Definitions of Operation

Most of the relays are restrained by spring so that they are at a pre-determined position when completely de-energized. At this position if the status of the contact is closed, then it is said “Normally closed” (NC) contact and if it is open it is said to be “NO” (Normally open) contact. Some terms related to the operation of relay are as follows :

• Pick-up value: The smallest value of the operating or actuating quantity for which the relay is at the verge of operation is called “pick-up” value of the relay.
• Reset value: The largest value of the operating or actuating quantity for which the relay is at the verge of non-operation is called “reset value” of the relay.
• Operating time: The time taken by the relay to operate the contacts is called the “operating time” of the relay.

Classification of Relay

The relays can be classified in different ways as follows:

According to the time of operation:

• Instantaneous relay: The operation of relay takes place after a very short duration of time (generally t ≤ 0.1 sec).
• Definite minimum time relay (DMT): The time of operation is fixed for these relays.
• Inverse time lag relay: Time of operation is inversely proportional to the magnitude of the current or other operating quantity.
• Inverse definite minimum time lag relay (IDMT): The time of operation is approximately inversely proportional to the smaller values of current or other quantity causing operation and tends to be a definite minimum time as the value increases without limit.

According to construction and principle of operation:

• Attracted armature type:
  (a) Attracted armature type: 
  Attracted armature type: In this type of relay operation depends on the movement of an armature under the influence of attractive force due to magnetic field set up by current flowing through the relay winding.
  (b) Solenoid type relay:
  Solenoid type relay: Operation of relay depends on the movement of an iron plunger core along the axis of a solenoid.
• Electromagnetic induction type: In this type of relay operation depends on the movement of a metallic disc or cylinder free to rotate by the inter-action of induced eddy currents and the alternating magnetic field.
• Electro-dynamic type relay
• Moving coil type relay
• Thermal type relay: The movement depends upon the action of heat produced by the current flowing through the element of the relay.

According to the applications:

• Over voltage or over current relay or over power relay
• Under voltage or under current relay or under power relay
• Directional or reverse current relay
• Directional or reverse power relay
• Differential relay
• Distance relay

Gas operated Buchholz relay:

It is used for the protection of oil immersed transformer from incipient fault or internal fault.

Microprocessor based relay/Static relay:

Static relays are capable of performing the same functions with the use of electronic circuit control which an electro-magnetic relay performs with the use of moving parts or elements.