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| the rated secondary voltage of the transformera precautionary safety measure) and the voltage measured between leads as shown; the voltage relations should be in agreement with the comparisons indicated. |

| Ratio Tests. |

| The ratio of a transformer is the ratio of voltage of the high-voltage winding to the low-voltage winding for two-winding transformers. When there are more than two windings, there are several ratios, all taken with respect to the high-voltage winding. The various voltages obtainable on a particular transformer are usually shown on the nameplate. Where two (or more) transformers are to be connected in parallel, it is essential that the voltage ratios of each should be correct. |

| There are two general methods for testing the ratio of a transformer: one for small transformers, and one for large ones. |

| For smaller transformers, used for general distribution purposes, the transformer to be tested is connected in parallel with a standard transformer; between the terminals of the secondaries in parallel, a voltmeter is connected as shown in Fig. 7-3. If the voltmeter reads zero, the two transformers have the same ratio and the same polarity; if the voltmeter reads double the secondary voltage, the polarities of the two transformers are opposite. Should the two transformers be alike in polarity but different in ratio, the voltmeter will indicate the difference. |

| For larger transformers, including station-type transformers, the test is made by connecting voltmeters to both the primary and secondary sides (usually employing potential transformers). The voltage on the high or primary side is held at a particular value, while the voltmeter on the secondary side is read. The voltmeters are interchanged to compensate for meter errors, and the test repeated. The average of the two is used to calculate the ratio. An ammeter is sometimes connected in the circuit to read the exciting current, |

| Figure 7-3 Connections for ratio (and polarity) tests using standard transformer. |

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