On Thu, 29 Oct 2020 10:19:10 +0000, Martin Brown wrote:

Despite the ring circuit being correctly wired and being quite
short,
and there being little load on it...when I switch on the laser
printer, the UPS shifts taps to compensate for the voltage drop -
it
is clearly audible.

That sort of surprises me, there will be an inrush as the filter
capacitor and PSU capacitors charge but wouldn't expect that to be
greatly different to a PC.

Laser printers are a brutal load to get the fuser element that melts the
toner onto the paper up to temperature quickly is a brief but very high
surge current from cold. Easily enough to trip a UPS out.

Ah, I read "switch on the laser printer" as powering up, rather than
sending something to print. Can't see why the fuser has to be heated
on power up.

--
Cheers
Dave.