- Isolator Definition: An isolator in electrical systems is a manually operated mechanical switch that separates a part of the circuit for safe maintenance.
- Purpose: The main purpose of an isolator is to ensure safety by isolating a part of the circuit; it should not be operated under load.
- Types: Isolators can be double break, single break, or pantograph types, depending on system requirements.
- Operation: Isolators must be opened after the circuit breaker is open and closed before the circuit breaker is closed to avoid arcing.
- Manual vs Motorized: solators can be hand-operated or motorized, with the choice depending on system voltage and cost considerations.
Definition of Isolator
A circuit breaker interrupts load or fault current, while a disconnector provides an isolating distance and reliable position indication. Opening either device alone does not make an electrical circuit safe to touch. Maintenance also needs the authorised isolation procedure, control of every energy source, lockout or tagging, absence-of-voltage testing and protective earthing where required. A disconnector, also called an isolator, separates part of an electrical power system after the interrupting device has removed load current. Manual and powered operators are both used.
Types of Electrical Isolators
Open-terminal disconnectors use several contact arrangements. The frozen list gives three examples, while current standards also classify devices by service duty, endurance, installation and switching capability.
- Double Break Isolator
- Single Break Isolator
- Pantograph type Isolator.
A substation drawing can also name a disconnector by the equipment on either side. The frozen list uses common bay-position labels.
- Bus side isolator – the isolator is directly connected with main bus
- Line side isolator – the isolator is situated at line side of any feeder
- Transfer bus side isolator – the isolator is directly connected with transfer bus.
Constructional Features of Double Break Isolators
The illustrated double-break disconnector has three post-insulator stacks. A rotating centre stack carries the moving blade, which bridges a fixed contact on each outer stack when closed.
The two breaks appear on either side of the centre blade when open. Contact fingers provide pressure on the closed blade. Other double-break designs can use different bearings, contacts and drives, so the figure is one construction rather than a universal arrangement.
A linkage at the base rotates the centre post. A tie rod can connect that linkage to a local operating handle or a motor operator. Interlocks, position indication and mechanical adjustment must meet the switch design and substation control scheme.
Constructional features of Single Break Isolators
In the illustrated single-break disconnector, one moving contact is mounted on each of two rotating post stacks. The contacts meet at one central break when the stacks rotate towards the closed position. Reverse rotation creates the isolating distance. A manual or motor operator may drive the mechanism according to the equipment specification.
Earthing Switches
An earthing switch connects an isolated conductor to earth. It may share a structure with a line-side disconnector or be a separate device. Its blade orientation varies by design. Mechanical or electrical interlocks normally prevent closing the earthing switch unless the associated disconnector is open, and prevent closing the disconnector while the earth switch is closed. Some earthing switches have a rated short-circuit making capability; others do not.
Operation of Electrical Isolator
A disconnector has no circuit-breaker arc quenching system and is not intended to interrupt load or fault current. A rated device may switch limited currents such as bus-transfer or charging current within the duties specified by IEC 62271-102. Interlocks normally require the circuit breaker to establish the correct condition before a disconnector moves. Actual switching must follow the approved site procedure, not a generic sequence from this page. Manual and motorised isolators exist across many voltage levels. Remote-control needs, operating frequency, layout, reliability and cost determine the operator type.
