Direct lightning strikes to the lightning protective system of a residential building: triggered-lightning experiments

Lightning triggered from natural thunderclouds using the rocket-and-wire technique was employed in order to subject to direct lightning strikes the lightning protective system of a test house at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL. The electrical circuit of the test house was connected to the secondary of a padmount distribution transformer located a distance of about 50 m from the house. The transformer primary was connected to a 650-m long unenergized underground power cable. The test house had two ground rods, one for the lightning protective system grounding and the other for the power supply system grounding. The two rods were about 3 m apart and were connected by a metallic cable. Lightning current was injected into the lightning protective system ground rod, and the currents and voltages at different points in the test system were measured. The waveshapes of currents in the ground rods of the test house differed markedly from the current waveshapes in other parts of the overall system. The ground rods at the test house appeared to filter out the higher frequency components of the lightning current, allowing the lower frequency components of the current to enter the house's electrical circuit, that is, the ground rods appeared to exhibit a capacitive behavior rather than the often expected resistive behavior. This effect was observed for DC grounding resistances of the rods (driven in sandy soil with conductivity of about 2.5/spl times/10/sup -4/ S/m) ranging from more than a thousand ohms to some tens of ohms. The peak values of (1) the current entering the test house's electrical circuit, (2) the current flowing to the distribution transformer secondary neutral, and (3) the current flowing through the surge protective devices at the test house's service entrance were observed to be greater than in either of the two scenarios suggested by the International Electrotechnical Commission.