Medium-Voltage Switchgear and Circuit Breakers:AC Control Power Equipment

AC Control Power Equipment

The following is a general guide for the selection and application of AC control power equipment.


For sizing the capacity of an AC control potential transformer, all the loads should be totaled. These loads may be steady state, breaker closing, and tripping. If the tripping is provided by a capacitor trip device, the tripping demand may not be included. The closing demand of breakers that require simultane- ous closing should be totaled and included in sizing the control transformer.

The total load should then be compared to the available sizes of control power transformers, and the next larger size should be selected.


It is recommended that the control power be supplied from a separate trans- former strictly used for control purposes. This will minimize inadvertent loss of control power. The transformer should be connected to the bus or the line side of the circuit to minimize control power interruption. For multiple services, each service should have its own control power source. Circuits that are not exclusively associated with either source should be supplied control power automatically from either of the control power transformers in case of loss of one control source.

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