In the shell-type construction the iron almost entirely surrounds the copper (Fig. 7). The core is made up of E-shaped or F-shaped laminations which are stacked to give a rectangular figure eight. All the windings are placed on the centre leg, and in order to reduce leakage, each high-side coil is adjacent to a low-side coil. The coils actually occupy the entire space of both windows, are flat or pencake in shape, and are usually constructed of strip copper
Again, to reduce the amount the high-voltage insulation required, the low-voltage coils are placed adjacent to the core.
Choice of Core-or Shell Type Construction. In general, the core-type has a longer mean length of core and a shorter mean length of coil turn. The core type also has a smaller cross-section of iron and so will need a greater number of turns of wire, since, in general, not as high a flux may be reached in the core. However, core type is better adopted for some high-voltage service since there is more room for insulation. The shell type has better provision for mechanically supporting and bracing the coils. This allows better resistance to the very high mechanical forces that develop during a high-current short circuit.
The choice of core- or shell type construction is usually one of cost, for similar characteristic can be obtained with both types.
Both core and shell forms are used, and selection is based upon many factors such as voltage rating, kVA rating, weight, insulation stress, mechanical stress, and heat distribution.