Disconnector
A mechanical switching device that provides visible isolation of a circuit for maintenance — distinct from a circuit breaker in that it cannot interrupt load or fault current.
Also: isolator, disconnect switch, disconnectors
A disconnector (also called an isolator or disconnect switch) is a mechanical switching device used to provide a visible, physical break in a circuit so that downstream equipment can be safely maintained.
How it differs from a circuit breaker
The critical distinction: a disconnector cannot interrupt current. It must only be opened or closed when the circuit is already de-energised (or carrying negligible current). A circuit breaker, by contrast, is designed to interrupt fault currents of tens of thousands of amps within milliseconds.
In practice, the operating sequence is always:
- Open the circuit breaker to interrupt the current.
- Open the disconnectors to provide visible isolation.
- Apply earth switches to ground the isolated section.
Reversing this sequence — opening a disconnector under load — creates an uncontrolled arc that can destroy the equipment and endanger personnel.
Role in a bay
Every bay in a substation has disconnectors on both sides of its circuit breaker, plus earth switches for grounding during maintenance. Together with the breaker, they define the switching topology of the bay — which circuits are connected to which busbars, and which sections are isolated.
IEC 61850 representation
In the IEC 61850 data model, a disconnector is represented by the XSWI logical node (as distinct from XCBR for the circuit breaker). GOOSE messages carrying XSWI position changes flow over the station bus, and the interlocking logic that prevents unsafe switching sequences is implemented in the bay-level IED or in the station-level automation system.