A technical study presented by the National Transmission Company South Africa NTCSA at the CIGRE Southern Africa Regional Conference in Pretoria in October 2025 has highlighted the structural implications of dynamic forces generated during circuit breaker operations. The work focused on the refurbishment of the Fordsburg substation in Johannesburg where equipment is supported on suspended concrete slabs rather than ground-founded structures.
The Fordsburg substation, a 275/88 kV indoor air-insulated installation, is undergoing an upgrade in which legacy horizontal puffer-type circuit breakers are being replaced with modern vertical disconnecting units, says Azhar Maye of the NTCSA who co-authored the study. While the new breakers improve electrical performance and reliability, their rapid mechanical operating time of approximately 30 milliseconds introduces significant inertial forces into the supporting structure.
Unlike most substations, where breakers are installed on ground-supported foundations, the Fordsburg breakers are mounted on a suspended reinforced concrete slab supported by beams and columns approximately 13 m high. This configuration prompted an investigation into the structural response of the slab to operational CB loads.
