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| circumstances. When transfer switch C is open and either transfer switch A or B is open at the same time during the short time of operation, the whole current flows through half of the preventive autotransformer winding. To prevent an undesirable voltage dip from occurring during this period, the preventive autotransformer is built with an air-gap in its core. This prevents the core from handling too great a magnetic field which would cause excessive voltage drop in the coil. |

| The operation of the tap changers may be made by hand or controlled automatically. For larger tap changers, the control mechanism is mounted outside the transformer case. A motor mechanism, actuated by relays, opens and closes both the selector switches located inside the transformer tank and the transfer switches located in a separate compartment. The mechanism is so arranged that the switches operate in their proper sequence. A time delay device, used in connection with the control relay prevents its operation during voltage dips of short duration. |

| Such TCUL transformers may be used to regulate voltage over a plus 10 percent to minus 10 percent range, and may have thirty-six or more steps incorporated in the tap changing mechanism. |

| Instrument Transformers |

| When values of voltage or current are large, or when it is desired to insulate the meter, relay or other control device from the circuit in which the electrical values are to be measured, an instrument transformer is used. |

| Instrument transformers (Fig. 6-7) are of two kinds, voltage or potential transformers, and current transformers. Both work on the same principle of the two-winding transformers. They differ from power or distribution |

| Figure 6-7 Typical instrument transformers. |

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