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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 of the following is closed first for supplying power after repair or maintenance?
Isolator
Earth switch
Circuit breaker
Relay
Isolator
In a power substation, the isolator is a mechanical switching device used for isolation, not for breaking load currents. When restoring power after maintenance, the isolator is closed first to establish a physical connection path before the circuit breaker is operated to energize the circuit.
In a power substation, the isolator is a mechanical switching device used for isolation, not for breaking load currents. When restoring power after maintenance, the isolator is closed first to establish a physical connection path before the circuit breaker is operated to energize the circuit.
Iload=ZV — Circuit current magnitude
Parc=Varc×Iarc — Power dissipated in arc during switching
The switching sequence in a substation is governed by safety interlocks. When energizing, the isolators are closed first because they are 'off-load' devices, meaning they cannot interrupt the arc. Once the isolator blades are locked in place, the circuit breaker is closed to allow current to flow safely, as the breaker is designed to quench the arc produced during closing or opening under load.
Isolators are 'off-load' devices and must not be operated under current.
Circuit breakers are 'on-load' devices designed to handle arcs during switching.
The golden rule of switching: Isolators Close First (ICF) and Open Last (IOL).
Earth switches must be opened before the isolator is closed to prevent short circuits.
Ensures safety by preventing arc formation at the isolator contacts.
Isolator provides visual confirmation of disconnection for maintenance personnel.
Manual or motorized operation of isolators is slower than circuit breakers.
Cannot clear faults; they rely on upstream breakers.
Substation busbar selection.
Isolating equipment like transformers or lines for inspection.
| Feature | Isolator | Circuit Breaker |
|---|---|---|
Operation | Off-load (No current) | On-load (With current) |
The isolator acts as a visible air-gap break to ensure complete isolation from high-voltage supply.
Operating an isolator under load can lead to catastrophic arcing and damage to the substation structure.
A is correct — The isolator is closed first during the power supply restoration process as it is an off-load device that establishes the electrical path prior to the circuit breaker being switched.
Remember the mnemonic 'ICF' (Isolator Close First) for restoration and 'IBL' (Isolator Break Last) for maintenance.