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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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Isolators can be open
After opening of circuit breaker
After closing of circuit breaker
Under closed position of circuit breaker
Any of above
After opening of circuit breaker
An isolator (or disconnecting switch) is a mechanical switching device capable of opening or closing a circuit only when the current is negligible or zero. It is specifically designed to provide visible isolation for maintenance purposes and must never be operated while the circuit breaker is carrying load current.
An isolator (or disconnecting switch) is a mechanical switching device capable of opening or closing a circuit only when the current is negligible or zero. It is specifically designed to provide visible isolation for maintenance purposes and must never be operated while the circuit breaker is carrying load current.
Ibreak=0 — Isolators must operate under zero current conditions to avoid catastrophic damage.
The switching sequence for isolating a circuit is strictly enforced: first, the circuit breaker (CB) is opened to interrupt the fault or load current. Only after the CB confirms a 'break' status are the isolators opened. This ensures no arcing occurs at the isolator contacts, as isolators lack the arc-quenching chambers present in circuit breakers.
Isolators are 'off-load' devices.
They provide physical isolation and safety for maintenance personnel.
Interlocking mechanisms are used in substations to prevent isolator operation while the CB is closed.
Circuit breakers are 'on-load' devices capable of arc quenching.
Provides a visible gap for safe working conditions
Cost-effective compared to circuit breakers
Highly reliable for sectionalizing systems
Cannot break fault currents
Does not possess automatic arc-extinguishing capability
Isolation of transformers for maintenance
Sectionalizing bus-bars in high-voltage substations
Isolating circuit breakers for repair
Operating an isolator under load creates a high-power arc, potentially causing explosions or equipment damage.
Option B and C are incorrect because closing the CB first would result in drawing a destructive arc if the isolator contacts were moved under load.
A is correct — Isolators must be operated only after the circuit breaker has interrupted the circuit to ensure no current flows through the isolator contacts.
Always remember the mnemonic 'Isolators are for isolation (Safety), Circuit Breakers are for current (Switching).' Never operate an isolator under load!