Increases (up to 6,000V), lasting less than a millisecond, which are induced onto power, data communication, video, signal or telephone lines during lightning. These currents are conducted along the cables, overhead or underground, seeking a path to earth, and damage any electronic equipment through which they flow.
Transients and damage
Components like microchips can be completely destroyed - burnt out - by the effects of transients. More insidiously, transients can degrade components, which does not cause the equipment to fail immediately, but reduces its lifetime, so that failures may occur days or weeks later. As electronic components - linked by power, video, signal and telephone cables - are widely used in alarm systems, transients can seriously compromise the reliability of security, monitoring or access systems.
Assessing the risk
The British Standard on structural lightning protection, BS 6651:1999, carries detailed risk assessments for both direct strikes and transients (Appendix C). Interestingly, if both these are applied to the same building, the risk of transient overvoltage damage is invariably greater than the risk of structural damage.
Why? Because to damage a building, lightning has to actually hit it, whilst to create transients, lightning only has to strike close to a building or the cables running into it. As a rule of thumb, a lightning strike up to 1 kilometre from a building can cause transients destructive to its electronic systems.
Increasing risk
With advances in technology, the threat from transients is becoming ever more pressing. In the security market, the use of sophisticated electronics has both improved existing technologies like presence or vibration detectors, as well as providing the basis for a new wave of security solutions, such as iris or fingerprint recognition systems for access control. Also, many mechanical operations have become computerised; car parks that used to have attendants, now have CCTV cameras and automatic barriers. And the electronics themselves are developing, becoming more powerful, but also smaller, and therefore more sensitive to transient overvoltages.
Not convinced? Recently an Oxfordshire company installed a new alarm system, which included a master and slave control panel, networked via underground cabling. During a lightning storm both panels and some associated equipment were damaged. They cost several thousand pounds to repair, but worse still was the associated cost in safety terms of the alarm being out of operation.
Perhaps this site was just unlucky, but with up to half a million lightning strikes to ground in the UK each year, it's more a question of when, not if, lightning strikes close enough to cause problems.
Protection
So how can vital electronic systems be protected? The theory is quite simple: protectors are installed between the equipment and the transient. This means that protectors need to be fitted on all power, data communication, signal, video or telephone lines which enter or leave the building. Fibre optic cables, which are not conductive, do not need transient overvoltage protection.
For example, power lines should be protected at the main LV distribution board, and also to any outgoing power supplies, eg to another building or to a CCTV camera. Additional protectors can be installed at local distribution boards to protect important pieces of equipment from transients injected onto the mains power after the main LV board.
A practical example was highlighted during a recent site survey at a prison in southern England. The engineer recommended protectors to be fitted to the video, telemetry and power lines entering the control room from the external CCTV cameras. In addition, mains power protectors were advised to protect the UPS and associated computer equipment, and another protector on the main power distribution board to the room. Each CCTV camera was also protected by protectors on its power, telemetry and video cables, mounted on the camera pole.
Conclusion
By its nature, this article has only the scope to introduce transient overvoltages, to indicate the nature and extent of the risk, and to highlight the key areas concerning the deployment of protectors. However, any reputable manufacturer will be able to provide more detailed information.
After all, it only takes one strike . . . . .
Source
SMT
Postscript
Mary Taylor is with Furse, a manufacturer of lightning and transient overvoltage protection.
