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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 sequence is followed first while closing a circuit breaker?
Close the isolator
Open earthing switch
Close circuit breaker
Any of these
Open earthing switch
In sub-station switching operations, the sequence is designed to ensure safety and prevent accidental energization of grounded equipment. The earthing switch must be opened first to remove the physical ground connection, followed by closing the isolator (disconnecting switch) to complete the path, and finally closing the circuit breaker to energize the load.
In sub-station switching operations, the sequence is designed to ensure safety and prevent accidental energization of grounded equipment. The earthing switch must be opened first to remove the physical ground connection, followed by closing the isolator (disconnecting switch) to complete the path, and finally closing the circuit breaker to energize the load.
Iarc=ZV — Importance of minimizing impedance before circuit completion
P=VIcosϕ — Power transfer considerations post-closing
The principle relies on the 'Make' and 'Break' safety interlocks. Opening the earthing switch eliminates the path to the ground. Closing the isolator creates a metallic path without current (off-load), and the circuit breaker closes last to perform the final connection under controlled conditions, minimizing potential arcing or fault risks.
Safety interlocks prevent closing the breaker if the earthing switch is closed.
Isolators are not designed for arc quenching and must only be operated under no-load conditions.
Circuit breakers are equipped with advanced arc-quenching media (SF6, Vacuum, Oil, or Air).
The standard 'Open-Isolator-Breaker' sequence follows the mechanical interlock system.
Prevents accidental energization of grounded circuits
Protects operators from electric shock
Minimizes equipment damage due to short-circuit currents
Requires complex mechanical or electrical interlocks
Strict operational sequence increases human factor error risks
High Voltage (HV) Substations
Switchgear maintenance protocols
Industrial power distribution boards
The correct sequence for closing is: Open Earth Switch -> Close Isolator -> Close Circuit Breaker.
The correct sequence for opening is: Open Circuit Breaker -> Open Isolator -> Close Earth Switch.
Option A (Close the isolator) is the second step, not the first.
Option C (Close circuit breaker) is the final step, as closing it first while grounded would cause a dead short.
B is correct — Opening the earthing switch is the essential first step to remove any ground connection before applying voltage to the system.
Remember the mnemonic 'E-I-B' for closing (Earth, Isolator, Breaker) and 'B-I-E' for opening (Breaker, Isolator, Earth) to avoid mistakes in exams.