Page 52 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
50 Chapter Two

load, the bus section breaker is tripped and, after a short delay, the
standby supply breaker is closed. Figure 2.2 indicates such a system;
for small installations interlocked changeover contactors may be used
in the manner indicated in Fig. 2.1.
For a large installation where there are several major distribution
boards, it may be expedient to equip other boards with busbars and cir-
cuit breakers as described above, and to provide a feed from the
standby supply. This would involve the addition of a standby supply
distribution board.
If a number of individual loads distributed within a building are clas-
sified as essential and have to be supplied from the standby supply, the
simplest procedure is to install a supply changeover contactor for each
of the loads. It is usually convenient to install the contactors near to
their loads but such a decision depends on where the normal and
standby supplies are available. The contactor control circuits may be
autonomous or may receive signals from the generator control panel. If
they are autonomous they should be biased to use the normal supply
whenever it is available, the standby supply being used only when the
normal supply is not available. For autonomous controls there should
be two timers, one to delay the closing of the standby supply contactor
so that the loads may be applied to the generating set in stages, and
another to delay the return to the normal supply for sufficient time to
prove its reliability.
Another consideration that sometimes arises is the geographical
nature of the site. If the installation is “compact,” such as a single build-
ing, the foregoing considerations will be applicable but if it is, for
instance, an airfield, it is “spread out” and other considerations arise. In
addition to using split bus distribution boards, a large, spread-out site
may use a signal distributed over the site by, for example, a telephone-
type cable network, the final shedding of the nonessential load being
achieved by using various devices such as:

■ Circuit breakers with shunt trips. These are applicable to both low-
voltage and high-voltage installations and will usually incorporate a
facility enabling them to be closed automatically or manually from a
remote control point.
■ Ring main units with shunt trips. These are applicable to high-volt-
age installations only. A remote closing facility may not be available
on ring main units and in such cases reclosing involves an operator
visiting the location of each ring main unit.
■ Outdoor-type fuse pillars having duplicate busbars (the nonessential
bars being supplied through a contactor). These are applicable to low-
voltage circuits only. The contactor coil is supplied through a normally

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