Protection & Maintenance: Power Shield

The earth is constantly losing electrons. Even though this current is
extremely small (2mA/sq km), yet 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, almost 2,000 thunderstorms are in progress on the earth.
They occur most frequently in the tropical and subtropical belts surrounding the
earth where the temperatures and the air humidity are very high. India is one of
the countries, which is subjected to relatively more frequent lightning strikes
and consequent hazards to human beings, high-rise buildings and electrical and
communication installations.

Potential Damages

There can be serious hazards for communication installations due to the
interference of direct lightning current injections and high surge voltages
induced by the electromagnetic field of the nearby lightning channels/down
conductors. The high energy and high-voltage transients generated by lightning,
called lightning electromagnetic pulses (LEMPs), cause flashovers and in turn,
damage electronic components leading to the failure of the subsystem or the
system itself. The resulting damages to telecommunication, mobile radio and
other communication installations are due to the following reasons:

• LEMPs produced at the
  above-mentioned sites by direct lightning strikes to antenna towers/overhead
  power lines and induction on to telecommunication or power cables

• Induced transient over voltages
  originating mostly from near strokes via capacitive or inductive coupling

• Communication stations at the end
  of long power distribution lines are prone to suffering equipment damage due
  to transients caused by switching surges

Threats: As per IEC 61312, LEMPs have a typical rise time of 10 ms
with the duration of 350 ms (half-value time), which is a good match to the
first-stroke of lightning. They 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, 1m distance).

This combination of high electric and magnetic fields can induce very
high-voltage pulses in unprotected communication lines and antennae from where
pulses would enter the communication equipment. Such high voltages in the range
of kilovolts are far above the damage threshold of most electronic components
used in the communication equipment.

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, which means on top of the
mountains/near tall towers/on top of tall buildings i.e. just where the
lightning is looking for the path of least resistance to ground.

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

Protection Strategy

No single technology or protection device could possibly provide immunity from
lightning and transients. What is needed is an integrated approach.

The whole concept of lightning protection is to control and direct the
lightning surge energy safely to the ground via a separate and a preferred safe
path, so that it does the least amount of harm/damage to installation, and in
the process reduces the electromagnetic field.

No one can predict if the installation will survive or not, since the
intensity of a hit is such a variable factor. Statistics show that around 50
percent of all strikes have a first strike of at least 16 kA, 10 percent exceed
65 kA level and only 1 percent exceeds 140 kA. The largest strike ever recorded
was almost 400 kA.

The three well-proven basic principles of protecting electronic equipment
against lightning are: shielding, bonding and grounding. 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 reduce the
electromagnetic field.

Bonding is provided to reduce the potential difference between the metal
parts and the systems inside the volume to be protected.

Four-Point Approach

Capture the direct lightning strike at the highest point
of the site:

The highest point on the antenna structure is the most likely point to
attract the direct lightning strike/discharge. Also, the tower sites are struck
by lightning more frequently than by any other site because of their being at
higher elevation than the surrounding terrain and also being a conductor.

One of the oldest and most commonly used methods is to
provide metal lightning protection rod on the highest point of the site to
attract the maximum amount of lightning discharge. Thereafter, this rod would
conduct the captured lightning energy safely to the tower ground through
purpose-designed grounding bars, which provides the path of least resistance for
movement of charges to ground. This preferred path must be located away from the
antennae and cabling in order to save them from lightning strokes.

After having provided protection in the first stage at the
highest point of the site, chances of damage to the structure and the equipment
from the direct force of lightning strike/discharge are minimized. The battle
continues as explained below:

• LEMPSand transients entering via overhead power
  lines/aircraft warning lights, getting induced on to power cables.

• LEMPs can also track via RF feeder cables from the tower
  to the transmission and telemetry equipment housed in the communication
  building. Some induction to the RF cables can also occur due to the magnetic
  and capacitive coupling from the air channel component of the lightning
  stroke.

Grounding

The grounding of the entire installation must be carried out in accordance
with the recommendations given in IEC 61024-1.

The grounding of the installed lightning protectors and their
connections to the bonding bar of the equipment have to be prepared very
carefully to achieve the lowest possible resistance and inductance to the
ground.

• Once lightning has been captured at a preferred point, it
  should be conducted to the ground safely through insulated and screened down
  conductor to minimize the induction to the nearby coaxial feeder cables and
  in turn, minimize the danger of side flashing.

• The captured lightning energy must be dissipated into the
  ground mass with minimal rise in the ground potential through a low
  impedance grounding system.

• An earthing ring should surround the communications
  facility with only a single-point connection to the earth bar, within the
  facility. A single ground rod is never enough to ground a tower for
  lightning. Basically, there are three types of grounds namely for RF,
  lightning and power return (which is safety ground for ground faults).

Protect power equipment from LEMPs and transients:

The incidence of damage to equipment in general, is higher from the power
line surges than by any other input/output. This is not to suggest that more
surges or transients come through the power line, but that the damage is just
more visible here.

To reduce the risk to the equipment, loss of operations and
consequent economic loss, it is essential that efficient clamping and filtering
of LEMPs and/or transients be done at the point of entry itself of power lines.

To protect the mains supply lines and the equipment from
LEMPs and transients, protection devices like surge protection filters (SRFs)
are connected in parallel to them. Normally, these protection devices are in
circuit-open position. However, in the case of LEMPs/transients hitting any one
of these power lines/equipment, the protection device will discharge them to the
ground.

Even when SRF is installed on the main power feed, protection
should be installed on external power feeds, to protect against LEMPs/transients
that could be conducted back to the equipment. Some of the examples of external
power feeds are power supply lines to aircraft warning lights at the top of the
tower and overhead power supplies.

Protect communications and signal lines against LEMPs:

The coaxial cable is the biggest path of the surge current due to LEMPs, as
it is the largest surface area connecting to the direct source of surge current
ie the tower. These LEMPs track directly via RF feeder cables to the
transmission and telemetry equipment in the communication equipment building.

Protective devices effectively minimize the maintenance and
repair requirements. The coaxial LEMP protectors would protect the RF and
microwave communication lines that enter communication building. During normal
operation, coaxial protector shall not influence the RF behavior of the
equipment, which has to be protected. In fact over voltage peaks generated by
LEMPs will be diverted by coaxial protectors to ground for a very short
duration, without affecting the operation of the radio equipment.

Vinod K Bhatnagar