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Which of the following is normally open in substations?
Circuit breaker
Earth switch
Isolator
None of above
Earth switch
An earth switch is a safety device in a substation designed to connect de-energized equipment to the ground potential. Under normal operating conditions, it remains in an open state to ensure no path exists between live high-voltage conductors and the earth, preventing short circuits.
An earth switch is a safety device in a substation designed to connect de-energized equipment to the ground potential. Under normal operating conditions, it remains in an open state to ensure no path exists between live high-voltage conductors and the earth, preventing short circuits.
VearthтАЛ=0V
IleakageтАЛтЙИ0A (for open earth switch)
The earth switch is mechanically or electrically interlocked with the main isolator. Before maintenance begins, the circuit is broken by the circuit breaker, the isolator is opened to provide a visual gap, and only then is the earth switch closed to drain any trapped residual charge and ensure the equipment is at zero potential. When the substation is in service, the earth switch is kept open to maintain isolation from the grounding system.
Earth switches are strictly for safety maintenance (earthing).
They are interlocked with isolators to prevent accidental closure while lines are live.
Circuit breakers and isolators are 'Normally Closed' (NC) in service; earth switches are 'Normally Open' (NO).
Essential for preventing 'induced voltage' hazards during maintenance.
Ensures safety of maintenance personnel.
Eliminates static charge on de-energized lines.
Prevents accidental re-energization effects.
Requires complex interlocking mechanisms.
Adds cost and physical space requirements in switchyards.
High Voltage (HV) and Extra High Voltage (EHV) substations.
GIS (Gas Insulated Switchgear) modules.
Maintenance of power transformers and busbars.
Circuit breakers are normally closed to conduct current, opening only during faults or switching operations.
Isolators (Disconnectors) are normally closed to provide a continuous conducting path.
The earth switch is the only device specifically designed to be normally open to prevent short-circuiting the system.
B is correct тАФ The earth switch is designed to be normally open to keep live high-voltage conductors isolated from the ground during normal operation.
Always remember that in power systems, 'Normal' refers to the state where power is being delivered; thus, any device providing a path to ground must be open.