On-site diagnostic of insulating down conductors

Insulating Lightning Protection Systems (LPS) are widely used in different applications like mobile radio, industrial equipment or process industries. For the erection of such insulating LPS very often GFRP components as well as insulating down conductors are used. The use of an insulating down conductor becomes more and more relevant in lightning protection of buildings where architectural restrictions apply. It seems to be logic that the functionality of the components used in those insulating LPS has to be checked periodically. In insulating LPS where GFRP components are used a simple visual inspection is checking the mechanical condition. This simple visual inspection may not be sufficient in insulating LPS with insulating down conductors where a potential insulation damage may be very small and possibly inside the down conductor. Insulating down conductors based on coaxial structure with a low conductive coating are different from aerial or energy cables. A good conducting cover acts as a coaxial back conductor and allows the propagation of travelling waves within the cables. These travelling waves, however, cannot exist on structures with low conducting cover. The analysis of these travelling waves are the basis for the application of fault location methods used at HF and power cables. For that reason these methods cannot be applied. Obviously all established fault detection procedures which use the propagation of signals in soil are not applicable. The paper presents different electric methods of on-site dielectric testing of insulation failures in insulating down conductors. According to today's state of technology these procedures require the provocation of a minimum insulation failure on-site. Numerous investigations on damaged insulating down conductors have shown that the existence of an insulation fault can be proved by impulse testing with comparatively low voltage and energy. Different methods of fault detection are discussed by means of lab tests. Finally the paper presents a simplified integral electrical test method to check the insulation capability of the insulating down conductor on-site. The test method is based on the evaluation of the time constant of the test circuit in presence of an insulation fault. The results of first on-site tests using this method are discussed.