Earthing

The out side Lightning conductor should act as a safety bypass for any possible discharge so as to prevent any surge related damage. The actual ohmic value of either should be the lowest value that can be achieved. The two systems should be completely isolated from one another so that any ground current can not flow back to the equipment earth. In the event of a storm blowing in and if it can be disconnected from both the power feed and earth the equipment should be isolated. Let the proper lightning conducter take the hit it should be robust enough to survive and give good protection to the structures it is there to protect. Over voltage protectors surge arresters should be deployed. VDR's, Spark gaps, etc. Faraday cages. What ever does the job.

Original question: "A communication tower(30m tall) has a lightning protection system of earth value 5 ohm. The communication equipment in an adjacent building (well within the protection of lightning) is provided with a distinct earth measuring 1.5 ohm. Is it true that the lightning earth value should be lower than the equipment earth. Why?"

Answer: This problem is one MCI Telecommunications solved in the mid 1980s after experiencing a major lightning strike outage in Atlanta. Lightning hit the microwave tower and followed the wave guides into the building to find a lower impedance ground. The tower ground had a higher resistance than the building ground and the best ground with the least resistance happened to be the DAC's ground grid below the concrete floor. There in is the answer. Lightning is unpredictable, however, the best bet is that it will seek the path of least total resistance. A rule of thumb is that for every thousand volts a one inch arc is likely to occur. This bit of knowledge comes in handy when trying to understand this problem. Lightning will seek the path of least resistance. Therefore the ability to jump from one ground system to another with lesser resistance is likely to occur when proximity allows that to happen. The cure to the problem was worked out and implemented at all MCI sites but how we did it remains proprietary... Excellent question by the way! JWS senior engineer

PS: I have a question... What does "(well within the protection of lightning)"in your statement mean? Might I suggest looking at Michael Faraday's work in particular the Faraday cage experiments... It is great work and is very helpful in understanding the control of lightning and RFI phenomena.

The relationship between the resistance of the lightning earth and the resistance of any other earth is almost irrelevant. The sole requirement for the resistance of the lightning earth is that insufficient potential is generated across it to cause arcing to other equipments (or people).

Note that ensuring low enough resistance is not of itself always sufficient to protect other equipment within the building. Induction effects can cause damage without any direct contact. This is where multiple routing of the lightning current grounds to form what is effectively a Faraday cage can be beneficial.

Where groups of equipment have to operate together, but not to interface to other areas, it has even been known to use a suitable local point on the lightning conductor as the local earth; but this should not be undertaken lightly, as every conductor (including at least heating ducts and often building reinforcements) within the area has to be connected to this same earth, and isolated from all others. The specification for the isolation transformer for mains can be quite spectacular. This is really only a sensible solution where there is no way to avoid exposing sensitive outputs to the local ambient potential.

As electricity always seeks the path of lowest resistance, the lower the resistance level of the lightning conductor earthing point, the more effective the protection it offers.

One thing we all are ignoring is that: "IT IS THE IMPEDANCE OF THE ELECTRICAL CIRCUIT & NOT ONLY THE RESISTANCE OF PATH TO GROUND" As the lay-out of Earthing conductor to ground & Type of conductor ie 1. Strip 2. Multi-strand twisted Wire 3. Braid etc Strip has minimum inductance; 3 or more strips making a circle around the Tower or Building; making Faraday Cage Effect. Another point is that a full length without a joint is a must