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Coex Training

Power System Switches

The main four different switches used in a power system are:

  • Circuit breakers are designed for controlled closing and controlled or automatic opening of load and fault current. Their duties are to close onto a fault and to automatically open when a fault occurs. Circuit breaker contacts are not usually visible.
  • Load breaking switches are similar to circuit breakers but without the automatic fault breaking features. Load break switches are not commonly used above
  • Isolators are designed for controlled closing and opening but at no-load. They have limited capacity to break and make busbar/line charging currents.  Their contacts are clearly visible to provide confidence that they have opened properly and that their associated circuit is disconnected from the live system.
  • Earthing switches are designed for controlled closing and opening but only when their associated circuit has been disconnected from the live system. Their duty is to be able to carry rated fault current should their associated circuit be accidentally connected to the live system.  Their contacts are clearly visible so there can be confidence that they have closed properly and that their associated circuit is connected to earth.

 

The table below provides an overview of the duty cycle of each type of switching equipment.

Type Duty Cycle
Circuit Breaker Load make; Load break; Fault make; Fault break
Switches Load make; Load break ; Fault make
Disconnectors Limited load make; Limited load break
Isolators No load break; No load make

 

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