Functional characteristics of Protection Relays

Any Protection Relay should fulfill the following functional Characteristics.
  • Reliability
  • Sensitivity
  • Selectivity and
  • Speed


Reliability means that the relay will act when it is required to act.  This is ensured by making sure


Sensitivity refers to the characteristic of the relay to act when the actual fault conditions occur.  Sensitivity is usually represented in terms of the minimum volt-amperes required for the relay operation. 


The relay should act according to the present time delay.  It should neither operate too fast or too slow.  If it is too slow it can cause damage to the equipment, if it operates too fast it may unnecessarily trip equipment for transient faults.


Selectivity refers to the ability of the relay to discriminate between faults.  This is critical as only the smallest possible section of the power system should be taken out of line in the event of a fault.  The relay should be able to discriminate between a transient fault and a through fault.  It should be able to differentiate between downstream faults and immediate faults.
that all the components of the protection from the voltage and current signals to the dc power supply for the trip circuit to the internal components of the relay are checked for for functionality and integrity.  The failure of any one of these components can result in the failure of the relay to act affecting the reliability.