Switching High Voltages

Switching 11 000 Volts and above requires special equipment because of electric arcs. High voltage circuit switching (11kV up) is done via circuit breakers which have arc extinguishing chambers and contacts that move very fast.

 The outdoor switchyard at Wairakei uses air as the insulator between circuits and therefore must cover a large area. The two Power Station buildings can be seen behind to the right. Station A is the blue building and Station B is the grey building to the left of it – Image: Heurisko Ltd.

The types of arc extinguishing chambers are those using:

  • sulphur hexafluoride gas (most common).
  • a vacuum (used with lower voltages).
  • very dry air to blow the arc out.
  • oil filled spaces.

 This modern switchgear is housed within the powerhouse of the Clyde Dam. The small space it occupies and absence of visible wires is possible because all circuits are insulated with Sulfur hexa-fluoride in the pipe work. Notice also the three phases colour coded red, yellow and blue – Image: Heurisko Ltd.

All circuit breakers (except those in a vacuum) use some sort of puffer mechanism to blow away the ions created by the arc. Therefore the path through the air that the arc has to travel, is lengthened and the arc can not be maintained. The circuit breakers are also designed to be able to break and withstand the many thousands of amperes (kA) that can occur during faults.

As High Voltage switches open or close, sparks (arcs) may occur between the switch surfaces.

These arcs:

  • damage the switch surfaces
  • damage other equipment, through voltage surges and spikes (transients)
  • become more damaging at high voltages.

Why can’t high voltages be switched by hand?

If a person tried to open a high voltage switch, with even a moderate amount of load on it using a simple lever switch (disconnector), they are likely to injure themselves and the switch will almost certainly be destroyed.

Even the fastest move would be too slow and an arc would be drawn right across the switch gap (opening). If the gap was large enough the heat generated by the arc would distort the arc so its length reaches a point where the voltage can no longer maintain it, and it would stop. By this stage there would be molten copper and porcelain sprayed everywhere.