The equipment installed in power system network is very costly. The main equipment of the power system network is power transformer which costs a lot. Reliability of electrical protection on such equipment should be high enough. Not only the transformer, the extra high voltage lines of the system demand extra reliability of protection. The backup relaying schemes provide this extra reliability to the system. Backup relays are extra relaying schemes attached to the equipment or part of the network with their own relaying system. The main function of backup relayis to operate in any failure of tripping of circuit breaker due to main relays. The relay attached to the system may fail due to the following:
- Mechanical defect of moving parts of the main relay,
- Failure of DC power supply to the relay,
- Failure of tripping pulse to the breaker from relay,
- Failure of current or voltage to the relay from CT or PT circuits etc.
In this typical situation there should be another line of protection called back up relaying. Hence, back up relaying essentially have everything separate from main relaying scheme. This is because backup relay must not fail to operate in the event of failure of main relays.
There are some situations when we have to disconnect main relays from the system for preventive maintenance or trouble shootings. In those cases due to presence of back up relays, we do not have to interrupt the equipment or circuit. During this time back up protection scheme takes care of the protection of the system. As the back relaying is second line of protection it must be slow in action than main relay so that it can only be operated when the main relaying scheme of the system/equipment fails.