Isolator function and application

An isolator (disconnector) is a mechanical switch that opens or closes a circuit only when no current, or negligible current, is flowing. It provides a visible, safe air gap between a live part and the part to be maintained. It has no arc-quenching arrangement, so it must never be opened on load.

Functions of an isolator

1. Safety isolation: disconnects equipment (CB, transformer, line, bus section) from all live parts so maintenance can be done safely.
2. Visible break: the open blades give an obvious, visible gap that confirms the circuit is dead.
3. Sectionalising: divides a busbar or a feeder into sections so that one part can be taken out while the rest stays in service.
4. Change-over / bus selection: in double-bus schemes, isolators select which bus a feeder is connected to.
5. Earthing: many isolators are combined with an earth switch to earth the dead line and discharge trapped charge.
6. Breaking small currents: can safely break the small charging current of busbars or short unloaded lines and the magnetising current of small transformers (where permitted).

Applications in power system

• On both sides of every circuit breaker in substations, so the CB can be maintained.
• At the incoming and outgoing line bays of substations (line isolator with earth switch).
• Bus coupler and bus sectionaliser isolators in single and double busbar arrangements.
• Transformer isolation on HV and LV sides of power and distribution transformers.
• Pole-mounted (gang-operated) AB switches on 11 kV and 33 kV distribution lines in Nepal for sectionalising feeders.
• Isolating potential transformers, lightning arresters and capacitor banks.
• Bypass isolators used to bypass a CB during maintenance.

Types (brief)

• By construction: double-break, single-break (centre-break), pantograph, vertical-break.
• By location: bus-side, line-side, transfer-bus isolators.
• By operation: manual (hook stick), gang-operated, motorised.

Example: in a 33/11 kV substation, the operator first opens the 33 kV breaker, then opens the isolators on both sides and closes the earth switch before work is done on the breaker.