Editing Distribution transformer (section)

===Primary===
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The high-voltage primary windings are brought out to [[Bushing (electrical)|bushings]] on the top of the case. 
*Single-phase transformers, generally used in the North American system, are attached to the overhead distribution wires with two different types of connections:
**Wye – A 'wye' or 'phase to neutral' transformer is used on a wye distribution circuit. A single-phase wye transformer usually has only one bushing on top, connected to one of the three primary phases. The other end of the primary winding is connected to the transformer case, which is connected to the neutral wire of the wye system and is also [[ground (electrical)|grounded]]. A wye distribution system is not preferred because the transformers present unbalanced loads on the line that cause currents in the neutral wire and are then grounded. However, with a delta distribution system, the unbalanced loads can cause variations in the voltages on the 3-phase wires.
**Delta – A 'delta' or 'phase to phase' transformer is used on a delta distribution circuit. A single-phase delta transformer has two bushings connected to two of the three primary wires, so the primary winding sees the phase-to-phase voltage; this avoids returning primary current through a neutral that must be solidly grounded to keep its voltage near earth's potential. Since the neutral is also provided to customers, this is a significant safety advantage in a dry area like California, where soil conductivity is low. The main disadvantage is higher cost, e.g., needing at least two insulated 'hot' phase wires even on a branch circuit. Another minor disadvantage is that if only one of the primary phases is disconnected upstream, it will remain live as the transformers try to return current. It could be a hazard to line workers.
*Transformers providing three-phase secondary power, used for residential service in the European system, have three primary windings attached to all three primary phase wires. The windings are almost always connected in a 'wye' configuration, with the ends connected and grounded.
The transformer is always connected to the primary distribution lines through protective [[Fuse (electrical)|fuses]] and disconnect [[switch]]es. For pole-mounted transformers, this is usually a '[[Fuse cutout|fused cutout]].' An electrical fault melts the fuse, and the device drops open to give a visual indication of trouble. [[lineworker (occupation)|Lineworker]]s can also manually open it while the line is energized using insulated [[hot stick]]s. In some cases, completely self-protected transformers are used, which have a circuit breaker built in, so a fused cutout isn't needed.