of circuit protection devices and to support the
conductors. The crossarms used in power distribution
come in three common sizes; namely, 3 1/4 inches by
4 1/4 inches, 3 1/2 inches by 4 1/2 inches, and 3 3/4
inches by 4 3/4 inches. The spacing for insulators on the
crossarms, as shown in figure 4-12, meets the minimum
standards for conductors at all distribution voltages
used by the Navy. In your naval service you may come
across larger wooden crossarms or crossarms made of
steel. These crossarms are designed to support
increased strain or to use with transmission lines
carrying higher voltages.
Single arms are used on straight lines when no
excessive strain needs to be provided (fig. 4-13). When
crossarms are installed, every other crossarm faces in
the same direction.
A double arm should be used at line terminals, at
corners, at angles, or at other points when there is an
excessive strain (fig. 4-14). When lines cross telephone
circuits or railroad crossings, double arms also should
be used, as more than ordinary safety is required at such
points. When two or more transformers are mounted on
the same pole, double arms, as a rule, are used for their
Figure 4-13.Single-arm construction.
Figure 4-14.Double-arm construction.