Lightning protection systems are the modern development of the innovation
pioneered by Benjamin Franklin: the lightning rod. Today, lightning protection
systems are in use on thousands of buildings, homes, factories, towers, and
even the Space Shuttle's launch pad. This article will examine why lightning
protection is necessary and what the systems can and cannot do.
Many buildings of
historical interest or tall buildings such as churches are
fitted with a lightning
protection system to protect the building and those
who use it from the
harmful effects of a lightning strike upon the building.
The average lightning
discharge current is in the order of 20,000 A and it is
this discharge of energy
through a structure that causes the damage.
Lighting protective
systems offer the lightning strike an alternative low
resistance path to the
general mass of earth. Such systems have three main
parts
● an air terminal, a
sharp copper spike;
● a system of down
conductors made of robust copper strip;
● an earth termination,
providing a solid and reliable connection with
the general mass of
earth so that the energy can be harmlessly dispersed
into the ground.
The building receives a
zone of protection from a lightning strike in the
shape of a cone,
starting at the highest point of the air terminal of the lightning
protective system.
Lightning protective
systems are discussed in detail in Advanced Electrical
Installation Work under
the sub-heading ‘ Protection of structures against
lightning ’ .
The IEE Regulations make
specific reference to many British Standard
specifications and
British Standard codes of practice in the 17th edition of
the Regulations.
The Electricity Supply
Regulations forbid electricity supply authorities
to connect electric
lines and apparatus to the supply system unless their
insulation is capable of
withstanding the tests prescribed by the appropriate
British Standards.
It is clearly in the
interests of the electrical contractor to be aware of,
and to comply with, any
regulations which are relevant to the particular
installation. IEE
Regulation 134.1.1 states that good workmanship by competent
persons and the use of
proper materials are essential for compliance
with the Regulations.
In order to try to
ensure that all electrical installation work is carried out
to a minimum standard,
the National Inspection Council for Electrical
Installation Contracting
(NICEIC) was established in 1956. NICEIC is supported
by all sections of the
electrical industry and its aims are to provide
consumers with
protection against faulty, unsafe or otherwise defective
electrical
installations. It maintains an approved roll of members who
regularly have their
premises, equipment and installations inspected by
NICEIC engineers.
Through this inspectorate the council is able to ensure
a minimum standard of
workmanship among its members. Electricians
employed by an
NICEIC-approved contractor are also, by association with
their employer, accepted
as being competent to carry out electrical installation
work to an approved
standard.
Lightning rods or 'air terminals' are only a small part of a complete
lightning protection system. In fact, the rods may play the least important
role in a system installation. A lightning protection system is composed of
three main components:
1. Rods or 'Air Terminals' - The small, vertical protrusions
designed to act as the 'terminal' for a lightning discharge. Rods can be found
in different shapes, sizes and designs. Most are topped with a tall, pointed
needle or a smooth, polished sphere. The funtionality of different types of
lightning rods, and even the neccessity of rods altogether, are subjects of
many scientific debates.
2. Conductor Cables - Heavy cables (right) that carry
lightning current from the rods to the ground. Cables are run along the tops
and around the edges of roofs, then down one or more corners of a building to
the ground rod(s).
3. Ground Rods - Long, thick, heavy rods buried
deep into the earth around a protected structure. The conductor cables are
connected to these rods to complete a safe path for a lightning discharge around a structure.
The conductor cables and ground rods are the most important components of a
lightning protection system, accomplishing the main objective of diverting
lightning current safely past a structure. The 'lightning rods' themselves,
that is, the pointy vertically-oriented terminals along the edges of roofs, do
not play much of a role in the functionality of the system. A full protection
setup, given good cable coverage and good grounding, would still work
sufficiently without the air terminals.
A lightning protection system's only purpose is to ensure safety to
a building and its occupants if lightning happens to hit it directly, a
task accomplished by providing a good, safe path to ground for the lightning to
follow. Contrary to the myths, lightning protection systems:
·
Don't attract lightning
·
Don't and cannot dissipate
or prevent lightning by 'draining' a storm of its charge
·
Most don't offer surge protection for sensitive
electronics
·
Do offer fire
protection and structural damage protection by preventing a hot, explosive
lightning channel from passing through building materials
Without a designated path to reach ground,
a lightning strike may choose to instead utilize any conductor available inside
a house or building. This may include the phone, cable, or electrical lines,
the water or gas pipes, or (in the case of a steel-framed building) the
structure itself. Lightning usually will follow one or more of these paths to
ground, sometimes jumping through the air via a side flash to
reach a better-grounded conductor (watch animation above). As a result,
lightning presents several hazards to any house or building:
·
Fire- Fire can start
anywhere the exposed lightning channel contacts,
penetrates or comes near flammable material (wood, paper, gas pipes, etc) in a
building - including structural lumber or insulation inside walls and roofs.
When lightning follows electrical wiring, it will often overheat or even
vaporize the wires, creating a fire hazard anywhere along affected circuits.
·
Side flashes - Side flashes can jump across rooms, possibly injuring anyone who
happens to be in the way. They can also ignite materials such as a gasoline can
in a garage.
·
Damage to building materials - The explosive shock wave created by a lightning
discharge can blow out sections of walls, fragment concrete and plaster, and
shatter nearby glass.
·
Damage to appliances - Televisions, VCRs, microwaves, phones, washers, lamps and just
about anything plugged into an affected circuit may be damaged beyond repair.
Electronic devices and computers are especially vulnerable.
Adding a protection system doesn't prevent a strike,
but gives it a better, safer path to ground. The air terminals, cables and
ground rods work together to carry the immense currents away from the
structure, preventing fire and most appliance damage:
Surge protectors and UPS units are not suitable lightning protection
devices. These appliances provide some degree of protection from voltage spikes
from everyday power surges and distant lightning strikes. But when lightning
strikes a structure directly or very close to it, lightning protection system
or not, all bets are off.
A common surge protector simply cannot have any effect on the violent,
catastrophic burst of current from a very close or direct lightning strike.
Direct lightning current is simply too big to protect with a little electronic
device inside a power strip, or even a hefty UPS unit. If your UPS or surge
protector is in the way of the lightning's path, all or part of the lightning
will just flash over or through the device - regardless of the amount of
capacitors and battery banks involved.
Even 'disconnects', or devices that physically switch off power to a device
by activating a set of contacts, will not guarantee protection. A small air gap
will not stop a lightning bolt that has already jumped across miles of air. It
won't think twice about jumping a few more inches, or even a few more feet,
especially if the 'path of least resistance' to ground is across the contacts
of the disconnect switch.
Not only that, but not even a full-fledged lightning protection system with rods, cables and grounds will guarantee against damage to
electronics and computers. For any system to provide 100% protection, it must
divert almost 100% of the lightning current from a direct strike, which is
nearly physically impossible: Ohm's Law states that for a set
of resistances connected in parallel, the current will be distributed across
ALL resistances, at levels inversely proportional to the different values of
resistance. A house or building is nothing more than a set of resistors
'connected' in parallel- the electrical wiring, plumbing, phone lines, steel
framework, etc. (Even though plumbing and electrical wiring, for instance, may
not be physically connected, lightning will use side flashes across
air gaps to effectively connect them). In a direct lightning strike, the
current will not follow only one path- it will distribute itself across all
paths to ground depending on each path's resistance.
Lightning current often peaks at 100,000 or more Amperes. With that in
mind, consider if you have a lightning protection system installed, and your
house is hit directly by lightning. If the protection system takes even 99.9%
of the current, then your electrical wiring may take the remaining 0.1%. 0.1%
of 100,000 Amperes is a 100 Amp surge through your lines- which may be enough
to take out your computer.
It is not uncommon for 'side flashes' to occur inside a house or building,
where all or a part of the lightning will jump across an entire room to reach
ground- such as from the electrical wiring system to well-grounded water pipes.
If your computer is in the way, it'll be time to shop for a new one, even if
you have the most expensive protection system installed.
Guarantees on the packaging of UPS/surge protection devices are somewhat
misleading when it comes to lightning protection, implying that the devices can
stop any effects of a strike. In some cases, they will - as long as they aren't
in or near the direct line of fire. But in reality, nothing can guarantee
absolute protection from a direct or very close strike.
All this doesn't mean that you shouldn't use a surge protector, UPS,
disconnect, or a full-fledged lightning rod system. Any device will provide some degree
of protection from everyday power line spikes and distant lightning strikes.
But when lightning hits nearby or directly, all bets are off.
The best, and cheapest, way to protect your stereo, television, computer,
or any electronic appliance is to unplug all power, telephone, cable,
(modem), and antenna connections during a thunderstorm.
Some could argue that the risk of a direct strike to any given house is too
low to justify unplugging everything for every storm that passes overhead.
There is some truth to that. It's wise then to make sure your homeowner's or
renter's insurance covers lightning damage, and all of your devices are
inventoried and covered by the policy. Insured expensive electronics can be
replaced, after all. However, consider irreplaceables such as the data saved on
your computer (photos, videos, work files, etc). You can mitigate that risk by
performing frequent offsite backups and/or storing data on an external hard drive
that you can unplug when needed.
Products called 'lightning elimination' or 'lightning dissipation' devices
have arisen as a result of two myths: one, that a thunderstorm's charge can be
drained or otherwise affected by objects on the ground, and two,
cloud-to-ground lightning discharges begin from the ground. These products,
that are still being sold today, claim to be able to prevent a direct lightning
strike to any object on which they are installed. The devices have widely
varying appearances, but usually are characterized by a metallic frame with
hundreds of sharp-pointed bristles, needles or thin rods. The frame designs
range from comb-like to umbrella-shaped.
The devices are said to prevent or reduce direct lightning strikes to
objects on which they are installed, using corona discharge to perform one or
more of the following: 1.) to drain a storm of its charge before lightning can
occur, 2.) to create a localized 'space charge' over the protected area that
diverts lightning strikes, or 3.) to make initiation of upward leaders from the
object more difficult, thereby reducing the chances of a direct stepped
leader-ground leader connection.
Cloud-to-ground lightning strokes initiate high in thunderstorms, miles
above the surface where ground objects have no effect. Even after initiation of
the discharge, the downward-moving stepped leader is 'blind' to objects on the
ground until it is very close to the ground, within 50 to 100 feet. At that
distance, lightning will strike within the very small area it is already
descending in, regardless of any devices nearby that claim to divert or prevent
the strike. For example, a photograph exists of a lightning strike to the Merchandise
Mart building in downtown Chicago. Merchandise Mart is very close to the 1,700
foot tall Sears Tower, yet not even the Sears Tower influenced the ground
connection of this close cloud-to-ground stroke.
In addition to the obvious scientific flaws with the concept of lightning
'dissipation' and 'elimination' devices, they have been proven to be
ineffective in real-world installations. Many 'lightning dissipation' devices
on towers and buildings have been struck directly. Despite the evidence, they continue
to be sold, installed and promoted.
Rods and protection systems don't
attract lightning, nor do they influence where lightning will strike. Rods or protection systems do not
and cannot prevent lightning, nor can they 'discharge' thunderstorms. Lightning protection systems
(including placement of rods, cables, and groundings) are custom-designed for
individual structures and require complex engineering to function properly.
They should only be installed by qualified contractors. Lightning protection systems do not
always prevent damage to electronics or computers. You should still unplug such
devices during thunderstorms to ensure sufficient protection
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