Transient overvoltages are significant increases in voltage that occur in fractions of a second and can cause devastating damage. But where do surges come from? Like with all threats, to manage them effectively it is vital to establish their origins. There are two main sources of transient overvoltages being the consequential impacts of lightning strikes (which can occur in excess of a kilometre away from the location of the surge); or through switching of sizeable loads be they transformers, large motors, light-rail systems, or through capacitive loads through the power supply.
Surge Protection & Lightning Protection Systems
Surge protection has always been closely linked to lightning protection systems due to the logical link between the management of, and protection against, substantial fault currents. It is pertinent to consider that a structure is usually at greater threat from lightning surge current damage than from the threat of a direct lightning strike.
The previous iteration of lightning protection standard, BS 6651, held information on integrating surge protection devices within an appendix; however BS EN (IEC) 62305 now holds surge protection as a distinct aspect of the standard (part 4), which must be addressed in any structure that claims to be compliant with this standard. BS EN (IEC) 62305-4 recommends a number of methods to protect against transient overvoltages cause by lightning, such as effective earthing and bonding, electromagnetic shielding and line routing, and importantly coordinated surge protection devices; these measures directly impact on the performance of the internal measures of a lightning protection system. The function of the internal lightning protection system is to prevent dangerous sparking between areas of differing electrical potentials, and to prevent the fault currents causing damage in the form of fires, explosions or equipment damage.
A philosophy which underpins the whole of BS EN 62305 is that of lightning protection zones. The concept is that any complex, facility, building or room can be split into zones which have different requirements for lightning protection, and then appropriate protection measures can be broken down into that particular zone. A building may have an external lightning protection system that will stop the building from being damaged, or catching fire. Inside the building there may be equipment such as a computer server within a dedicated room. The server will need further protection to ensure that it is protected from surges, therefore being a lightning protection zone, within a lightning protection zone. It is always worth remembering to attempt to surge protect locally to equipment to ensure that it is managed effectively, and to avoid allowing fault currents inducting downstream of the surge protection device.
Surges can carry through data, power and telephone lines and it is prudent to install surge protection measures on the local power supply to vital equipment, any electronic equipment outside the main structure; particular attention should be given to cables which penetrate the external envelope of the structure. Examples of such items can include CCTV systems, air handling units, power, data and telecom lines, meteorological sensors, aerials, antennas or satellite dish receivers.