Some notes on oil circuit-breaker design and performance

The principal cause of inconsistency in the short-circuit performance of oil circuit-breakers is considered to be the erratic movement of the oil. Quick-break circuit-breakers are advantageous for the higher voltages, and still further improved performance and consistency is obtained by the use of suitably-designed contact shrouds. Reference is made to published tests on some of the recent designs. The principal features of design affecting arc rupture are briefly reviewed and are followed by notes on the causes and effects of the magnitude and the rate of rise of recovery voltage. The importance of short-circuit testing is emphasized, and a plea is made for facilities for testing on site. A description of short-circuit testing arrangements is given, including a detailed description of the oscillograph equipment; also general notes on the calculation of 3-phase kVA. Reports are included of recent short-circuit tests carried out on oil circuit-breakers fitted with a new type of shrouded contact.