Some recent advances in the design of relays for the protection of alternating-current systems

The paper postulates that the success of any protective system depends upon the reliability and suitability of the relays employed. The requirements of an ideal protective relay are formulated, and the constructional and operatingcharacteristics of some novel designs of protective relays are compared therewith. For convenience of treatment, the relays¿all of the induction type¿are divided into four groups in the order of their development. It is submitted that the improved differential or balance type relay (Group A) and the novel time-selective relay (Group U)described in the paper, approach very closely to the ideal aimed at. Some novel working principles are described, but only one form of construction, capableof universal application, is followed throughout the paper, the aim being economical production and uniformity of performance.