The design of apparatus for the protection of alternating-current circuits

The paper deals with differential protective systems, particularly those suitable for use where there is a neutral earthing resistance. The operating conditions, and their influence on methods of design and construction, are discussed, and improved means of practically applying the various protective circuits are described. The superiority of biased systems, in principle, is accepted as having been established, but the objections to a more complicated relay are recognized. The author advocates the use of simple relays only, in order to restrict to a minimum the employment of moving parts associated with small amounts of power. The necessary functions peculiar to biased and certain other protective systems have led to the use of balanced beam, dynamometer, and other special forms of relay The paper describes how these features may be achieved with a simple relay and a static device called a ¿biasing transformer.¿ Current transformers and simple types of relay are briefly dealt with, and a further static device called a ¿phantom auxiliary switch¿ is described. The application of the foregoing apparatus to actual protective circuits is described in the second portion of the paper, and attention is drawn to the advantages of dealing with earth faults and line faults independently.