Layout and rupturing capacity of protective devices in motor circuits

The paper deals with some of the practical considerations that will determine the decisions on technical points taken by engineers who are responsible for the installation of motor control gear and sub-distribution switchgear, for isolating and protecting motor circuits. The functions of isolation and protection are shared by motor control gear and sub-distribution switchgear, especially when motors function in groups. Various grouping arrangements are compared. The use of 3 300 volts in motor circuits is discussed, and a reference is made to the use of 3 300-volt contactors. The increasing use of contactors emphasizes the importance of further study of the process of making and breaking circuit by means of contactors, which can be examined in more detail by a demonstration with slow-motion films. After a note on thermal overload trips, the rupturing capacity of starters is discussed with particular reference to the short-circuit values which can be fed from the transformers and supply cables usual in l.t. circuits. The rupturing capacities of rewireable and cartridge fuses are discussed, with particular reference to the time of operation of the latter on severe short-circuits. The rupturing capacity of l.t. circuit breakers is shown to involve problems akin to those which have been studied in h.t. switchgear design. Finally, the problem of the anticipation of short-circuits by leakage indication is stated in terms of practical application.