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.

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.

"andy12345" wrote in message
...
Hello,
It has come to my attention recently - after using computer
communications voice software that I can reduce the hum of the
microphone recording by lightly touching my pc case.
I was always under the impression that the earthing worked in my room.
However, I have bypassed the psu earth temporarily simply by
connecting a wire from a clamp in the pc case(unpainted surface) to
the earth contact hole of the mains socket.
This does not reduce the hum at all. However, by touching the case
and touching the floor with the other hand I can reduce the hum on
the signal. Therefore, is the earthing failing at some point in the
house or is there something that I am ignorant of?

I can supply any information if you require more detail.
thanks
goodbye