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| Test for Polarity |

| If the polarity is unknown or not specified, it may be determined by a simple voltage measurement, as follows: |

| 1. Place a connection between the high-voltage and low-voltage terminals on the right, when facing the low-voltage bushings. |

| 2. Apply a low voltage, say 120 volts, to the two high-voltage terminals; measure this voltage with a voltmeter. |

| 3. Measure the voltage from the left-hand, high-voltage terminal to the left-hand, low-voltage terminal. |

| If the latter voltage is lower than the voltage across the high-voltage terminals, the transformer has subtractive polarity. If it is higher, the transformer has additive polarity. |

| Importance of Impedance |

| In addition to the polarity consideration, transformers connected in parallel should have the same high-voltage rating, the same low-voltage rating, and very nearly the same impedance. |

| When transformers are connected in parallel, the largest current will flow in the one with the least impedance and that transformer may become overloaded. It is important, therefore, that the impedance of all transformers in parallel be close to the same, within a fraction of a percent. |

| Single-Phase Transformer Connection.. |

| The simplest and by far the most widespread of all transformer connections used is that of one single-phase transformer. |

| A typical arrangement of the method of bringing the terminals or leads of the primary and secondary coils out through the tank of a single-phase distribution transformer is shown in Fig. 5-2. To provide for flexibility of connection, the primary and secondary coils are usually each arranged in two sections, each section of a coil having the same number of turns and consequently generating the same voltage. The two primary sections are usually connected together inside the tank and only two primary leads are brought out from one side of the tank through bushings, which insulate them from the tank casing. Four secondary leads may similarly be brought out through insulating bushings from the opposite side of the tank, two leads being brought out from each of the secondary coil. Transformers of older vintage have the four secondary leads brought out of the tank, with the two inner leads |

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