In article ,
stu_e wrote:
i believe you are seeing two separate issues:

1. the 58vac hot to ground and neutral to grd are because when you
unplug the input power cord you lose your reference ground. during an
outage, with input and load power cord present, your safety ground
reference would still be present....the voltage would be close to 0
neutral to ground.

I'd say by the Maplin (bit like RS) reference Andy is in the UK.

In the UK, neutral and earth may either be connected at the substation, or
at the home in many modern installations.

On an older installation where the neutral and earth aren't connected in
the home, it's common to see several volts between them.

2. the voltage change:

when input utility is present the apc just passes that through to the
load with some isolation. when the input is lost, the dc to ac
conversion is not perfect and the load does not really care. you would
see this change with an o-scope.

--
*I get enough exercise just pushing my luck.

Dave Plowman London SW
To e-mail, change noise into sound.