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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Which is used for back up protection?
Relay
Circuit breaker
Both A and B
None of above
Both A and B
Quick Summary: Back-up protection is a secondary protection system designed to operate only if the primary protective device fails or remains unresponsive during a fault. Since both relays (which detect the fault) and circuit breakers (which isolate the circuit) are essential parts of the protection scheme, a failure in either requires a back-up system for overall power system security.
Back-up protection is a secondary protection system designed to operate only if the primary protective device fails or remains unresponsive during a fault. Since both relays (which detect the fault) and circuit breakers (which isolate the circuit) are essential parts of the protection scheme, a failure in either requires a back-up system for overall power system security.
Tbackup=Tprimary+Δt — where Δt is the coordination time interval to ensure selectivity.
The protection system operates in zones. Primary protection is the first line of defense. Back-up protection is typically time-delayed so that it does not operate unnecessarily if the primary protection successfully clears the fault. If the primary relay fails to send a trip signal, or if the primary circuit breaker fails to open, the back-up relay triggers a back-up breaker to isolate the faulty section.
Primary protection provides instantaneous clearance of faults within its zone.
Back-up protection is essential for avoiding catastrophic failures during hardware malfunctions.
Coordination is required to prevent nuisance tripping of healthy sections.
Back-up can be remote or local depending on the implementation architecture.
Increases reliability of the power system
Prevents system-wide blackouts due to localized equipment failure
Increased cost due to redundant hardware
Complex relay coordination settings
Transmission lines
Generator protection
Transformer protection
| Feature | Fast response for local fault | Delayed response for system security |
|---|---|---|
Functionality | Primary Protection | Back-up Protection |
Common back-up relays include Inverse Definite Minimum Time (IDMT) relays.
Option A and B are both necessary because a relay is useless without the breaker, and a breaker is useless without the relay signal.
C is correct — Back-up protection necessitates redundant relaying and switching devices to ensure fault isolation even if the primary equipment malfunctions.
Always remember that back-up protection must be set with a time margin, typically 0.2 to 0.5 seconds, to allow the primary system to clear the fault first.