larry moe 'n curly wrote:

I have a Conext (by APC) model CNB-325 battery backup power supply (not
a real UPS) that shows funny voltages when the power cord is
disconnected.

Backup supply plugged into AC outlet;

hot-neutral: 120 VAC
hot-ground: 120 VAC
neutral-ground: 0.7 VAC

Suggests the output is connected directly to the mains.

Backup supply unplugged from AC outlet, computer used as load:

hot-neutral: 115 VAC
hot-ground: 58 VAC
neutral-ground: 58 VAC

This is not unusual; the output is symmetrical about earth for safety
(as the biggest shock you can get is 58 volts). Most isolation
transformers are wired this way.

Backup supply unplugged form AC outlet, no load:

hot-neutral: 115 VAC
hot-ground: 19 VAC
neutral-ground: 60 VAC

A different model Conext, model CNB-300, behaved similarly, only the
no-load voltage on battery power was 90 VAC neutral-ground and 40 VAC
hot-ground.

It sounds like the lack of load is doing strange things to the
regulation, and probably the waveform. Many supplies that are derived
electronically exhibit this effect - and remember that uneven
waveforms will play havoc with a meter because it won't know which bit
it's supposed to be measuring.

Look at it on a scope and see what's actually coming out.

--

rgds
LAurence

...."Dial only the last eight digits for calls within the London area"