Switchgear. A review of progress

In British switchgear, development during the period under review has been directed mainly towards the improvement of interrupting performance, measured by reliability and speed of interruption of short-circuits, and by the prevention of over-voltages when disconnecting open-ended lines or small reactive currents. It has also been directed towards reducing the size and cost of switchgear and the buildings in which indoor gear is mounted. The techniques of oil, air-break and air-blast interruption, and of multi-break design, with various forms of voltage division, have all been exploited to assist in achieving these aims. There is now little room for further reduction in the minimum size of switchgear for a given voltage, but the MVA rating for the smallest frames, already doubled in this period, will no doubt continue to increase in the future. The upper limit of MVA has increased seven times in the period under review, and there are no obvious limitations to further progress; it is to be expected that the MVA rating available in circuit-breakers of British design will continue to exceed those required on contemporary networks.