Mains Protection
Mains protection is the essential protection provided for protecting an equipment.
Backup Protection
As a precautionary measure, an additional protection is generally provided and is called backup protection. The primary protection is the first to act and the backup protection is the next in the line of defense. If primary protection fails, the backup protection comes in action and removes the faulty part.
For 220KV and 500KV transmission lines, two identical sets of protection are used. They are fed from two different DC supply sources. Main protection with backup protection along with all auxiliary relays makes one set of protection.
To ensure the isolation of fault, two sets of complete protection are used in EHV(Extra High Voltage) system.
