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Lighting And Surge Protection
8/1/2010Lightning has long fascinated the technical community. Ben Franklin studied lightning's electrical nature over two centuries ago and Charles R Steinmetz generated artificial lightning in his General Electric laboratory in the 1920's. As someone concerned with premises data communications you need to worry about lightning. Here I will elaborate on why, where and when you should worry about lightning. I'll then discuss how to get protection fromit.
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Coming right down to it, a lot can be done as far as protection is concerned. However, it is best to begin by describing the magnitude of the threat from which you need protection.
The first stroke of lightning during a thunderstorm can produce peak currents ranging from 1,000 to 100,000 Amperes with rise times of 1 microsecond. It is hard to conceive of, let alone protect against, such enormous magnitudes. Fortunately, such threats only apply to direct hits on overhead lines. Hopefully, this is a rare phenomenon.
More common is the induced surge on a buried cable. In one test, lightning-induced voltages caused by strokes in ground flashes at distances of about 5 km were measured at both ends of a 448 meter long, unenergized power distribution line.
80 Volts is well within the range of affordable protection.Conceptually, lightning protection devices are switches to ground. Once a threatening surge is detected, a lightning protection device grounds the incoming signal connection point of the equipment being protected. Thus, redirecting the threatening surge on a path-of-least resistance (impedance) to ground where it is absorbed.
Any lightning protection device must be composed of two "subsystems," a switch which is essentially some type of switching circuitry and a good ground connection-to allow dissipation of the surge energy. The switch, of course, dominates the design and the cost. Yet, the need for a good ground connection can not be emphasized enough. Computer equipment has been damaged by lightning, not because of the absence of a protection device, but because inadequate attention was paid to grounding the device properly.
The basic elements used as protective switches are: gas tubes, metal oxide varistors and silicon avalanche diodes (transorbs). Each has certain advantages and disadvantages.
Because they can withstand many kilovolts and hundreds of Amperes, gas tubes have traditionally been used to suppress lightning surges on telecommunications lines. This is just what is needed to protect against a direct strike. Because gas tubes have a relatively slow response time, this slowness lets enough energy to pass to destroy typical solid state circuits.
Metal oxide varistors (MOVS) provide an improvement over the response time problem of gas tubes. But, operational life is a drawback. MOVs protection characteristic decays and fails completely when subjected to prolonged over voltages.
Silicon avalanche diodes have proven to be the most effective means of protecting computer equipment against over voltage transients. Silicon avalanche diodes are able to withstand thousands of high voltage, high current and transient surges without failure. While they can not deal with the surge peaks that gas tubes can, silicon avalanche diodes do provide the fastest response time. Thus, depending upon the principal threat being protected against, devices can be found employing gas tubes, MOVS, or silicon avalanche diodes. This may be awkward, since the threat is never really known in advance. Ideally, the protection device selected should be robust, using all three basic circuit breaker elements.
