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Page 253

| Figure 13-13 Typical circuit breaker in d-c control circuit. |

| 3. To make the circuit trip-free so that it cannot be held closed on a fault; these sometimes energize electrical interlock circuits. |

| Since the supply of control power is usually dc from storage batteries, breakers may still be operated if the main ac supply sources are de-energized. These may be 6-, 12-, 24-, 48-, or 120-V systems, although the 24-, 48-, and 120-V systems are usually preferred because possible voltage drops across relays or from poor connections will still leave sufficient voltage to operate the relays, breakers, and auxiliaries. |

| If only small amounts of control power are required and a dc source is not economically justified, an ac source is sometimes used in conjunction with a capacitor, as shown in Fig. 13-14. A combination of a copper-oxide rectifier (to obtain a dc) in series with a capacitor is energized from a potential transformer. The charged capacitor furnishes current in place of the battery, but since it is quickly discharged, it must be recharged before it can be used to trip a breaker again. It will retain sufficient charge to trip a breaker for only some five or six seconds after the charging potential is removed. |

| In some circuits, control circuit switches are normally closed and initiate action when contacts are opened. More often, however, control circuit switches are normally de-energized and initiate action when contacts are closed. |

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