4.3 RELAYS. This section provides a general discussion of protective relay systems and
describes the various types of relays used for short-circuit protection.
4.3.1 Protective Relay Systems. Protective relay systems are intended to detect abnormal
conditions and to isolate them by initiating the operation of circuit breakers or other devices.
Normally, relays operate power circuit breakers rated above 600 V. The most common condition
for which protection is required is the short circuit. There are many other conditions, however,
which also require protection. These conditions include undervoltage, overvoltage, open-phase,
overcurrent, unbalanced phase currents, reverse power flow, underfrequency, overfrequency, and
The basic relay types most commonly used in power systems are: overcurrent, directional,
differential, current phase-balance, ground-fault, synchronism check and synchronizing,
pilot-wire, voltage, distance, phase-sequence or reverse-phase, frequency, temperature, pressure,
and auxiliary relays. Following is a brief description of the characteristics and applications of
the various relay types.
4.3.2 Overcurrent Relays. The most common relay for short-circuit protection of the
industrial power system is the overcurrent relay. Overcurrent relays used in the industry are
mostly of the electromagnetic attraction, induction, and solid-state types.
220.127.116.11 Electromagnetic Attraction. The simplest overcurrent relay using the
electromagnetic attraction principle is the solenoid type. The basic elements of this relay are a
solenoid wound around an iron core and steel plunger or armature which moves inside the
solenoid and supports the moving contacts.
18.104.22.168 Induction. The induction overcurrent relay is similar to a watt-hour meter since it
consists of an electromagnet and a movable armature which is usually a metal disk on a vertical
shaft restrained by a coiled spring. The relay contacts are operated by the movable armature.
22.214.171.124 Operating Current. The pickup or operating current for all overcurrent relays is
adjustable. When the current through the relay coil exceeds a given setting, the relay contacts
close and initiate the circuit breaker tripping operation. The relay operates as a burden to a
126.96.36.199 Operating Time. If the current operates the relay without intentional time delay,
the protection is called instantaneous overcurrent protection. When the overcurrent is of a
transient nature, such as caused by the starting of a motor or some sudden overload of brief
duration, the circuit breaker should not open. For this reason most overcurrent relays are
equipped with a time delay which permits a current several times the relay setting to persist for a
limited period of time without closing the contacts. If a relay operates faster as current
increases, it is said to have an inverse-time characteristic. Overcurrent relays are available with