Figure 4-6.Single transformer hung with crossarm
Figure 4-8.Three 37.5 kVA transformers mounted with
through-bolt type of brackets.
Figure 4-7.Three-phase bank of transformers hung on a
Formerly, all banks of three transformers were
hung with crossarm brackets or mounted on a platform
between two poles. Because of improved materials,
however, transformer capacities have been enlarged
without increasing their size and weight. This
improvement means that banks of three large trans-
formers can now be hung on a pole with a through-bolt
bracket type of suspension, as shown in figure 4-8.
The old method of mounting transformers on a
platform required an extra pole and the added cost of
building the platform. This method is still used when
installing large transformers and in special job
applications. Figure 4-9 shows the platform method of
mounting a bank of three single-phase, 25 kVA
transformers-Y-connected to obtain single-phase
and three-phase power.
Grounding the distribution system helps to prevent
accidents to personnel and damage to property in the
event of insulation breakdown, accidental shorting of
high- and low-voltage lines, or a lightning strike.
If a high-voltage line is accidentally shorted with a
low-voltage line, the current will flow through the
secondary coil of the transformer to the secondary
ground that will then cause the primary protective
device to open the circuit. In this case, the primary
protective device functions as the substation circuit
breaker. An accidental shorting of the primary and
secondary windings in the transformer will cause the
primary fuse to open.