Effects of Lightning Punctures on the Core-Shield Voltage of Buried Cable

Abstract

A common form of lightning-inducedcable damage occurs when a lightning surge travels down the shield of a cable, inducing large voltages between the cable core and the shield and leading to possible insulation breakdown and arcing. The magnitude of this core-shield voltage is a function of how quickly the lightning current is shunted to the surrounding soil, either through a ground or by puncturing the protective jacket of the cable. Core-shield voltages are calculated for various combinations of cable length, cable diameter, soil conductiv ity, lightning pulse shape, and resistance of the cable jacket to puncturing and compared with results obtained by assuming that the cable jacket is punctured along its entire length. This latter assump tion, the "bare shield" model, has generally been used to predict coreshield voltages. We have found that the "bare shield" model can lead to a significant underestimation of the core-shield voltage, particu larly for small diameter cables with long distances between grounds (on the order of 10 kilometers).We present curves that provide an estimate of the peak core-shield voltage for a cable in which the jacket punctures for a portion of its length.

Recent progress of ITU-T recommendations on lightning protection

“…However, experiments carried out by Douglass [5] showed that it is necessary a minimum value of current to puncture the cable and Ungar [6] demonstrated that, as the current leaves the cable through the punctures, there comes a point beyond which there is no more punctures. The results of Ungar's model show that the voltage developed between the core and the sheath of a buried cable may attain values significantly higher than previously calculated by Sunde.…”