Page 86 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
84 Chapter Two

which to operate switchgear under the stressful conditions applying after
a power or equipment failure. There are advantages to installing the
switchgear in a separate room away from the noise and high ambient tem-
perature of the engine room; if there are problems to be resolved it will cer-
tainly be easier to discuss the matter or to exchange technical information.

The Cable Connecting the Switchgear to
the Generator
The cable entering the generator terminal box should be a flexible cable
and should be so arranged and run that it does not come under stress
due to vibration or movement of the set. If the cable is not flexible or is
not properly installed, it may restrict the freedom of movement of the
set, affect the performance of the antivibration mounts, and cause
vibration problems.
If the length of the cable run between the switchgear and the gener-
ator is short, say not exceeding 10 m, the entire run can be in flexible
cable. If the run is longer a cable change box may be installed so that
the majority of the cable is of standard distribution type and only a
short length of flexible cable is necessary.
For some small sets the main circuit breaker is mounted on a frame-
work fixed directly to the generating set base frame, the manufacturer
providing the interconnections between the circuit breaker and the
generator. For such an arrangement the interconnections should be in
flexible conductors and the cable taking power from the circuit breaker
may be of standard distribution type.
For fault protection purposes, this cable may be regarded as an exten-
sion of the stator windings, and if the switchgear is remote from the gen-
erator some additional protection may be desirable. The probability of a
fault developing in this cable is small but the condition can be detected
by installing a restricted earth fault protection system as described in
the previous section of this chapter. In addition to providing cable fault
protection, this system provides protection against internal winding
faults and limits the resulting damage to the machine.
For installations of more than one set, the restricted earth fault pro-
tection should be provided on each set.

Low-Voltage Switchgear
For low-voltage installations, the switchgear should be a type tested
assembly (TTA) or a partially type tested assembly (PTTA) complying
with BS EN 60439-1. The switchgear and controlgear components within
the switchboard should comply with BS EN 60947. The enclosures should
protect the equipment against the ingress of solid foreign objects, and per-
sons against touching hazardous parts, to one of the degrees of protection
in BS EN 60529 which is invoked by BS EN 60439-1.

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