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Page 164

| storage tank are necessary. The controls on the compressed air lines, however, are electrically operated. When the closing coil in the control air valve is energized, the valve opens, permitting air from the storage tank to flow into a cylinder, the piston of which closes the breaker contacts by means of a mechanical linkage (at the same time compressing a spring, if the mechanism is so equipped); they are locked in the closed position by a latch. When the breaker contacts close, the coil controlling the air valve is de-energized and the valve closes and cuts off the air supply. Auxiliary devices then permit any air trapped in the system to escape; as the air escapes, a spring returns the piston and linkage to their original positions. A system of interlocks prevents the piston from operating if the breaker is closed in a fault or abnormal condition. It is important that leaks be discovered and repaired immediately since the breaker will not operate (to close, and to open if so designed) without proper air pressure. |

| Control Indicators.. |

| Located at or near the circuit breaker control handles or levers are signs labeled ON and OFF or CLOSED and TRIP to indicate the operating position of the breaker. Many breakers have additional indicators in the form of semaphores or targets connected to the handle. They usually show the color red for the closed position and green for the trip position. Some breakers also have a red and green light mounted above the handle and governed by auxiliary breaker switches to indicate position. Sometimes a white light is included to indicate a disagreement between the breaker position and the semaphore or targets at the control handle. It is extremely important that the position of the breaker contacts be known at all times. |

| Maintenance |

| Before any work is performed on circuit breakers, care must be taken to isolate them completely from the circuit. They may be provided with disconnecting switches or devices on both sides, and these should be locked open and grounded if necessary. When such devices are not available, it may be necessary to disconnect the breakers physically from the circuit. |

| For most breakers, the tank must be removed before contacts and other parts can be inspected and maintained. In oil circuit breakers, the oil must be removed before the tanks are lowered. Such breakers, especially large outdoor types, are equipped with tank lifters, either as part of the mounting frame or as an accessory device, because of the weight of the tanks. These lifters may be operated manually by handcranks or driven by electric motors. Tanks are usually equipped with a valve at the bottom through which the oil may be drained. |

| Three main elements of circuit breakers require special attention: |

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