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Category: Furse Lightning Protection and Transient Overvoltage ProtectionFurse Structural Lightning ProtectionBS 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: Internal Lightning Protection SystemThe 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. Lightning Equipotential BondingEquipotential 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 Classes of LPSThere 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 ConductorsThe down conductor spacing range from 10m for a class I LPS up to 20m
for a class IV LPS. Earth Termination SystemThe 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. Protection of Electrical & Electronic SystemsBS 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 & BondingSensitive 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 RoutingThe following measures will significantly reduce the surges/transient
overvoltages entering the structure. Effective EarthingThe 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. Using & Choosing Transient Overvoltage ProtectionDamaging transient overvoltages caused by lightning can be conducted
into electronic equipment on mains power, data communication, signal and
telephone lines. ‘Let-through’ VoltageThe 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. CompatibilityIt 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 FailureWhen 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 InstallationThe 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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