the following three basic principles of protection:

Shielding: It is the basic measure to reduce the electromagnetic interference
through Faraday cage, Reinforced concrete, or Screened cables.

Bonding: It is provided to reduce the potential difference between the metal
parts and the systems inside the volume to be protected against lightning. This
shall be achieved through bonding at the boundaries of the lightning protection
zones as well as between subsequent lightning protection zones, bonding of
internal conductive parts, and finally bonding of information systems to create
an equi -potential plane.

Grounding: It conducts the lightning strike to the ground safely, thus
dissipating the lightning energy.

Lightning Protection Zones

The volume to be protected against lightning at any site should be divided
into the lightning protection zones in order to define and control volumes of
different LEMP severities and thereafter designating locations for bonding
points on the zonal boundaries as per IEC 61312-1. A typical communication site
can be divided into lightning protection zones for the purpose of defining the
threat.

LPZOA-It is located outdoors on top of the antenna tower, which is vulnerable
to direct lightning strikes. Here objects must be capable of carrying the full
lightning current. Since the un-attenuated electromagnetic field (lightning
current test pulse of first stroke 0/350µs) is very dangerous in this zone, the
protection must be provided to protect the entire outdoor installation against
LEMPs at the top of the antenna. Lightning striking the communication site is
captured by the lightning protection rod provided at the highest point of the
site.

LPZOB-It is located outdoors on the antenna tower and around it on the site.
A direct hit is not possible here. But the un-attenuated electromagnetic field
is present. This zone is then determined by the external lightning protection
system consisting of the air termination, down conductor and earth termination
system. Since the various antennas, aircraft warning lights on the tower,
overhead power lines, etc are located outdoors they would be provided with
protection against LEMPs and transients and classified as in LPZOB.

LPZ1-It is located inside the communication building, where a direct hit is
not possible. The currents in all conductive components are lower than that in
LPZOA and LPZOB. Also, the electromagnetic field in this zone is attenuated due
to the screening measures applied. In order to protect the electronic equipment
in the communication building, all the incoming and outgoing lines and media
shall form part of the bonding system.

Water, gas and air conditioning pipes, outer conductors of coaxial cables and
wave guides shall be connected directly through grounding cable to the bonding
bar so that they are continuously connected. Signal carrying lines like data,
telephone and RF lines and power lines shall be earthed through their respective
lightning protection devices, which would short to ground in the event of being
struck by LEMPs. RF signal and power supply lines leading into this zone can
thus be protected safely by lightning protection devices.

Inner Shield 2 shall be provided at the entrance of the computer room in and
bonded to local Bonding Bar 2 (BB2), which is the interface provided at the
boundaries of LPZ1 and LPZ22. Additional surge protection devices connected to
BB2 shall equalize the potential difference between LPZ 1 and LPZ2.

VK Bhatnagar

vadmail@cybermedia.co.in