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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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While opening the circuit, which device opened first?
Circuit breaker
Isolator
Earthing switch
None of above
Circuit breaker
Quick Summary: In a power system protection sequence, the circuit breaker must open first to interrupt the fault current under load. The isolator is a no-load device and should only be operated after the circuit breaker has successfully disconnected the current path to prevent arcing and damage to the isolator contacts.
In a power system protection sequence, the circuit breaker must open first to interrupt the fault current under load. The isolator is a no-load device and should only be operated after the circuit breaker has successfully disconnected the current path to prevent arcing and damage to the isolator contacts.
P=3VLILcosϕ — represents the power interrupted during opening
Q=∫t1t2i(t)v(t)dt — energy dissipated as arc energy during breaker operation
The circuit breaker is designed with arc quenching media (SF6, vacuum, or oil) to extinguish high-energy electric arcs during fault interruption. The isolator acts as a physical air-gap separation device that provides visual confirmation of isolation for maintenance. Operating the isolator under load would cause a massive arc, destroying the device and causing a phase-to-ground fault.
Circuit breakers operate under load to quench arcs.
Isolators are 'off-load' devices for isolation only.
Interlocking mechanisms are used to ensure the correct sequence (CB first, then Isolator).
Operating an isolator under load is a critical safety violation.
Prevents dangerous arching during switching
Ensures safe maintenance environment
Requires complex interlocking logic
Physical space requirements for separate devices
Substation transmission lines
Industrial motor control centers
| Feature | Circuit Breaker | Isolator |
|---|---|---|
Load Handling | Capable of breaking fault current | No-load operation only |
Arc Control | Integrated arc quenching | No internal arc quenching |
The switching sequence is strictly enforced by mechanical or electrical interlocks.
Option C (Earthing Switch) is operated only after the circuit is fully isolated and verified to be de-energized to protect personnel during maintenance.
A is correct — The circuit breaker must open first to quench the arc created by fault currents before the isolator can be opened safely.
Always remember the mnemonic 'Isolator is NOT for load'—if you open an isolator while current is flowing, it acts as a short circuit path and can explode.