UPS gives funny voltages - why?

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

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

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

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.

I didn't get such wierd voltages on battery operation with a much older
APC BK-300 (much larger battery, all-metal case); hot-neutral and
hot-ground both measured 118 VAC, neutral-ground was slightly under 1
VAC.

I measured with both a Fluke 73 digital meter (true RMP) and an analog
meter with 10K/volt sensitivity. The analog meter gave very different
voltages with each backup supply running off battery, but I assume it
was caused by the AC not being a real sine wave.

 

Suggests the output is connected directly to the mains.

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.

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"

 

the chances are when its not conected to an ac suply the output is floating
with respect to ground, so any voltages relative to gnd are due to leakage,
with a load conected there is more balanced leakage.

Colin =^.^=

 

Backup is line powered and is just passing thru the relationship that
your AC line has.

As someone else said, ground is floating and the load leakages are
pretty nicely balanced, and lower impedance than your Fluke.

Here the output is completely floating, but you can see that the
leakages in the backup are not particularly balanced. The 10 MOhm
impedance of the Fluke is significant here; it has pulled each one
down a bit so that the last 2 numbers no longer add up to the first.
If you had a second Fluke so that you could actually make those 2
measurements simultaneously, you would get numbers that add up right.

-

 

The PC PSU has RF suppression (?) capacitors between A-E and N-E. If
the earth pin is disconnected, then its potential floats to a point
midway between A and N.


- Franc Zabkar