A disconnecting switch- Isolator

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Isolator is an offload mechanical switch which use to disconnect an electrical circuit to ensure electrical safety during maintenance. Isolators are used in substations to isolate apparatus such as circuit breaker, transformer and transmission line at a time of maintenance. It is usually not intended for normal control of the circuit but only for safe isolation.

There is mainly four types of isolators.
  1. Centre break isolator
  2. Double break isolator
  3. Pantograph isolator
  4. Knee type isolator
Centre Break (CB) isolator designed with two rotating insulators mounted on an integrated base frame. Both insulators hold current carrying aluminium rods. In this, current carrying path is between the vertical cylinder rod and horizontal contact fingers. Rotation of insulators are opposite to each other. The current path opening to the side creates a bigger phase distance in parallel arrangements than with other types.
Fig. Centre break isolator
Double Break (DB) isolator has three supporting insulators. The centre insulator can rotate and carries a moving aluminium rod, which is conducting path. Outer supporting insulators carry fixed contact. The fixed contacts are generally in the form of spring-loaded finger contacts. When isolator operates, it breaks the circuit from two different places. Rotation of the central insulator is done by a driving lever mechanism at the base of the insulator and it is connected with operating handle or motor through a mechanical tie rod. It is used for high mechanical tensile loads due to its compact and stable design.
Pantograph (PG) isolator is used for busbar connection. It is used for 400kV and above on the busbar side to save space and busbar is quite high. Rotating insulator is connected with operating arms of isolator through gear box. Busbar is connected between the two arms and the other conductor is connected through gear box. Gear box is conducting which carry current from busbar to conductor. Pantograph isolator is used where tapping is required from busbar.

Fig. Pantograph isolator
Knee type isolator is a vertical type isolator. It consists of one folding arm which moves horizontally on the plane of the insulators. It closes the circuit on a fixed contact located on the side insulator. Since the movement of the arm takes place on the same plane as that of the insulators, the phase-to-phase spacing is minimized.

Fig. Knee type isolator
In next post, Earth switch will be discussed which is used to ground residual current present in conductor connected to isolator.

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