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
Isolators are designed to open a live circuit under
On load
No load
Full load
Any load
No load
An isolator is a mechanical switching device used to isolate a part of a power system for maintenance or repair. Because it lacks arc-quenching chambers (unlike a Circuit Breaker), it is strictly designed to be operated under no-load conditions to prevent the formation of a dangerous high-voltage electric arc.
An isolator is a mechanical switching device used to isolate a part of a power system for maintenance or repair. Because it lacks arc-quenching chambers (unlike a Circuit Breaker), it is strictly designed to be operated under no-load conditions to prevent the formation of a dangerous high-voltage electric arc.
Varc≈Ebreakdown×d — voltage required to maintain arc across contact gap d
When an isolator is opened under load, the current flowing through the circuit creates an electric arc between the contacts as they separate. Since there is no mechanism to extinguish this arc (such as compressed air, vacuum, or SF6 gas), the arc can cause severe flashover to adjacent parts or permanent damage to the isolator contacts.
Isolators are 'off-load' devices.
Circuit breakers are 'on-load' devices equipped with arc-quenching media.
The operational sequence is: Open Circuit Breaker first, then open Isolator.
Isolators provide a visible physical break in the circuit for safety.
Provides physical safety gap
Cost-effective compared to circuit breakers
Simple mechanical construction
Cannot break fault currents
Cannot switch capacitive or inductive loads safely
Requires sequential interlocking with circuit breakers
Substation bus-bar isolation
Maintenance of transformer banks
Isolating circuit breakers for servicing
Always check the status of the upstream/downstream circuit breaker before operating an isolator.
Option A and C are incorrect because opening an isolator under load would lead to an uncontrolled arc, likely resulting in a phase-to-ground or phase-to-phase short circuit.
B is correct — Isolators are strictly 'no-load' devices, as they lack the arc-extinguishing capability required to interrupt current safely.
Always remember the mnemonic 'Isolators for isolation, Circuit Breakers for interruption.' You never open an isolator unless the current is zero.