Join 60,000+ competitive exam aspirants
or with email
Your Cart
Subtotal₹0
Total₹0
Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
Join 60,000+ competitive exam aspirants
While closing the circuit, which device opened first?
Circuit breaker
Isolator
Earthing switch
None of above
Earthing switch
Quick Summary: To ensure electrical safety during maintenance, the Earthing Switch must be opened first when energizing a circuit that has been grounded. This prevents the intentional short-circuit path to ground from being active while the primary circuit components are being engaged.
To ensure electrical safety during maintenance, the Earthing Switch must be opened first when energizing a circuit that has been grounded. This prevents the intentional short-circuit path to ground from being active while the primary circuit components are being engaged.
Ifault=ZgroundVphase — represents the fault current if the circuit is closed while grounded
The switching sequence follows a strict 'Last In, First Out' (LIFO) protocol regarding grounds. When closing a circuit (energizing): 1. Open the Earthing Switch (removes ground path). 2. Close the Isolator (establishes physical connection). 3. Close the Circuit Breaker (establishes current flow). This sequence avoids fault conditions caused by energizing a grounded circuit.
Switchgear operation sequences are safety-interlocked to prevent equipment damage.
The earthing switch provides a metallic path to earth for safe maintenance.
Energizing a circuit with an active ground leads to an immediate short-circuit fault.
Isolators are for visible disconnection and are not designed to break load current.
Prevents personnel electrocution during maintenance
Protects primary switchgear from short-circuit damage during startup
Requires complex mechanical or electrical interlocking systems
Human error in sequence leads to catastrophic failure
High Voltage Substations
Switchgear panels for Industrial Power Distribution
Transmission lines maintenance
Interlocks (Key or Electrical) are mandatory to enforce this specific switching sequence.
Circuit breaker is the only device rated to interrupt fault currents; isolators and earthing switches are not.
C is correct — The earthing switch must be opened first to remove the intentional ground before the circuit is energized.
Always remember: 'Open Earth, Close Power' during energization; 'Open Power, Close Earth' during de-energization for maintenance.