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A circuit is disconnected by isolators when
Line is on full load
Circuit breaker is not open
There is no current in the line
Line is energized
There is no current in the line
An isolator (or disconnecting switch) is a mechanical switching device capable of opening or closing a circuit under 'no-load' conditions. It is primarily used to isolate a section of the electrical network for maintenance and does not have any arc-quenching capability, unlike a circuit breaker.
An isolator (or disconnecting switch) is a mechanical switching device capable of opening or closing a circuit under 'no-load' conditions. It is primarily used to isolate a section of the electrical network for maintenance and does not have any arc-quenching capability, unlike a circuit breaker.
IswitchingтАЛтЙИ0 тАФ Isolators are designed for off-load switching where the current to be interrupted is negligible.
Isolators operate by providing a visible physical gap in the circuit, ensuring safety during maintenance. Because they lack arc-quenching media (like SF6тАЛ gas or vacuum), attempting to operate them while current is flowing would result in a dangerous, sustained electric arc between the contacts. Therefore, a circuit breaker must always be opened first to interrupt the current flow before the isolator is operated.
Isolators are 'off-load' devices, not 'on-load' devices.
They provide visual proof of disconnection for personnel safety.
They are used in series with circuit breakers.
An interlocking mechanism usually prevents the operation of the isolator until the circuit breaker is in the 'OFF' position.
Ensures absolute safety for maintenance staff.
Cost-effective compared to circuit breakers.
Provides clear visual indication of physical isolation.
Cannot break fault currents.
Risk of damage and flashover if operated under load.
Substations for bus-bar arrangement.
Transmission lines for sectioning.
Distribution systems for maintenance isolation.
The sequence of operation is 'Open CB -> Open Isolator' for disconnection, and 'Close Isolator -> Close CB' for connection.
Option B is incorrect because if the circuit breaker is not open, the line is still carrying current, making isolator operation extremely dangerous due to arc formation.
C is correct тАФ Isolators are designed strictly for off-load operation where there is no current flowing, ensuring no arc is created during switching.
Always remember the mnemonic: 'Isolators are for Isolation, not Interruption'. They isolate an already interrupted circuit.