Page 253 - Uninterruptible Power Supplies

Some System Failures: The Light of Experience!
Some System Failures: The Light of Experience! 251

Lack of Cooling Air
A large city bank installed a multimodule parallel redundant UPS sys-
tem which was commissioned without problems but failed within a
week. The load included a large number of office PCs and, as the fail-
ure was caused by overheating of a large output transformer, it was
suspected that the harmonic currents drawn by the PCs had caused
overheating. Measurements of the load current taken adjacent to the
transformer indicated a 3:1 crest factor which is within the capabilities
of most static UPS.
Further investigation revealed that the overheating had nothing to
do with harmonic currents, it was due to a lack of cooling air. The UPS
units had cool air fed to them from under the floor, exhaust air being
emitted from the top of the cubicles. At a central T junction the under-
floor duct was joined by the main cool air duct, and the transformer was
mounted above the T junction. It was intended that the cool air would
flow left and right to the cubicles and upwards to the transformer, but
it was found that very little air flowed upwards hence the overheating
of the transformer. Modifying the T junction improved the air flow and
resulted in satisfactory operation of the UPS.
It is not easy to check every detail at the commissioning stage, but
the air flow rates should really have been checked at some time before
putting the equipment into service.

An Inadequate Supporting Structure
These two incidents concern the close-coupled diesel/clutch/kinetic
store/generator type of UPS, both being installed within steel-framed
buildings and supported on steel joists.
At one site problems were experienced during installation due to the
deflection and curvature of the supporting joists. To overcome the prob-
lem it was decided to install steel columnar supports beneath the set.
Fortunately there was a firm base able to take the load, but the columns
were inconvenient, restrictive, and expensive.
At the other site the set was installed without any known problems
but there followed a series of bearing failures which were attributed to
the resilience of the mounting.
The size and weight of these machines makes major repair or main-
tenance very difficult and structural problems are best avoided. These
events occurred long ago and lessons have been learned. However, if
such machines are to be supported on other than a solid foundation, it
would be wise to discuss the installation with the manufacturer.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.