Page 251 - Uninterruptible Power Supplies

Some System Failures: The Light of Experience!
Some System Failures: The Light of Experience! 249

An Overenthusiastic Charging Regime
This event occurred many years ago when flooded cells were in use. The
conditions are unlikely to be repeated nowadays, but a description may
serve a useful purpose.
The installation included a conventional UPS using a nickel-cad-
mium battery as its energy store. The purchasers of the equipment
had, against strong advice from the manufacturer, insisted on an unre-
alistic charging regime which required a boost charge after every sup-
ply failure. It is for this reason that nickel-cadmium cells were used
instead of the usual lead acid cells.
One weekend (when the installation was not attended) the supply
authority was working on the local distribution system and there was
a series of interruptions of short duration. As a consequence, the
already fully charged battery was subjected to a continuous boost
charging rate.
After many hours there was an impressive failure, one or more cells
exploded and devastated the battery room. The electrolyte level may
have been low at the commencement of the boost charging. There was
no way of knowing, but it is believed that the electrolyte level dropped
during the charging due to the predictable electrolysis. As a result of
the electrolysis, the cells became full of an explosive mixture of hydro-
gen and oxygen and the atmosphere within the battery room was prob-
ably overloaded with a hydrogen/oxygen mixture.
As the electrolyte level continued to fall, there came a time when the
electrolyte was level with the bottom of the plates; the inevitable spark
occurred causing the explosion.
The situation would not have arisen if valve-regulated recombination
cells had been used because boost charging of such cells is not allowed.
The incident is described here because it indicates a hazard of which
readers should be aware; it also indicates the danger of failing to con-
sider the probable effect of using an unconventional charging regime.
Loose Intercell Connections On
a UPS Battery
A series of winter storms resulted in an irregular mains supply to a
large computer network in a city center. As the disturbances were
erratic and of short duration, the supporting generating set was not
started, thus over a period of a few hours the battery was called on to
support the load whenever the mains was absent.
This resulted in a serious fire enveloping the entire battery room.
Site personnel were made aware of the situation by the UPS in a sepa-
rate room; in fact the UPS registered wide dc voltage variations which

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