Page 246 - Uninterruptible Power Supplies

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

A Bypassed Radiator
This failure occurred at a conventional single-set installation having a
rated output of about 1 MW. The installation was one of many similar
but not identical installations at various locations spread over the
United Kingdom.
On loss of the normal supply the set started and supplied power but
after a short time tripped out on coolant overtemperature. As in the
preceding example, the reason was simple but was less easily explained.
The coolant pipework had been repaired recently and had been installed
incorrectly; the coolant flow bypassed the radiator!
This failure probably occurred during a test run but it demonstrates
the need to conduct a test run after any major work has been completed.

Lack of Fuel

This failure is so simple that it barely seems worth recording but it is
a real-life situation, an example of what actually happens! An impor-
tant set was regularly tested on load, but when it was required to start
following a mains failure it went through its multiple cranking sequence
and registered Fail to Start.
The daily service tank was not automatically topped up and test runs
in the past had drained it. There was no other fault. The daily service
tank should include clear, visible indication of its contents, and its con-
tents should be checked after each period of running.

Changeover of Supplies without a Break
There was a large number of identical small sets installed at locations
spread over the United Kingdom. Some sites experienced frequent trip-
ping at the start of test runs whereas others had no such problems.
It was found that at sites which experienced failures the test proce-
dure involved starting the standby set, opening the normal supply
switch, and immediately closing the standby supply switch. At the sites
which had successful test runs the first two operations were conducted
in reverse order so that the sequence was to open the normal supply
switch, start the standby set, and close the standby supply switch.
At the sites which had successful test runs the starting of the
standby set ensured a delay between opening the normal supply and
closing the standby supply. The load at all of these sites included an ac
electrical rotating machine and the failures demonstrated the need for
a short delay (a few seconds) to allow magnetic fluxes to decay when
changing from one supply to another unless the supplies are synchro-
nized. Some machines will take a very heavy transient current if they
are connected to a supply while running at or near synchronous speed,

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