[c-nsp] Redundancy vs. Paranoia

[Bill Wichers]

> The larger sites I have seen have had three phase power supplies, and run
> one
> phase to each PSU.  As each phase has it's own transformer/fuse/wiring
> setup
> from the supplier, it helps provide some redundancy in case power is
> partially
> lost.

That's not exactly true... Three-phase service usually is delivered with a
single transformer with three windings (which saves on iron for the core,
size, weight, etc.). For circuit breakers, code actually *requires* that
all three phases be interrupted if any one phase has a fault. As such, 3
phase service doesn't really provide "redundancy", although I have seen in
some of our suburban POPs where not all phases will drop from the utility
during storms so it can sometimes help.

I would recommend each power supply be placed on a seperate circuit
though, since I have seen many cases where a single circuit will trip and
if your power supplies are on different circuits you won't go down if a
single circuit fails. Best is to have each supply on a seperate UPS and
generator, but that's not always possible. Redundant supplies also have
the advantage of allowing you to move the electrical load of a given
device between circuits/UPSes/etc. without taking the device out of
service. This is sometimes helpful if things need to be rearranged in a
rack.

     -Bill

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Waveform Technology
UNIX Systems Administrator