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| Whether the taps are on the high voltage or low-voltage position does not make any difference. If they are on the high-voltage side and the circuit is open, the current will drop to only the exciting current in the primary produced by the (impedance) voltage being applied. |

| Temperature Test or Heat Run |

| This test is made to determine the temperature rise of the transformer under rated load. Thermometers or thermocouples are installed to measure ambient temperatures, oil surface temperatures in the transformer tank and at some point (perhaps 2 or 3 inches) below the oil surface where maximum temperatures may be expected. Readings on these are taken before the test is started to obtain base temperatures on which to determine the rise at rated load. These are also read periodically during the test, and continued until temperatures do not vary more than a few degrees over a period of hours, and hence may be considered as stabilized. |

| Loading Transformers Under Test |

| There are three ways of loading the transformer under test: the direct method; the opposition method; and the compromise short-circuit method. |

| The Direct Method |

| Used for very small transformers, which may be energized in the normal manner and loaded by means of a lamp bank or other resistance load. For most transformers, this is not practical. |

| The Opposition Method... |

| The unit under test is connected to a duplicate transformer as shown in Fig. 7-7. The two transformers are connected in parallel on both the high voltage and low voltage sides. Full rated voltage is applied to the low voltage or secondary side, and a voltage from an external source is inserted in the high voltage or primaries in parallel as shown, just large enough to circulate rated current through the primaries of the two transformers. The two transformers are thus both carrying their normal currents in both their primary and secondary windings. The loading transformer should be interposed between the test source and the primaries of the transformers under test to avoid imposing a high voltage and possible insulation breakdown on equipment constituting a test source. |

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