Page 252 - Uninterruptible Power Supplies

Some System Failures: The Light of Experience!
250 Chapter Ten

the system interpreted as a battery failure. Sufficient warning was
given to ensure that the fire did not spread from the battery room.
Needless to say, the entire computer network was shut down.
The resulting investigation established that the three parallel
strings of 400-V 400-Ah flooded lead acid cells had not been properly
maintained. Maintenance logs only recorded that the cells had merely
had electrolyte levels topped up. There was no record of torque testing
the cell terminals. It was found that the terminals on some cells were
only finger tight! The heating and cooling caused by the discharge
cycles had caused further deterioration of the already bad connections
and eventually arcing had occurred, initiating a serious fire.
This incident indicates the importance of proper battery mainte-
nance. Before passing on to the next incident, spare a thought for the
individual who had to enter that battery room to disconnect the system!

An Unsuccessful Attempt at Cost Reduction
A project engineer had to design a system comprising a static UPS, a
generating set, and the usual ancillary equipment to complete the
installation. After the system had been installed it was found that the
generating set was unable to support the load in bypass mode.
In unraveling the problem it was discovered that the project engineer
had approached several suppliers for a complete installation and had
chosen a particular supplier, only to be told by his management that
the cost was too high. As a result he decided to design the system him-
self, but with insufficient knowledge of the component parts. The load
included a large number of office-type PCs with a high harmonic con-
tent and a high crest factor, measured at the generator as 3:1. The gen-
erator and its voltage regulator had been chosen without reference to
the harmonic loading.
This failure indicates the hazards of not employing a main contractor
with responsibility for coordinating all aspects of the installation. An
experienced contractor, before accepting responsibility, would undoubt-
edly have asked questions about the load and its harmonic content.

Empty Sumps
Exporting generating sets to developing countries can lead to interest-
ing problems! Two sets were shipped for a computer center and
installed by local staff. On commissioning, both sets failed due to there
being no lubricating oil in the sumps. Generating sets are shipped with
drained sumps, and the need to add oil had been overlooked.

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