AUDIOCLINIC (Q and A) (May 1975)

by Joseph Giovanelli

More About Lightning

As a former distribution engineer for a power company in New England, I thought you've like to hear about our experiences with the grounding of electric pole lightning arrestors.

If a ground rod is not driven into a really good ground, it is of little value. In fact, it can represent a real hazard. I have seen the results of a lightning stroke where the lightning hit the high-voltage line, followed the line to a lightning arrestor and then to the arrestor ground. This arrestor ground was a six-foot copper rod, about one and a quarter inches in diameter, driven into the ground at the foot of the utility pole, with a No. 4 weather wire connected from the arrestor to the ground rod with a ground clamp.

When the ground rod was driven into a good ground, no trouble resulted from the stroke. When the ground was not a good ground, anything could happen. The pole could be damaged, the transformer could be burned out, or it could be blown right off the pole. Because of a poor ground, I have seen where lightning struck a pole, splintered it, and opened up the earth for as much as 100 feet, trying to find a good ground in which to dissipate.

Every spring we sent a team to check the lightning arrestors for leakage and to see how effective our driven ground rods were, especially in areas most susceptible to lightning strokes, such as high elevation, hilly, rocky, or open areas. We used a megohmeter with a long wire to ground the meter to a known good ground and a quantity of rock salt.

We would disconnect the arrestor from the high-voltage line and check the resistance of the arrestor with the megohmeter. It was supposed to test infinity. If not, we replaced it. The resistance of the arrestor ground would then be measured--from it to a good ground; either a water faucet tap of the lead sheath of a telephone cable.

Too many times we found the ground resistance also read infinity! When this occurred we dug a hole around the ground rod to a depth of about three feet. Half a bucket of rock salt and a bucket of water was poured into the hole. Then we filled up the hole with dirt. After waiting a few minutes, a second resistance reading was taken, which in most cases turned out to be between 100 and 1,500 ohms.

Most of these grounds required this kind of treatment every Spring, and in southern states which are subjected to lightning storms most of the year, more frequent checks were required. Rock salt doesn't dissolve away quickly because it needs a fairly constant flow of water to eliminate it, and if plenty of water were present already, then we'd also have a good ground anyways.

Once we were testing an arrestor on a pole in an abandoned rock quarry. We found that the arrestor resistance had broken down. It should have read 2,300 volts relative to ground, and we read 550 volts between the two metal conduits driven into the ground at the foot of the pole (poor ground, and how!). A man who was watching us told us that after it had rained, any cat or dog walking up to the pole would pick its feet up very gingerly until it got too close to the pole, and then would leave the scene, immediately. Obviously it was feeling the voltage differential between its front and back feet increasingly as it neared the pole, first a tiny tingling, and later, a nice big voltage difference!

This man told us about one dog who had been visiting the pole pretty often for many months, but had never previously been there just after a rainstorm. He went quickly up to the pole, cocked up his leg, and then suddenly dashed off frantically, "kikying," over the hill into the distance. That dog was never seen anywhere around there, ever again! We replaced the faulty lightning arrestor.

-Guy Crooker, Portland, Maine.

( Audio magazine, May 1975, JOSEPH GIOVANELLI)

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