Protective Relays

  1. A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the faulty section from the rest of the system.

Fundamental Requirements of Protective Relays

  1. Selectivity: it is the ability to select the faulty section.
  2. Speed : speed of operation
  3. Sensitivity : it is the ability to sense small quantity fault current
  4. Reliability: it is the ability of the relay system to operate the pre-determined conditions
  5. Simplicity: so that it can be easily maintained
  6. Economy: should be cheap

Classification of Relays

Electromagnetic relay

According to principle and protection

  1. Electromagnetic attraction relay
  2. Electromagnetic induction type relay
  3. Electro-dynamic type relay
  4. Moving coil type relay
  5. Thermal type relay: these relays operate on the principle of thermal effect of electric current.


  1. Buchhholz relay is a gas actuated relay. This relay is used in oil immersed transformer (rating above 500KVA).


Important terms

  1. Pick-up current: it is the minimum current in the relay coil at which the relay starts to operate.
  2. Current Setting:

Pick-up current = rated secondary current of C.T. *current setting

  1. Plug setting multiplier (P.S.M)

P.S.M. = Fault current in the relay/Pick-up current

  1. Time-setting multiplier:

Static relays employ thermionic valves, transistors or amplifiers

According to their function

  1. Main relay
  2. Auxiliary or supplementary relay
  3. Signal relay

According to applications the relays may be classified as :

  1. Over-voltage, over-current and Over-power relay
  2. Under-voltage, under-current and under-power relay
  3. Directional current relay
  4. Directional power relay
  5. Differential relay
  6. Distance relay
  7. Instantaneous relay : no intentional time delay is provided for operation.
  8. Inverse-time relay: the operating time is approximately inversely proportional to the magnitude of the actuating quantity.
  9. Definite time lag relay
  10. Inverse definite minimum Time (IDMT) relay

Distance Protection

  1. Impedance relay (used for medium transmission lines)
  2. Reactance relay (used for small Transmission lines)
  3. Mho Relay (used for Long transmission lines)