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| Figure 5-3 Single-phase transformers connected for: (A) 3 wire, 120/240 volt secondary. (B) 2 wire, 240 volt secondary. (C) 2 wire, 120 volt secondary. |

| from this secondary transposed before being brought out through the casing. In later units, these two transposed secondary leads are connected together inside the tank and only one common lead brought out through the casing. The center secondary bushing is generally called the neutral bushing and, in many cases, is a stud connected also to the tank casing providing a means also for grounding the transformer tank. Three methods of connecting such a transformer are illustrated in Fig. 5-3. |

| Polyphase Systems. |

| There are two types of ac circuits: (1) single-phase circuits and (2) polyphase (multi-phase or more than one phase) circuits. In single-phase circuits, only one phase or set of voltages of sine-wave form is applied to the circuits and only one phase of sine-wave current flows in the circuits. In the polyphase circuits, two or more phases or sets of sine-wave voltages are applied to the different portions of the circuits, and a corresponding number of sine-wave currents flow in those portions of the circuits. Each different portion of the polyphase circuit is usually called a phase and these are usually lettered for identification, i.e., A phase, B phase, C phase. The voltages applied to the separate phases of the circuits are correspondingly referred to as the A-phase voltage, the B-phase voltage, etc. The phase currents of the different portions are also correspondingly identified as the A-phase current, the B-phase current, etc. |

| The voltages for polyphase systems are supplied from polyphase generators, each phase of voltage being generated in a separate winding. A three-phase generator has three such separate windings. These three separate coils may be arranged for connection in different ways to form the three-phase system. |

| Two commonly used methods of connecting the coils of three-phase |

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