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EM Induction of a Lightning Strike

Question: If lightning strikes a lightning rod, or a TV/telecomm tower, it will also produce EM fields in the surrounding area. This indirect effect can cause damage to any sensitive electronic equipments. My questions are:

  1. What is the average radius of the area within which EM fields can damage electronic equipment?
  2. What protection should be provided to avoid the damage?

Kind regards,
Rinaldi A. Rianda
Network Quality Assurance
PT. Telkomsel
Indonesia

Answer:
1) A nearby lightning stroke will cause significant electromagnetic (EM) fields regardless of whether it terminates on a lightning rod, a nearby telecommunication tower, a tree, the building itself, or any other ground object.

2) In case of the lightning rods of typical length, installing these does not materially change the frequency of lightning strikes to the building. Hence their existence is not relevant to the question of induction. In other words, lightning would have struck the building regardless of their existence, and the same inductive effects would have been encountered. The only difference is that providing protection against direct strokes improves the situation by eliminating the risk of fire to the structure itself.

3) In case of tall towers, they collect many of the strokes, which would otherwise have terminated on the building itself. Hence the increase in the frequency of induction incidents is at least partially compensated by the elimination of some direct strike events.

4) The bigger issue in case of telecommunication towers is that the ground potential rise generated by lightning strikes may be transmitted into nearby buildings if the power supply system of the tower is not isolated from the power supply to adjacent buildings.

5) The extent of the area within which the EM field will exceed a certain value will depend on magnitude of the stroke, which varies in the range 2-200 kiloampere. In dealing with protection of equipment, average values do not help. You need to know either the maximum value or the value, which will not be exceeded, say, 90% of the time.

6) The question of risk of damage to electronics involves both magnitude of the EM field and susceptibility of the equipment. The latter, and more important factor, depends on extent of the wiring loops within which induction will take place and also on the withstand voltage of the equipment.

7) Induction aside, the equipment will still be open to damage via the surges that enter the building over the power and communication lines. Hence surge protection will be necessary regardless.

8) From the practical point of view, the protection provided to deal with incoming surges will be quite adequate to deal with induced surges as well. Hence no special provision is really needed to address the issue of induction.

9) In summary:
a) The issue of induction, with all its complexities and uncertainties, can be ignored where the protection of equipment within a building is concerned, and,
b) In case of telecommunication towers, their power supply must be isolated from the general power distribution system.

Abdul M. Mousa, Ph.D., P. Eng., Fellow IEEE

Dear Mr. Holt:

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