Chapter 13: Lightning Protection

Lightning is an electrical discharge pulse in the atmosphere, which averages 20 kA or more in current. Commonly, these discharges are viewed as a flash from cloud to ground, although it can flash from the ground to the cloud. Once ionization of the atmosphere occurs, this becomes a luminescent, conductive plasma (a lightning "bolt") sometimes reaching 60,000 degrees F. Lightning can deliver a tremendous discharge of energy at any grounded object. It can easily explode a tree with a direct strike.

Lightning strikes are somewhat predictable over a geographical region. The isokeraunic level is the number of thunderstorm days per year. This isokeraunic number varies from over 100 along the Gulf coast of Florida to less than 5 in the Pacific Northwest. Nevertheless, virtually all areas in North America are subject to lightning strikes to some degree. Exterior equipment, rooftop equipment, and equipment connected to aerial and buried copper cables are subject to possible damage. In low lightning areas, protected-type line repeaters are used along aerial cable and unprotected-type line repeaters are used along buried cable. In contrast, in areas of heavy lightning activity, it is quite common to use protected-type line repeaters regardless of whether the cable is aerial or buried.

Aerial cables are especially susceptible to lightning strikes. The huge energy pulse from lightning momentarily raises the potential of ground at that strike point and then travels along the copper pairs to the central office where it finds a lower ground potential. At the central office, primary protection consists of three-element gas tube protectors, typically installed at the well-grounded protection frame or near the cable vault. In some central offices, gas tubes are installed at the top of the relay rack with office repeaters.

Primary protectors must be present and grounded properly. Many items of transmission equipment, such as office repeaters, have secondary protection in the form of solid state surge limiters, but they are not effective if primary protection is bad. When the lightning voltage causes the primary gas tube to conduct, it represents a short circuit to the span powering regulator.

In a typical T1 transmission line, span power is fed only from the Central Office. In some cases, additional power comes from the far end unit. During normal operation, the 60 mA simplex current flows normally, the gas tubes sit idly, waiting for a lightning strike, and the DS1 traffic moves along. In the instant of a lightning strike (somewhere mid-span), the lightning acts as a huge current pulse that raises the ground potential at that strike location. If the cable is not properly grounded everywhere, the lightning can enter the copper pairs and flow toward wherever the best ground point might be, which might be toward the nearest Central Office or, in the other direction, toward the remote end. The lightning might be in the form of a metallic voltage appearing from tip to ring on a pair. It might be in the form of a metallic voltage appearing from tip to ground or from ring to ground. Or, it might be in the form of a longitudinal current surge.

If a metallic voltage appears at a three-element gas tube protector (called the primary protector), the tube will fire either tip to ground, ring to ground, or tip to ring. This assumes that the gas tube is both working and grounded correctly. Some companies, however, fail to periodically test their gas tubes with a gas tube checker. If measured with a simple meter, gas tubes appear to be an open circuit whether they are working or not working, making it unreliable. If the gas tube has an improper voltage rating, it will not work correctly. If the voltage rating is too high, lightning voltage can seep in before it fires (thereby stressing equipment). If the voltage rating is too low, the normal DC voltage applied, at the simplex power feed end of the span, is enough to set it off prematurely, or at least, to hold the gas tube in "glow" mode after the strike (thereby forcing a failure situation after the lightning strike).

When the gas tube fires on schedule, it is effectively producing a short to ground. If there is some span power feed repeater nearby, this acts as a dead short on its current loop, which causes a big current surge. Various T1 products have built-in secondary surge protectors to withstand this secondary surge, called the current surge. But if the primary surge protector is not doing its job, it will most likely burn out the equipment or anything around it. In some cases, this surge protector is in the form of extra series resistance to limit current peaks. In other cases, this protector is in the form of a fuse that will open up at a high current point. In yet others, the protector is a combination resistor and fuse.

As a general rule, however, two-element protectors are not recommended for T1 circuits. Two-element protectors are suitable for plain old telephone service (POTS) on a two-wire circuit. Due to the nature of T1 and its simplex current loop, equipment may be damaged through the use of anything other than three-element gas tubes.

Some telephone operating companies have the policy that T1 is only placed on new cables, dedicated for T1. This is because the headaches of rehabilitating old exchange cable can be severe. The job of eliminating every last bridge tap is difficult on some older cables. Impulse noise from ordinary analog subscriber loops can become a problem when mixed side-by-side with T1.

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