Category: Furse Lightning Protection and Transient Overvoltage
Protection
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Furse Lightning Protection and 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.
Bonding can also be accomplished by the use of surge protection devices
(SPD's) 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 spacing range from 10m for a class I LPS up to 20m
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 0.5m and about 1m 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 and bonding magnetic shielding and line routeing surge
protective device set.
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 minimises 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 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, and 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.
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 and 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 200kA,
transient overvoltages caused by the secondary effects of lightning are
unlikely to have currents exceeding the 10kA worst case indicated in BS
6651:1999 and IEEE C62.41. The protector should therefore be rated for a
peak discharge current of not less than 10kA. 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.
Information associated with:
Furse - Lightning protection
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