Relays , CT/PT and Circuit Breakers together forms a protection triangle. Mal-operation in any one component can lead to failure of whole system, hence each component shall be tested individually.
Primary function of the protective system is to detect and isolate all failed or faulted components as quickly as possible, thereby minimizing the disruption to the remainder of the electric system. Accordingly the protection system should be dependable (operate when required), secure (not operate unnecessarily), selective (only the minimum number of devices should operate) and as fast as required. Without this primary requirement protection system would be largely ineffective and may even become liability.
Reliability of Protection Factors affecting reliability are as follows;
i) Quality of relays
ii) Component and circuits involved in fault clearance e.g. circuit breaker trip and control circuits, instrument transformers
iii) Maintenance of protection equipment
iv) Quality of maintenance operating staff
Failure records indicate the following order of likelihood of relays failure, breaker, wiring, current transformers, voltage transformers and D C. battery. Accordingly local and remote backup arrangement are required to be provided.