Current Carrying Capacity of an Earthing Switch for a Generator Circuit Breaker

In this study we present the contact system of an earthing switch used in a Generator Circuit Breaker (GCB) System focusing on the current carrying capacity. For this special application the contact system is required to withstand short-circuit currents of 210kA_rms@ 1s. The safe and reliable operation of the system is achieved thanks to a tulip contact with silver plated contact fingers. The focus of this study is the relationship between the total force of the contact, the welding current and the contact resistance. During operation, the resulting force of the contact, due to the electromagnetic forces, can rise from a nominal contact force minor than 50N to peaks of more than 1kN. In this study, a FEA simulation of the total contact forces is presented. The simulation results were then validated by means of dedicated experimental tests. The experimental campaign enabled the determination of the current carrying capacity and the welding threshold currents over a wide range of contact forces. Thanks to this new development, the presented earthing switch was successfully type tested (acc. to IEC 62271-1) at a peak current of 575 kA.