Different Types of Protective Relays

Posted On: January 24, 2018
Power Plant Operators

Electrical relays are used in many types of technological systems from cars and appliances to medical equipment and aircraft. Power devices in power plants must have protective relays to operate. Protective relaying is a crucial part of electric power systems. Three important aspects of power systems in relation to relaying are:

  1. Normal operation
  2. Failure prevention
  3. Mitigating the effects of failure


A relay isolates or alters an electric circuit. Relay classification depends on what they’re used for. Types of relays include:

  • Monitoring
  • Regulating
  • Auxiliary
  • Reclosing
  • Protective

Protective Relays

It is the job of the protective relay to monitor power, voltage, and current and detect problems. If any of those parameters go beyond set limits, an alarm is tripped or the circuit is isolated. This device causes a circuit breaker to isolate the bad element from the rest of the system. Protective relaying results in the removal of abnormal or short-circuiting power system elements.

To protect against short circuits there is primary relaying, the first line of defense, and back-up relaying, which springs into action when primary relaying fails. Protective relaying equipment is described with the words “sensitivity,” “selectivity” and “speed.” These characteristics allow the relay to accurately detect problems and quickly disconnect faulty elements.

Types of Protective Relay

There are two operating principles: electromagnetic attraction and electromagnetic induction. Basic classification of protective relays includes:

  • Electromagnetic Relays:
    • Armature
    • Induction cup / induction disc
  • Static Relays: Analog input signals are processed by solid state devices
  • Digital / Numerical Relays: Uses programmable solid state devices based on digital signal processing

Electromagnetic Relays

Electromagnetic relays have mechanical, electrical, and magnetic elements as well as operating coil and mechanical contacts. The mechanical contacts are opened or closed when a coil is activated. DC relays de-energize the coil with a diode. AC relays have laminated cores that prevent losses. Attraction electromagnetic relays attract metal if the coil is supplied power with a plunger moving towards a solenoid or the attraction of an armature toward an electromagnet’s poles. Induction relays are used in AC systems by developing actuating force when a conductor (cup or disc) moves into the electromagnetic fluxes interaction.

Solid State Relays

The switching operation is performed with solid state components without moving the parts. The less necessary control energy compared to the output power results in higher power gain than with an electromagnetic relay. Types of solid state relays include reed relay coupled SSR, transformer coupled SSR, and photo-coupled SSR.

Thermal Relays

Thermal relays use the effects of heat. Temperature increase makes the contacts switch positions. These relays have elements like temperature sensors and control elements. Thermal relays are often used in motor protection.

Hybrid Relays

Hybrid relays combine electromagnetic relay and electronic components. Rectification is performed by electronic circuitry, while the electromagnetic relay is the output part. Reed Relays These relays have magnetic strips (“reeds”) in a glass tube acting as a contact blade and armature. Switching occurs when a magnetic field causes the reeds to move.

Relay Differentiation

There are a few ways relays can be classified:

  • Dimensions:
    • Miniature
    • Subminiature
    • Micro miniature
  • Construction:
    • Open
    • Sealed
    • Hermetic
  • Load operating range:
    • High power
    • Intermediate power
    • Low power
    • Micro power

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