The greatest fear when lightning occurs is that expensive electronic equipment, computers and important networked control systems will be damaged making all aspects of the system vulnerable to certain elements.
South Africa does not have the highest lightning strikes or storms in the world - as some think. Johannesburg experiences approximately six to seven strikes per km2 and around 80 lightning storm days per year, while Uganda has recorded 30 strikes per km2 and around 260 lightning storm days per year.
Systems can be damaged as a result of direct strikes to buildings, overhead lines or high mast lighting. It can also be a result of the effects of lightning, inducing large surge voltages and currents into electrical and electronic equipment up to 1 km away from the point of strike. There is no possible means to prevent direct lightning strikes to equipment or buildings. However it is possible to safely conduct the lightning currents to earth. Lightning masts that are used to protect hazardous areas such as explosive magazines, explosive gases, switchyards or thatch-roofed buildings, do not prevent lightning strikes but act as a preferential point of strike.
This means that the lightning strike should strike the mast rather than the structure within its zone of protection. No external protection system can prevent damage to sensitive electrical or electronic equipment. It is often suggested that by putting up a mast one can prevent damage to equipment, but in actual fact, the problem can be worsened. A 30 m mast has an attractive radius of 108 m which means that lightning that would have struck 108 m away will now strike next to one's equipment.
In order to prevent damage to equipment, one requires an external protection system to provide a controlled path to earth in the event of a direct lightning strike as well as internal protection to protect against the induced voltages and currents.
Power protection
Due to the fact that the Eskom network is so vast, the system is very exposed to lightning and overvoltage damage via the grid. All systems receive power either directly or via a transformer or battery which needs a charger to keep it operational. The choice of the surge arresters for the power is also important in order to offer the correct level of protection.
It must also be noted that two surge arresters must be fitted for single-phase supplies and four surge arresters for three-phase supplies. The protection should be capable of withstanding nominal surge currents of 10 kA and limiting the voltage to less than 950 V. They should also have visual indication should they ever be over-stressed. The DEHNguard 275 and HDO 280/T are suitable for this level of protection.
Networked electronics
As more and more systems are becoming computer or microprocessor-controlled, they are exposed not only to lightning and overvoltage damage via the power, but also via the data or signal cables. These cables must be protected with protection modules capable of withstanding the expected surge currents. For cables that do not leave the building but are fairly long (around 50 m), surge arresters capable of repeatedly withstanding surges of 2,5 kA should be installed. For cables that leave the building, surge arresters capable of repeatedly withstanding surges of 10 kA should be installed.
Typical cables that require protection include intercom, data, signal and video systems. Each system requires different protection techniques. For example, a four-wire RS232 protector cannot be used on a four-wire RS422 system as the protection levels differ totally, as well as the protection layouts.
Surge Technology has developed over 200 specific protection modules for the various computer and instrumentation protocols, video/intercom, radio and other related systems. Alarms are linked via telephone lines or modem lines, which also require protection as they are exposed to damage via the telecommunication network. Telkom and most suppliers supply some form of protection built into their units, but experience has shown that these units cannot withstand very large surges often experienced on these lines. With the advent of fibre-optic cables, telecommunication lines are becoming less exposed to overvoltage damage.
Installation
Lightning and overvoltage protection can only operate if correctly installed and earthed. In most cases the electrical earth is adequate, as the electrical regulations are well enforced. The system is normally referenced to the electrical earth and most voltages developed will be with reference to earth (common mode). There will also be voltages developed between wires (transverse mode) but these are not normally so large.
It is important to follow manufacturers' instructions when installing the lightning and overvoltage protection modules. Clean, protected wires and dirty, unprotected wires must be kept separate as to limit the effects of induction which can be as high as 300 V/m under surge conditions.
The lightning induced effects can be considerably reduced by correct screening and shielding techniques and by fitting good quality surge arresters. All too often the cables are installed in plastic conduit which offers no screening or shielding. If plastic conduit is to be used, a screened cable should limit the electromagnetic influences of the lightning surges.
Cable screens, racks, pipes and conduits must be earthed to offer the correct screening and shielding. If a screen is earthed at one end only, it can only act as an electrostatic screen. In cases where there are problems created by circulating earth currents, one end of the screen can be earthed via a spark gap to give a controlled path to earth under surge conditions. Double-screened cables are designed with the outer screen earthed at both ends and individual screens earthed at one end only.
Conclusion
There is no reason why electrical or electronic equipment should suffer damage as a result of lightning or overvoltage as the technology exists to protect against the effects of both direct and induced lightning surge currents. For the protection to be effective, it must be correctly chosen for the application and then correctly installed and maintained.
Surge Technology, (011) 792 1303
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