What is a Relay?

  • Relays are widely used for protection of system which makes the system aware whether it has abnormal condition or a fault.
  • If the system is under abnormal condition, Relay gives an alarm signal and if the system is under fault it gives signal to circuit breaker trip coil to trip the circuit.
  • Relays acts as a brain in this whole process, which provides necessary signals on the occasion of abnormal conditions or a fault.
  • The basic difference between a relay and circuit breaker is that relay acts as a sensing element but cannot trip the circuit whereas the circuit breaker can trip the circuit only if overload current is flowing but cannot judge between a fault and abnormal condition.

Operation of relay:-

It works under the principle of either electromagnetic attraction or electromagnetic induction. A plunger(Fig (a)) gets attracted into a solenoid in electromagnetic attraction type relays and can be operated by ac or either dc.

Principle of induction motor is used and sufficient torque is developed by electromagnetic induction in case of electromagnetic induction relays(example of watthour type induction disc relay is shown).

Relay components:-

  • Sensing element: – It senses the abnormal conditions and faults in the protected zone of the system.
  • Comparison element: – Compares the abnormality of system with a preselected relay setting.
  • Control element: – It’s usually a switch which gets closed and completes the circuit to trigger the circuit breaker / make an alarm signal.

When you purchase relays, you generally have control over several variables:

  • The voltage and current that is needed to activate the armature
  • The maximum voltage and current that can run through the armature and the armature contacts
  • The number of armatures (generally one or two)
  • The number of contacts for the armature (generally one or two — the relay shown here has two, one of which is unused)
  • Whether the contact (if only one contact is provided) is normally open (NO) or normally closed (NC).

Terms related to protective relay:-

  1. Pickup Level of Actuating Signal:

The value of actuating quantity (voltage or current) which is on threshold above which the relay initiates to be operated. If the value of actuating quantity is increased, the electromagnetic effect of the relay coil is increased and above a certain level of actuating quantity the moving mechanism of the relay just starts to move.

  1. Reset Level:

The value of current or voltage below which a relay opens its contacts and comes in original position.

  1. Operating Time of Relay:

Just after exceeding pickup level of actuating quantity the moving mechanism (for example rotating disc) of relay starts moving and it ultimately close the relay contacts at the end of its journey. The time which elapses between the instant when actuating quantity exceeds the pickup value to the instant when the relay contacts close.

  1. Reset Time of Relay:

The time which elapses between the instant when the actuating quantity becomes less than the reset value to the instant when the relay contacts returns to its normal position.

  1. Reach of Relay:

A distance relay operates whenever the distance seen by the relay is less than the pre-specified impedance. The actuating impedance in the relay is the function of distance in a distance protection relay. This impedance or corresponding distance is called reach of the relay.

For a secured protection from faults and abnormalities, backup relays are used which acts as a secondary defense of line, if at all primary relay fails to operate.

By the use of relay with circuit breaker, the following advantagesare obtained:-

  1. Transient state stability limit is improved.
  2. There is not permanent damage to any apparatus in the line.
  3. Small faults cannot get converted into bigger faults.
  4. There is continuity of supply through other good sections.
  5. System can be repaired in less amount of time.


Types of relaysavailable in industry:-

  • Overcurrent relay
  • Under-voltage relay
  • Under-frequency relay
  • Directional relay
  • Thermal relay
  • Phase sequence relay
  • Differential relay
  • Pilot relay
  • Distance relay
The best protection scheme is in which various types of relays are used together in the system line.



  1. Relays are best equipment which can save other apparatus from being damaged and it ensures the safety of the line at every instant of time.
  2. Relays can protect the system from dangerous fault currents by making aware the other protective devices to trip the circuit.

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