Primary and Secondary Protection Schemes

Protection Relay systems are classified into two types.

• Primary relaying Equipment and
• Secondary relaying Equipment
  

The Primary relay protection equipment is the first line of defence.  The secondary relay scheme comes in line when the primary relay system fails to act.

The Primary relay protection scheme can fail due to reasons such as

Failure of DC tripping voltage supply.
Failure of Current or voltage signal to the relays.
Failure of the Circuit Breaker.
Failure of the internal mechanism of the Protection Relays. 

The Secondary relay Protection scheme is intended to operate in the event of a failure of the primary supply.  Hence, the secondary relay protection scheme should be totally independent of the primary.  The current and voltage signals, the power supply of the relay, the output to the breaker should all be independent of the primary protection scheme.  The secondary protection scheme has a time delay greater than that of the primary relay protection scheme.