Backup Power

[Bill Frantz]

Over the years I have observed the attempts to keep several mainframe
computer centers running in spite of power problems.  In general, most of
these centers have installed battery operated UPS systems.  In my
experience, the first year after the UPS was installed, it failed about as
often as the power utility power.  After a year the UPS reliability was
significantly better than the power company, but still not 100%.

The best story I know occurred at a large computer center in the
mid-western part of the United States.  They had 4 way redundant power:

2 power company feeds from separate substations.
Battery operated UPS
3 Diesel generators (all needed to carry the load).

One day, one of the power company feeds was offline for maintenance.  A
thunder storm knocked out the other power company feed.  The UPS gracefully
took over and the generators fired up.  A bad diode in one of the
generators caused it to fail to start.  The power controller, seeing that
it didn't have enough generator capacity to carry the load, shut down the
other 2 generators, and shut down the UPS as well.  Things were very dark
and quiet.

One month later, the same thing happened again.  Management wasn't happy.


The point of these stories is that 100% just is not possible.  You can
extend the 99.99.... for as many nines as you can afford, but you can't
reach 100%.  Practically, you are sometimes lucky if you can reach 99.0%.

6

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