Chapter 7 Protection Against Lightning - Fujitsu M10 Series Installation Planning Manual

CHAPTER 7 Protection Against Lightning

1
If a low-voltage distribution cable that feeds power directly to devices or an interface
cable is to be laid outdoors, safeguards are needed to protect against possible
destruction caused by lightning surges.
If a device is damaged by a lightning, the direct cause is a surge (abnormal voltages
and currents). Lightning surges can be classified into four cases:
1 Direct inflow of current into cables or devices caused by a direct lightning strike to
the cable or device or by a lightning strike to the ground.
2 Generation of a surge voltage or current resulting from a large ground potential
difference between devices caused by lightning near any one of these devices or any
interconnecting cable.
3 The induction of a current surge through a cable resulting from lightning near the
cable.
4 The release of charge which has been captured by thunder cloud and accumulated
on a cable, and which flows as a surge.
Generally, the phenomena outlined 1 and 2 are called direct strikes, while those
outlined in 3 and 4 are called indirect strikes. Direct strikes have such a huge
destructive energy that protection against them is extremely difficult to achieve.
Indirect strikes, on the other hand, have by far a less destructive energy, and surge
absorbers will usually provide protection against them.
Protection, however, will not be available against surges that are beyond the
performance limits of the surge absorbers.
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