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A Switchyard or Substation, consisting of large breakers and towers, is usually located in an area close to the plant. The substation is used as the distribution center where:
• electrical power is supplied to the plant from the outside, and
• electrical power is sent from the plant
Often there are at least 2 main Buses. Very high voltages (typically 220,000 or 345,000 volts) are present. Gas and oil circuit breakers are used. The gas (e.g. sulfur hexaflouride) or oil is used to extinguish the arc caused when a breaker is opened, either by a control switch or due to a fault. Manually or motor operated disconnects are provided on either side of the breaker to allow the breaker to be electrically isolated so that maintenance work can be performed.