Furse Lightning Protection
Lightning Protection & Transient Overvoltage Protection
Structural Lightning Protection
BS EN 62305:2006 (Protection against lightning)
clearly advises strict adherence to the provision of a conventional
(or Faraday Cage) lightning protection system (LPS).
External Lightning Protection System (LPS)
An external LPS is termed:
a) 'Isolated' - typically a catenary system suspended over the
structure
b) 'Non-isolated' - typically a mesh system located directly on the
roof of the structure
An external LPS consists of:
a) Air termination system
b) Down conductors
c) Earth termination system
These individual elements of an LPS should be
connected together using appropriate lightning protection
components (LPC) complying with BS EN 50164 series. This will
ensure that in the event of a lightning current discharge to the
structure, the correct design and choice of components will minimize
any potential damage.
Internal Lightning Protection System
The fundamental role of the internal LPS is to
ensure the avoidance of dangerous sparking occurring within the
structure to be protected. This could be due, following a lightning
discharge, to lightning current flowing in the external LPS or
indeed other conductive parts of the structure and attempting to
flash or spark over to internal metallic installations.
Carrying out appropriate equipotential bonding
measures or ensuring there is a sufficient electrical insulation
distance between the metallic parts can avoid dangerous sparking
between different metallic parts.
Lightning Equipotential Bonding
Equipotential bonding is simply the electrical
interconnection of all appropriate metallic installations/parts,
such that in the event of lightning currents flowing, no metallic
part is at a different voltage potential with respect to one
another. If the metallic parts are essentially at the same potential
then the risk of sparking or flash over is nullified.
This electrical interconnection can be achieved
by natural/fortuitous bonding or by using specific bonding
conductors. In accordance with BS EN 62305, the use of lightning
current/equipotential bonding surge protection devices (SPDs) is
required where the direct connection with bonding conductors is not
suitable - for example metallic power and telecommunication lines.
Classes of LPS
There are four classes of LPS (I, II, III, IV)
which have corresponding mesh conductor sizes, down conductor
spacings and where appropriate different rolling sphere radii.
Down Conductors
The down conductor spacings range from 10 m for a
class I LPS up to 20 m for a class IV LPS.
BS EN 62305:2006 permits the use of 'natural
conductors' such as rebars and structural steelwork, provided that
they are electrically continuous and adequately earthed.
Earth Termination System
The earth termination system is vital for the
dispersion of the lightning current safely and effectively into the
ground. Although lightning current discharges are a high frequency
event, at present most measurements taken of the earthing system are
carried out using low frequency proprietary instruments. The
standard advocates a low earthing resistance requirement and points
out that can be achieved with an overall earth termination system of
10 ohms or less.
BS EN 62305-3 recommends a single integrated
earth termination system for a structure, combining lightning
protection, power and telecommunication systems.
Two basic earth electrode arrangements are used.
(i) Type A arrangement - This consists of
horizontal or vertical earth electrodes, connected to each down
conductor fixed on the outside of the structure. This is in essence
the earthing system used in BS6651 where each down conductor has an
earth electrode (rod) connected to it.
(ii) Type B arrangement - This arrangement is
essentially a ring earth electrode that is sited around the
periphery of the structure and is in contact with the surrounding
soil for a minimum 80% of its total length (ie 20% of its overall
length may be housed in say the basement of the structure and not in
direct contact with the earth).
The ring electrode should preferably be buried at
a minimum depth of 0.5 m and about 1 m away from the external walls of
the structure.
Where bare solid rock conditions are encountered,
the type B earthing arrangement should be used.
The type B ring earth electrode is highly
suitable for:
-
Conducting the lightning current safely to
earth
-
Providing a means of equipotential bonding
between down conductors at ground level
-
Controlling the potential in the vicinity of
conductive building walls
Protection of Electrical & Electronic Systems
BS EN 62305-4 defines the protection against
Lightning Electromagnetic Impulse (LEMP). The basic protection measures
include:
Earthing & Bonding
Sensitive electronic systems housed within a structure
require a type B earthing arrangement. A low impedance-bonding network is
required to avoid dangerous potential differences between all equipment
housed within the inner zones of the structure.
Magnetic Shielding & Line Routing
The following measures will significantly reduce the
surges/transient overvoltages entering the structure.
The use of reinforcing bars, stanchions etc to create a
spatial shield or screen.
Suitable routeing of internal lines minimizes induction
loops and reduces internal surges.
The shielding of cabling and equipment by use of metallic
cable ducts and metallic enclosures.
Effective Earthing
The dangers posed to both life and equipment by poor
earthing make effective earthing essential. Given the complexity of national
and international standards in earthing system design, material
specification and installation, it makes sense to talk to Furse.
Furse design and model earthing electrode systems in
compliance with BS 7430:1998, IEEE standard 80:2000, BS 7354:1992, EATS
41-24:1992 and other accepted standards. The comprehensive Furse range of
earthing equipment includes solid copper, stainless steel and copperbond
earth rods and accessories, high copper alloy bonds and clamps, earth pits,
solid copper plates, lattice mats, earth rod seals and the FurseWELD
exothermic welding system.
Using & Choosing Transient Overvoltage Protection
Damaging transient overvoltages caused by lightning (or
electrical switching) can be conducted into electronic equipment on mains
power, data communication, signal and telephone lines. Therefore, as a
general rule, protectors should be fitted to all cables that enter a
building, or travel between buildings. The exception is fibre optic cabling,
which, of course, is a non-conductive medium.
A number of products are available which claim to protect
electronic equipment against lightning induced transient overvoltages.
However, tests have shown that many of these units have unacceptably high
‘let-through’ voltages, which leave the electronic equipment open to damage.
In order to provide effective protection, a transient
overvoltage protector should:
-
Have a low ‘let-through’ voltage for all combinations
of conductors
-
Be compatible with the system it is protecting
-
Survive the transient
-
Not leave the user unprotected as a result of failure, & be
properly installed
‘Let-through’ Voltage
The larger the transient overvoltage reaching the electronic
equipment, the greater the risk of disruption, degradation or physical damage to
its components. Thus the let-through voltage of the protector should be lower
than the level at which interference or component degradation may occur.
The let-through voltage should be equally low between any two
conductors. Because transients can exist between any pair of conductors (phase
and neutral, phase and earth, neutral and earth on mains power supplies, and
line to line and line to screen/earth on data communication, signal and
telephone lines), the let-through voltage between any pair should be below the
level at which the system can suffer damage. BS EN 62305 recognizes protectors
with low let-through voltages as “enhanced” protectors, which further minimize
the risk of damage and disruption.
Compatibility
It is important that the protector does not interfere with or
restrict the system’s normal operation.
-
Mains power protectors should not disrupt or corrupt the
continuity of the supply, nor introduce high earth leakage currents
-
Data communication, signal & telephone protectors should not
impair or restrict the system’s data or signal transmission, as a result, for
example, of the protector’s maximum operating voltage, current rating, in-line
impedance or bandwidth
Survival
Although lightning discharges can have currents of 200 kA,
transient overvoltages caused by the secondary effects of lightning are unlikely
to have currents exceeding the 10 kA (8/20 waveform). The protector should
therefore be rated for a peak discharge current of not less than 10 kA. Lightning
is a multiple pulse event, and so the protector should not fail after the first
transient.
Protection Failure
When in-line protectors fail - for example those used on data
communication, signal and telephone lines - they take the line out of
commission, thereby preventing damage to the system. However, it is unacceptable
for protectors on mains power distribution systems to fail by short circuit. It
is therefore important that protectors for mains power distribution systems have
a properly indicated pre-failure warning, whilst protection is still present.
Installation
The performance of transient overvoltage protectors is
heavily dependent upon their correct installation. For example, the length and
configuration of connecting cables can increase the protector’s let-through
voltage. Thus, transient overvoltage protectors must be supplied with detailed
installation instructions, and the person performing the installation should
conform to these installation instructions.
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