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| Current transformers on each side of the piece of equipment being protected are connected to a differential relay. As long as the current flowing to the equipment through the one current transformer is equal to the current leaving the equipment through the other current transformer, the differential relay will not operate. A fault or short circuit between the two current transformers will result in more current flowing into the area of equipment being protected than out of that area, thereby causing the differential relay to operate. |

| As differential relay operation is entirely independent of conditions outside of the protected area, it can operate very quickly; no time delay is needed to provide selectivity or coordination with other relays or protective devices. |

| Since the operation of a differential relay often requires tripping more than one circuit breaker, it is usually provided with an auxiliary relay, which in turn is provided with a sufficient number of contacts to trip a number of circuit breakers. |

| To obtain satisfactory differential protection during normal operation, it is essential that current from the current transformers (CTs) on the high-voltage side of the power transformer be equal to, and in the same direction as, the current from the CTs on the low-voltage side. Because of the power-transformer ratio of transformation, the losses in the transformer, and the use of standard-ratio CTs on both sides of the transformer, the currents may not be equal under normal conditions. To balance these currents, auxiliary autotransformers are connected in the circuit from the CTs. In one type of relay, the operating coils are provided with taps for balancing the currents of the CTs so that auxiliary autotransformers are unnecessary. Under these conditions, the currents normally balance each other, and no current flows in the relay. Relay action, therefore, does not occur even under overload and short-circuit conditions on the circuits associated with the power transformer being protected. |

| If there is an internal fault in the power transformer, the currents between the circuits from the CTs become unbalanced. This difference of current flowing through the relay causes its contacts to close, completing the trip circuit of the circuit breakers. |

| Reclosing Relays. |

| Some circuit breakers are equipped with automatic reclosing relays that automatically close the breakers after they have opened as a result of fault conditions (Fig. 13-11). The reclosing operation takes place in accordance with a predetermined cycle. The first automatic reclosure usually takes place immediately after the fault causes automatic tripping. If the fault persists, the breaker opens automatically again and after a time interval recloses a second |

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