Lightning surges on a control cable incoming through a grounding lead

Abstract

The present paper has investigated the effect of sheath grounding on a control cable based on EMTP simulations, when a lightning current flows into counterpoises representing a part of a grounding mesh. Voltages and currents observed on the control cable are given as a superposition of an well-known induced voltage due to the lightning current on the counterpoise, of a voltage transferred from the counterpoise to which the cable metalic sheath is grounded, and of a lightning current circulating in a closed loop composed of the metalic sheath and the counterpoise when the cable both ends are grounded. The induced voltage from the counterpoise is not necessarily dominant even in a long cable because the current along the counterpoise decreases rapidly as the distance from the sending end increases. Although the sheath grounding decreases the node voltage due to the induced voltage, but at the same time increases a voltage transferred from the counterpoise. The inductance of a grounding lead reduces a current flowing into the cable sheath during a high frequency transient, and thus a more current flows through the counterpoise. This results in a higher induced voltage to the control cable from the counterpoise.