Page 85 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 83

In the previous paragraph the characteristics described as standard
inverse, very inverse, and extremely inverse are the operating charac-
teristics described in BS EN 60255. The characteristics are inverse
with a definite minimum time and are known in full as inverse definite
minimum time characteristics, sometimes abbreviated to IDMT.

Protection of the Distribution System
In a normal situation the overload relay characteristic of the supply
would appear on the time grading graph to the right of and above the
characteristics of the distribution system. Any fault on the distribution
system would be cleared, enabling the healthy parts of the system to
continue in operation. However, the prospective fault current obtain-
able from the standby supply will be much less than that obtainable
from the normal supply, and will probably be of the order of three times
the rated current. On the protection time grading graph the effect is to
move to the left, sometimes drastically, the supply characteristic. This
characteristic may intersect some of the existing operating character-
istics, thus lengthening their operating times or, worse, may leave
them isolated in a high current section of the graph where the standby
supply cannot operate.
If the standby supply feeds a number of loads through dedicated
changeover contactors or circuit breakers there should be no problem
in providing protection because the distribution system is in effect
duplicated. Where a small standby supply feeds a large distribution
system and uses the same switchboard as the normal supply, it will be
unlikely to be able to clear faults in the main distribution system and
a cable fault near the power source may not be cleared, thus rendering
the standby supply inoperative, until it can be restored by manual
switching or repair. Such an event is unlikely and the risk may be
acceptable; provided that the generator rating is large in relation to the
final circuit protective devices, disconnection times complying with BS
7671 should be achievable for the final circuits. The alternative is to
connect the standby supply to a downstream point where the protection
will be set at a lower level.

Switchgear
For the smaller and simpler generating sets, the switchgear may comprise
a single molded-case circuit breaker mounted on the generating set base
frame. For larger sets the switchgear is installed separately, and for mul-
tiple-set installations the switchgear will comprise several cubicles with
busbars and interconnecting wiring. Sometimes, such switchgear is
installed in the engine room, but this is an extremely noisy location in

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