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In October 1947, F. A. Lane and B. W. Wyman1 described before the Institute the switchgear equipment developed for the Tidd High-Voltage Test Line. This equipment included a low oil content impulse-type circuit breaker rated 10,000,000 kva at 360 kv. The unit described at that time was part of an over-all development program of higher capacity switchgear equipments which will be needed to control and protect properly the rapidly expanding power systems of this country. This paper describes the development of the high-capacity impulse-type circuit breaker to be used in these equipments, both at existing transmission voltages and at the higher transmission voltages projected for the future. In addition, it presents substantiating laboratory and field test data indicating the ample margin of safety possessed by this circuit breaker to meet any requirements the future might dictate.