Thursday, 20 February 2020

What Is Relay Protection? Construction, Operation, Advantages, Disadvantages, Applications.


  • Relay is an electrical switch which is used to open or close a circuit with electromagnetic relay or electronically. It operates manually as well as automatically.
  • Relay is used in circuit for protection of electrical circuit, system and electrical equipment. In any electrical system protection of the system is given utmost priority.
  • In domestic electrical system for protection of the equipment and also for the protection of the operator relay is provided which immediately turn off automatically or fuse blown in case of any abnormality in the equipment and we frequently face it home.
  • Now when it comes to comparatively large power consumption such as in industry for the protection of system and equipment we use relay it sense the current and voltage through transformer and if it crosses limits then it generates trip command. 
  • Relays protection can also be used as logic device. They can have timers build into them. They can be wired so that they have a set-reset function.

Construction Of Relay

Construction Of Relay
Construction Of Relay

Coil of Wire
  • Originally a relay is a device that has a coil of wire, electromagnet, that when energize a contact is used make or break a connection. That is to open or close a switch or switches. 

  • Their are two poles in it. Normally close and Normally open. One pole of the contact remains stationary. When the magnet energizes it pulls the other pole contact to or away from the stationary contact.
  • Normally Open means if there is no voltage present at the coil of relay the contact will act as open switch with respect to common. 
  • Similarly Normally closed means it will act as short with common while there is no voltage present at the coil of Relay. Contacts NO & NC are firmly mounted while common is mounted in a way it can move. A spring is attached to common for damping that is Getting back normal state once coil is de-energized.
  • There are solid state devices called solid state relay that effectively perform the same function. That is it has a set of contacts that when energized open or close isolated contact, or solid state device that act as contacts.
  • The purpose is have one circuit open or close contacts in another circuit. This isolates the two circuits. 
  • The isolation can be so that a circuit of lesser or greater voltage controls another circuit of greater or lesser voltage. Tt can be used to isolate a DC or AC from each other. 

Operation of Relay

Relay Operation
Operation of Relay 

  • The power source is given to the electromagnet through a control switch and through contacts to the load. When current starts flowing through the control coil, the electromagnet starts energizing and thus intensifies the magnetic field. 
  • Thus the upper contact arm starts to be attracted to the lower fixed arm and thus closes the contacts causing a short circuit for the power to the load. On the other hand, if the relay was already de-energized when the contacts were closed, then the contact move oppositely and make an open circuit.
  • As soon as the coil current is off, the movable armature will be returned by a force back to its initial position. This force will be almost equal to half the strength of the magnetic force. This force is mainly provided by two factors. They are the spring and also gravity.
  • Relays are mainly made for two basic operations. One is low voltage application and the other is high voltage. For low voltage applications, more preference will be given to reduce the noise of the whole circuit. For high voltage applications, they are mainly designed to reduce a phenomenon called arcing.

Types Of  Relay

Their are main two types of the Relays
  • Electromechanical Relay- Constructed with mechanical moving parts.
  • Solid State Relay- It is integrated electronic device.


  • Fast operation and fast reset
  • They can use both AC and DC system for protection of AC and DC  equipment.
  • They have properties such as simple, robust, compact in size, reliable.
  • Very low contact voltage drop thus no heat sink is required.
  • High resistance to voltage transient.


  • Low speed of operation.
  • Directional feature is not available in electromechanical type relays.
  • Component failure leading to relay failure.
  • Change in characteristics over a period due to aging effect.


  • Protection of varies AC and DC equipment's.
  • The over/under current and voltage protection of various AC and DC equipments.
  • For fast heater switching.
  • To control variable resistance heater.
  • solenoid activation control.

   Related Terms:


Post a comment

Popular Posts