← All revision guidesSEMESTER 5 · DI05009011
6FLASHCARDSSwitchgear and Protection
Fault protection, relays, circuit breakers and instrument transformers.
Protection principles
Q1What are the essential qualities of protection?
Reliability, selectivity, sensitivity, speed, stability and adequate simplicity.
Q2What is relay coordination?
Selecting settings and operating times so the device nearest the fault clears it first while upstream protection backs it up.
Q3Why must a CT secondary never be open-circuited on load?
Dangerously high secondary voltage and core heating can occur because ampere-turn balance is lost.
Relays and breakers
Q1How does an overcurrent relay operate?
It issues a trip when measured current exceeds its pickup setting for the selected characteristic time.
Q2What is differential protection?
It compares currents entering and leaving a protected zone and operates when the difference indicates an internal fault.
Q3What is circuit-breaker making capacity?
The maximum peak current a breaker can safely close onto under specified conditions.