Hands-on Protection against Lightning

Lightning strikes the earth around 6,000 times every minute. In this
scenario, countries like India, which are located relatively near the equator,
are subject to more frequent and more severe degree of hazards from lightning
strikes. Lightning strikes cause damages worth millions of dollars every year to
telecommunications, cellular radio and other communications installations, with
the high-energy and high-voltage transients called Lightning Electromagnetic
Pulses (LEMPs).

LEMPs are produced by:

• direct lightning strikes to antenna towers or overhead power lines
• induction on to telecom or power cables

On the other hand, induced Transient over Voltages (ToVs)
mostly originate from near strikes via capacitive or inductive coupling.

Communication stations at the end of long power distribution
lines are also prone to equipment damage due to ToVs caused by switching surges
or poor power supply regulation.

LEMPs and ToVs

We shall now go into the details of the consequences of LEMPs
and ToVs on communication installations.

Threat

As per IEC standards 1024-1, LEMP has a typical rise time of 10 ms with a pulse
duration of up to 350 ms. During this time, the electric-field intensity can
reach 500 kV/m, while the magnetic-field intensity can reach 30kA/m.

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 communications equipment. These high voltages,
in the range of kilovolts, are far above the damage threshold of most electronic
components used in the communications equipment. In fact, multi-pulse surges are
experienced in over seventy percent of direct strike situations. This is a
naturally occurring phenomenon where around twenty strikes may follow the path
of the main discharge, at intervals of 10-200 milliseconds.

Targets

Telecom, cellular radio, broadcasting, television and military communication
sites are particularly vulnerable to LEMPs and ToVs strikes. This is because
they are, for the sake of the best propagation, sited normally in exposed and
elevated locations, which means–on top of the mountains/near tall towers/on
top of tall buildings, i.e. just where lightning is looking for the path of
least resistance.

Damages

LEMPs/ToVs striking on any of the above mentioned sites could mean death or
injury to personnel and damage to equipment, stored data, etc. All these
situations would result in loss of revenue due to operational downtime and loss
of image in the ever-competitive operator environment, which are the most
serious consequences in any business.

To get a true picture of the extent of damage due to LEMPs
and ToVs, it would be necessary to add the cost of replacement of electronic
components or systems to the cost of operational downtime, lost business, lost
customers, lost image and finally lost commercial opportunities.

Protection Plan

This would lead to an action plan to provide protection
against LEMPs and ToVs from day one of operations.

Although, it is not possible to provide and guarantee
complete protection from the above-mentioned dangers, they can, however be
considerably reduced by careful attention to protection devices, grounding and
selection and layout of the site.

Protection Strategy

• Capture the direct lightning strike at the highest point on the antenna
  tower, as it is most likely to attract the maximum amount of lightning
  discharge at this point and thereafter discharge the captured lightning
  energy safely to the ground.

• Prevent the above efforts being
  circumvented by ToV and LEMP entering from the back door. For this purpose,
  all the conductors that enter the communications equipment building i.e.
  power lines/cables and RF feeder cables for communication and signal lines
  must be screened for LEMP and ToV, and the energy captured from them should
  be conducted safely to the ground.

• Minimize the damage through a comprehensive protection plan, with a backup
  to provide maximum level of protection against the attacks from LEMP and ToV,
  and in the process, also save on downtime.

Four Point Plan

The figure above is the schematic representation of the protection plan
against LEMPs and ToVs for a typical communications site for telecom, cellular
or radio. The protection plan for broadcasting, television and military sites
will be simplified as applicable.

Each link in the proposed protection chain is equally important. For this
reason, adequate attention has been given to grounding, which is normally taken
for granted.

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

Without any protection, the highest point on the antenna structure is the most
likely point to attract a direct lightning strike or discharge. Because of their
elevated location, antenna frames, microwave dishes, TV translator antennae and
large antennae systems, including their control equipment, are particularly
vulnerable to direct lightning strikes.

One of the oldest and most commonly used methods is to
provide a metal lightning protection rod on top of the antenna tower, i.e. the
highest point of the site, to attract the lightning discharge. Thereafter, this
rod conducts the lightning strike current safely to the tower ground, through
the purpose-designed grounding bars. Since lightning tends to find the path of
least resistance for movement of charges to the ground; it would follow this
preferred path, which is located away from the antennae and cables to save them
from the lightning strikes.

As a result, the chances of damage to the structure and
equipment from the direct force of the lightning strike are minimized.

Grounding

• Conduct the strike to the ground
  safely: Once the lightning has been captured at a preferred point, it is
  conducted to the ground safely through insulated and screened down
  conductors, to minimize the induction to the nearby coaxial feeder cables
  and, in turn, to minimize the danger of side-flashing.

• Dissipate the lightning energy to
  earth: The captured lightning energy must be dissipated into the ground mass
  as effectively as possible i.e. with a minimal rise in ground potential,
  through a low impedance grounding system.

• Eliminate earth loops and
  differentials: Grounding potential, equalization and bonding should be
  ensured between the lightning protection, telecom, radio, power and tower
  grounding systems, otherwise the destructive transient currents could flow
  in unintended directions due to the potential difference between unbonded
  grounding systems.

• To clarify this point let us
  take, for example, the case of a telecom tower struck by lightning. Let us
  say, if the tower ground is 10 ohms and communications ground is 1 ohm, a
  high proportion of the earth current will track down the wave guide to reach
  the communication facility earth. In this process of earth current passing
  through the equipment room, it would destroy the electronic equipment and
  points to the necessity of eliminating earth loops and differentials.

• An effective grounding system
  must be provided where towers are located on rooftops. The down conductor
  from the lightning protection terminal should be bonded and integrated with
  all other buildings and utilities grounding systems.

• Communication equipment building:
  The conductors entering this building and referred to above must be grounded
  preferably in the exterior.

• Regular measurement of ground
  resistance must be done to maintain its value within safe limits and thus
  avoid any major problem.

Protect equipment from surges and transients on power
lines

Even if a structure is provided with an integrated protection
against direct strike system, there remains the risk of the following:

• LEMPs and ToVs tracking via overhead power lines or
  getting induced on to power cables or aircraft warning lights, etc., enter
  the communications equipment building.
  
  
  
  To reduce the risk of physical damage to the equipment, loss of operations
  and consequent economic loss, it is essential that efficient clamping and
  filtering of LEMPs and ToVs should be done at the point of entry of power
  lines to the building itself. To protect the power lines and equipment from
  LEMPs/ToVs, protection devices like surge protection filters are connected
  in parallel to them. Normally, these protection devices are in circuit open
  position. However, in case of LEMPs/ToVs hitting any one of these lines, the
  protection device will discharge them to the ground. This will result in the
  power supply to the communications equipment getting shorted to the ground
  for a very short duration, which however would not be significant enough to
  affect the operation of the communication equipment. In fact, the power line
  is designed to withstand this momentary short to the ground.
  
  
  
  However, in addition to the above protection, power lines are also provided
  with circuit breakers or HRC fuses for isolation as well as to provide
  backup protection.

Protect equipment from surges and transients on
communications and signal lines

Despite taking all the above mentioned measures, the
following possibilities are still there:

• LEMPs tracking via RF feeder cables from the tower
  directly to transmission and telemetry equipment for telecom, cellular
  radio, mobile telephones, TV, FM broadcast, etc., which are housed in the
  communications equipment building.

• Even if double or triple insulated down conductor cables
  are provided for confining most of the current of the captured LEMPs to the
  cable, some induction to the RF cables can also occur due to the magnetic
  and capacitive coupling from the air channel component of the lightning
  strike.
  
  
  
  For the above reasons, additional measures are required to reduce, if it is
  not possible to eliminate altogether, LEMP on transmission lines. This is
  done by means of specially designed coaxial LEMP protectors.

  Devices using one of the
  following two coaxial protection technologies, as suited for the
  application, would protect all the RF and microwave communication lines that
  enter the radio communication equipment building.

• Gas Discharge Tubes, which are
  used from DC to 2.5 GHz.

• l/4 Shorted Stub Tuner, which
  presents a short for all frequencies except that of operation and are used
  from 0.4 to 12 GHz.

During normal operation, a coaxial protector shall not
influence the RF behavior of the equipment, which has to be protected. In fact,
high voltage peaks generated by LEMPs will be diverted by coaxial protectors to
the ground for a very short duration without affecting the operation of the
radio equipment.

An analysis of damage due to lightning strikes shows that,
till date, no single protection device or technology can provide immunity from
the damage due to lightning and transients. The integrated protection plan, with
backup, is one such approach which would provide comprehensive protection at a
fraction of the cost of the protected equipment.

Vinod K Bhatnagar,
consultant and ex-VP, VXL Engineers