On Monday, 29 July 2019 17:06:34 UTC+1, MM wrote:
On Fri, 26 Jul 2019 15:21:32 -0700 (PDT), wrote:

On Friday, 26 July 2019 12:27:41 UTC+1, Andrew wrote:

If there are two separate incomers (?originally 2 houses) then if they
are on separate phases, be careful if they resort to running long
extension leads from the unaffected half into the 'dead' half, because
of the voltage difference.

Some houses use three-phase power. Nobody seems to worry about that.
Does anyone take special precautions when using a UPS whose output
phase may be random relative to other unprotected mains circuits nearby?

This is a new one for me. *I* have two UPSs, one for each PC. They are
both by APC. I just plug 'em into the mains.

MM

Just like everyone else does. However, if the mains fails the two UPSs
will be outputting asynchronously generated waveforms, so some of the
time the outputs will almost certainly be out of phase with each other
and the relative peak voltages will be even higher than with two phases
from a three-phase mains supply.
In other words, your situation with two UPSs is potentially more
"dangerous" than running an extension lead from one phase of a
three-phase mains supply to an area with a different phase.

In reality, there is no problem, just as there is no problem with
having different phases of a three-phase supply near each other.
This is because each live connection is insulated and to suffer
the effects of the higher voltage difference there would have to be
multiple simultaneous insulation failures.

(I know that three phase mains will deliver much higher fault
currents in a phase to phase fault than UPSs, but for that to happen
there still need to be multiple simultaneous insulation failures.)

John