Synthetic testing of circuit-breakers

Circuit-breaker test plants installed during the last few years have sufficed for practically all commercial commitments except in the h.v. ranges, where high interrupting capacity is needed. For these limiting cases, variations of the direct method, and ¿synthetic¿ testing, in which the arc current is obtained from one source and the restriking voltage from a separate source, have often been advocated. In particular cases, where the tests have been confined to a single breaker of special type, such methods have been employed with apparently satisfactory results. An investigation was carried out to determine whether synthetic testing as a general means of extending the range of a test plant was economically preferable to a straight extension of the plant. This embraced a study of the requirements of the case and an estimate of the cost of the plant to meet these requirements. A circuit suitable for use with all types of breaker was devised. The restriking voltage was considered as of (1-cosine) form, rising to its peak value in times of 20¿100 microsec. The results show that there is little difference in capital cost as between synthetic injection testing and the use of full plant kVA. Moreover, it is by no means certain that the synthetic method is fully equivalent to the direct test and, as clearing up this important question would entail the comparative application of both methods, it is concluded that synthetic methods cannot generally be justified economically.