B. Polarity Testing IEEE 57.12.90 lays out a clear way to test transformers polarity. As shown in the diagram below, add a jumper to connect the H1 bushing to the adjacent low voltage terminal A convenient low voltage AC source such as 120VAC is applied to the high voltage terminals. WARNING – DO NOT CONNECT TEST VOLTAGE TO THE LOW VOLTAGE TERMINALS OF THE TRANSFORMER AS THIS WILL CREATE PRIMARY LEVEL VOLTAGES ON THE HIGH VOLTAGE TERMINALS. While energized with the test voltage, read the voltage between the H2 terminal and the adjacent low voltage terminal as shown below. If the resulting voltage is higher than the test voltage, the transformer polarity is additive. If the resulting voltage is lower, the transformer is subtractive polarity.
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