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The operation of the relay which is most affected due to arc resistance is
1. Mho relay
2. Reactance relay
3. Impedance relay
4. All are equally affected 

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Correct Answer - Option 1 : Mho relay

Distance Relay:

  • A distance protection relay is a name given to the protection, whose action depends on the distance of the feeding point to the fault.
  • The time of operation of such protection is a function of the ratio of voltage and current, i.e., impedance.
  • This impedance between the relay and the fault depends on the electrical distance between them.
  • Types of distance relay are impedance relay, reactance relay, and the mho relay.


Mho Realy:

  • A Mho relay or admittance relay measures a component of admittance |Y| ∠ θ. But it's characteristic when plotted on the impedance diagram is a circle passing through the origin.
  • The relay is called the Admittance relay because its characteristic is a straight line when plotted on an admittance diagram.
  • It is inherently a directional relay as it detects the fault only in the forward direction.
  • The operation of the Mho relay is most affected due to arc resistance.

  • A reactance relay is an overcurrent relay with directional restraint.
  • Mho relay is a voltage restrained directional relay.
  • A reactance relay is suitable for the protection of a short transmission line because its operation is independent of arc resistance.
  • The relay which is selected for a long transmission line should be less affected due to power swings. Hence Mho relay is preferred.
  • Impedance relay is suitable for medium transmission lines.


Synchronizing power surge effect: 
Reactance relay > Impedance relay > Mho relay

Arc fault effect:
Mho relay > Imedance relay > Reactance relay > over current relay

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