Protective relay


Protective relay

  • A relay is a device which can senses the faulty condition of electrical circuit and isolate its contacts. This contacts in turns to isolate and complete trip coil of the circuit breaker hence make the circuit breaker tripped and disconnect the faulty portion of the electrical circuit.
  • Protective relay senses the faulty condition in a power system and gives to blow siron or isolates that part from normal condition of system.
protective Relay
fig: protective Relay
fig: basic relay
  • Basic connections of tripping the circuit breaker using the C.T, relay coil and trip coil. when fault is occur in power system. the line has over current. so C.T is sensing the value of over current and the relay coil has energize. when coil  has energized the plunger is attract and contact has closed. and trip coil has attract the plunger of circuit breaker and contact had open and line is isolate.


There are two fundamentally different operating principles:

  • Electromagnetic attraction

This type of relays operate by virtue of a plunger being drawn into a solenoid, or an armature being attracted to the poles of an electromagnet. Such relays may be acting by DC or AC supply.

The electromagnetic force exerted on the moving element is proportional to the square of the flux in the air gap. If we neglect the effect of saturation, the total actuating force may be expressed:

F = K1 I² – K2

K1 I² = K2

current constant

where F =  force.

K1 = a force constant.

I = the magnitude of the current in the actuating coil.

K2 = the restraining force.

  • Electromagnetic induction

Electromagnetic induction relays use the principle of the induction motor where torque is developed by rotor. this operating principle applies only to relays actuated by alternating current, and in dealing with those relays we shall call them simply “induction-type” relays.

Type of Protective Relays

Based on Characteristics

  1. Definite time relays
  2. IDMT (Inverse definite minimum time Relays)
  3. Instantaneous relays.
  4. IDMT with Instantaneous
  5. Programmed switches.
  6. Over current relay.

Based on logic:

  1. Differential
  2. Directional
  3. Neutral Displacement
  4. Unbalance
  5. Restricted Earth Fault sensing relays
  6. Over Fluxing
  7. Distance Schemes
  8. Busbar Protection
  9. Reverse Power Relays
  10. Loss of excitation
  11. Negative Phase Sequence Relays

Based on Different quantity parameter

  1. Voltage relays
  2. Current relays
  3. Power relays
  4. Frequency relays

Based on Operation Mechanism:

  •  Static Relays
  1. Analog Relay
  2. Digital Relay
  •  Mechanical relays
  1. OT Trip (Oil Temperature Trip)
  2. WT Trip (Winding Temperature Trip)
  3. Bearing Temp Trip etc.
  4. Buchholz relays
  5. OSR(oil surge)
  6. PRV
  7. Water level Controls
  • Pressure Switchs
  • Mechanical Interlocks
  • Pole discrepancy Relays

Based on Applications

  1. Primary Relays
  2. Backup Relays

Let us consider for the moment only the relaying equipment for the protection against short circuits. There are two groups of such equipment one which we shall call “primary” relaying, and the other “back-up” relaying. Primary relaying is the first line of defense, whereas back-up relaying functions only when primary relaying fails.


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