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| Resistance Tests.. |

| These tests are made to measure the resistance of each winding to calculate power (I2R) loss, the resistance component of the voltage drop under load, to determine the temperature rise under load, and to ascertain that the internal connections are correct. A dc source should be used, together with ammeters and voltmeters of proper ranges. Temperature measurements of the windings should be taken while the resistance measurements are madethese may be thermometers or thermocouples. As a precaution, to avoid possible damage from voltages induced by the dc current causing the magnetic field to build up and collapse on being switched on and off, the winding whose resistance is not being measured should be temporarily short-circuited (Fig. 7-1). The re- |

| Figure 7-1 Connections for measuring transformer winding resistance. |

| sistance of each winding is obtained from the Ohm's Law derived equation: R = E/I or voltage value (volts) divided by the current flowing (amperes). |

| Polarity Tests |

| These tests are made to determine how the coils of a transformer are wound in relation to each other, so that the direction of the secondary voltage may be known when connecting transformers in parallel or in polyphase banks. In general, the leads are marked on the high side with the letters H1, H2, etc., reading from right to left; on the low side with the letters X1, X2, etc., reading from left to right for subtractive polarity and from right to left for additive polarity. |

| Single-phase Transformers |

| A single polarity check may be made by connecting a low-voltage ac to the high side of the transformer and having the H and X windings connected in series, one H lead being connected to the adjacent X lead (Fig. 7-2). If the |

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