Overcurrent and earth-fault protection

Abstract

Overcurrent and earth-fault protection are the oldest and most commonly used types of protection. Until the late 1980s almost all of these relays were electromagnetic in design, whereas new installations are predominantly solid state; even so, many of the simple, robust and reliable electromagnetic relays already installed are likely to remain on the system for many years to come. This chapter examines these relays from an electromagnetic design stand point, since it provides a clearer understanding of how this type of relay functions. The design, application, commissioning and maintenance of the following relays will be considered: a.) IDMTL overcurrent and earth fault; b.) directional overcurrent and earth fault; and c.) instantaneous overcurrent.