Running 3 phase 780s on single phase power (was RE: hams on
bob at jfcl.com
Mon Jan 19 14:35:21 CST 2009
>Roy J. Tellason (rtellason at verizon.net) wrote:
>Ah, THAT was the missing piece...
>Now I understand what the concern is with this stuff!
Right - the issue is with the original PDU, which Ian specifically said
they did not rewire. That means one neutral, both inside the PDU, in the
power cord, and in the power plug. The loads inside the 780 (which are
single phase, as Ian said) have nothing to do with the issue.
So, the 780 PDU has a three phase wye power cord (five conductors,
including ground). I'm betting that they have a 220V split phase circuit to
run it from, so they've wired two of the three phases together for one side
of the 220V circuit, and the third phase to the other side. Best case,
assuming the loads inside the 780 are balanced, that would make the neutral
current the same as one of the phase currents.
But that's not really a safe situation - if any of the internal loads
within the 780 is turned off, disconnected, or blows its own breaker, then
the neutral current will _increase_ due to the imbalance. Worst case in
this situation the neutral current would be 2x normal. They probably get
away with this because the PDU isn't loaded to anywhere near it's rated
And let's hope they really aren't running it from three separate circuits
on the same phase.
Were it my 780, I'd rewire the PDU to be 220V split phase (just
redistribute the third phase's outlets amongst the other two). The result
would only have 2/3rds the current carrying capacity of the original, but
it'd be a safe arrangement.
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