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Page 35

| Bushings and Terminals |

| Leads from the windings must be brought outside the transformer enclosure or tank. Leads operating at low voltages (usually less than 5000 volts), may consist of insulated flexible wires brought out through insulating collars, made of porcelain or plastic, with a terminal lug soldered or mechanically fastened to the wire (Fig. 2-17). For higher voltages (usually up to about 25,000 volts), stiff leads or bars are used, projecting through porcelain cylinders of varying thickness; the assembly is known as a bushing. Internally the coil is connected to the lead or bar of the bushing by suitably insulated wire or bar conductor; externally, the lead or bar contains some terminal arrangement which may be a simple thread and nut on the end of a bar, or a more complicated device involving springs and interlocks. Externally, the distance over the porcelain surface may be increased by installing petticoats that increase the creepage distance from the line terminal and the tank; this reduces the possibility of flashover over the surface due to rain and dirt. |

| For transformers operating at greater voltages (usually up to about 75,000 volts) (Fig. 2-18 on page 36), the conductor may be mounted inside porcelain cylindrical insulators filled with oil, and the oil space may be divided by thin insulating cylinders, with an oil gauge at the top to indicate the level of oil in the bushing. The conductor is generally a metallic rod which also serves to strengthen the construction of the bushing. |

| For even higher voltages (generally over 75,000 volts), the rod type conductor may be insulated with layers of oil-impregnated special paper, with metal foil inserted at several locations among the layers. The layers of insulation at the center are subjected to greater electrostatic stresses than the outer layers. The metal foils inserted between layers form a series of condensers which tend to even out and equalize the stresses among the layers. Such construction permits operation at higher voltages with less material, at lower cost, and with better performance. Refer to Appendix A for polymer insulation. |

| Figure 2-17 Sidewall mounted bushings. (A) Primary bushing. (B) Secondary bushing. |

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