Page 239 - Uninterruptible Power Supplies

Notes on Systems Installation
Notes on Systems Installation 237

680

660

640

620

600

580
Figure 9.1 Wave trace.

Where wide input voltage variations are to be expected, a normal by-
pass system may not be used. In such cases the bypass is obtained from
a standby UPS system. If this is the chosen design, then the standby
system must be in an operative mode that is inverter functioning.
Other problems to be expected with such systems are the need to
remember that most such systems are usually adjacent to the rail track
and may suffer from EMC emitted by the raw power from the overhead
lines or from passing traction units.
Many such installations clearly have to contend with wide tempera-
ture variations and other problems such as dust. Dust may contain iron
from brake shoes. Air-conditioned steel-clad containers are often used
to house such equipment. The steel cladding assists in providing a fara-
day cage to inhibit EMC problems.
Whereas the main market is endeavoring to reduce the utilization of
wound components, an area where the use of full isolation from the
incoming supply is still essential is in the medical field. UPS units are
often provided to guarantee power to respirators, heart pumps, and
other medical apparatus that is connected to the patient and any power
problems may affect the sensitive medical monitoring systems.
The UPS under such conditions should comply with EN 60601 or UL
2601; this implies a 4000 V test and also that earth leakage is nil. To
comply with this the solution usually adopted is to use a low-impedance

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