High-voltage disconnectors are used in electrical substations to allow isolation of apparatus such as circuit breakers, transformers, and transmission lines, for maintenance. The disconnector is usually not intended for normal control of the circuit, but only for safety isolation. Unlike load switches and circuit breakers, disconnectors lack a mechanism for suppression of electric arcs which occur when conductors carrying high currents are mechanically interrupted. Thus, they are off-load devices, with very low breaking capacity, intended to be opened only after the current has been interrupted by some other control device. Safety regulations of the utility must prevent any attempt to open the disconnector while it supplies a circuit. Standards in some countries for safety may require either local motor isolators or lockable handles (which can be padlocked).
IEC standard 62271-102 defines the functionality and features of a disconnector.
Disconnectors have provisions for a lockout-tagout so that inadvertent operation is not possible. In high-voltage or complex systems, these locks may be part of a trapped-key interlock system to ensure proper sequence of operation. In some designs, the disconnector has the additional ability to earth the isolated circuit thereby providing additional safety. Such an arrangement would apply to circuits that inter-connect power distribution systems where both ends of the circuit need to be isolated.
Types of disconnectors
Disconnectors can be classified into various types based upon their constructional features and mounting arrangement. The major types of disconnectors are:
- Centre-break disconnectors
- Double-break disconnectors
- Pantograph disconnectors
- Horizontal break knee disconnectors
- Vertical break disconnectors
- Coaxial disconnectors
These are selected based upon the sub-station layout, clearances available and space constraints.