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| gets its power from closing coils connected on the source side of the device; these closing coils also compress the springs used for tripping. Another coil in series with the closing coil trips a latch to open the breaker. A relay reenergizes the closing coils, closing the breaker automatically three more times before locking out. |

| The sequence of operation when a fault occurs is similar to that in any other breaker. Usually, the first trip occurs quickly after the fault is sensed; the second, third, and fourth may occur with time-delays inserted in the operation cycle (Fig. 8-24B). The unit may also be operated manually. |

| These units have a smaller interrupting capacity than regular circuit breakers and are usually installed at some critical or sensitive location in the circuit. The three reclosings serve to restore power to a circuit (or portion of a circuit) subject to a temporary failure such as might occur when a tree limb falls across power lines but drops to the ground immediately or when wires whip together in a severe wind. |

| Grounds |

| Tanks, metallic supporting structures, and other internal and external mechanical parts not associated with the current carrying or interrupting electrical function of the circuit breaker should be connected (or bonded) together and grounded to one or more grounds. Under fault or lightning conditions, surge voltages may develop in these parts that might prove unsafe for workmen or cause damage to the insulating materials in the breaker or to the metallic parts themselves. Ground connections from any associated lightning or surge arresters should also be interconnected with these ground connections to ensure proper functioning of the devices. The grounds should be checked periodically to make sure not only that the circuit connections are unbroken but also that their resistance is reasonably low. |

| Review |

| 1. Opening and closing of electric circuits is accomplished by equipment that varies from very simple devices for opening low-voltage circuits that carry small currents to large and very elaborate devices for interrupting high-voltage circuits that transport large currents and great amounts of energy. The term circuit breaker, however, is usually applied to heavy duty devices capable of interrupting comparatively large electric currents safely. |

| 2. Two or more adjacent conductors create magnetic fields that tend to attract or repel each other (Fig. 8-1). When the currents are extraordinarily high, such as during short circuit or fault conditions, mechanical |

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