LIGHTNING ARRESTERS

If lightning - a gigantic surge of electricity hit an unprotected power line, it would take the easiest path to ground, which leads through the transformer windings in the substations, and not only the transformers, but the whole substation would be burned to cinders.

To prevent such damage, power lines are provided with "lightning arresters." The name is misleading, for they do not arrest the lightning; they provide it with a quick and short path to ground, so that the surge of electricity will bypass the equipment at the end of the lines. The line is usually disconnected at the same time. The second, and more difficult, job is to reconnect the line again when the surge has been successfully diverted.

In all cases, lightning arresters involve a mechanism that will pass little or no current between line and ground when the voltage (electrical pressure) is normal, but that turns into a short circuit when there is a voltage surge. The simplest method is the spark gap - in normal operation, there is no current across the gap between two electrodes, but the high voltage of the lightning will provide its own short by sparking across the gap. There are many other such devices, up to metal- oxide rectifiers which will "avalanched in a heavy stream of electrons when the reverse voltage is high enough. At the same time, the surge caused by the lightning will trap circuit breakers, far bigger than those found in domestic fuse boxess but employing the same (electromagnetic) principle.

The first part of the job, preventing damage, is the easier of the two, and for that part, Con Ed's lightning arresters worked (or several substations would have burned out). Reconnecting the line and bringing it into balance again is another matter. The circuit breakers will automatically reconnect after a lightning stroke, but they may immediately trip again.

Without going into details, consider this analogy: If an elephant steps onto a rowing boat, it will sink; now if the boat is protected against boat-happy elephants by slippery grease that will slide them off into the water, the subsequent rocking of the boat may cause it to capsize, even though the elephant is no longer on it.

Particularly so when there are a lot of elephants, that is, when a line is hit by several strokes of lightning in different locations within a short time. Then circuit breakers may trip all over, and though there may be no physical damage the line must be reconnected manually by stages.