Paging electronic buffs

Had a wierd problem yesterday.

I've just been setting up a new server PC.

1 UPS to which PC, switch, wifi router are connected. Also one old TFT
monitor, in half its case, screwed to shelf.

This monitors PSU is dead. Not a problem, I ripped out dead PSU bits
and added a simple cable which takes 12V DC. Dug out an ancient 12V 5A
CB power supply, the old stylee transformer, cheapo reg/rect and big
heatsink jobby. Tested in garage, fine, 14V DC output, bit of AC ripple
but WGAF, it'll do.

Plugged it into UPS and monitor.

Turn monitor on.

bzzzzzzz... but monitor works.

Boot PC so monitor displays something.

BZZZZZZZZZ!!!!

<shrug> Its an ancient PSU, old transformers get noisy, it'll still
work.

2 mins later I smell burning and see smoke coming from PSU. Obviously
turn off and unplug.



Now, the (very) odd thing is that the mains cable was damn warm. The
mains cable was connected to the UPS (which didn't bat an eyelid or get
warm).

I know the PSU didn't output the kind of power needed to heat up its
mains cable, because if it had the little 12V supply cable I've put on
it would have exploded.

I know the monitor works.

I know the PSU works (on a normal 240V supply).

What the hell is going on between the UPS and PSU?

On a UPS' output does earth come from a centre tapping of the output
transformer, live one side and neutral the other? So if you short
neutral to earth you effectively short out half the output transformer?

Very worrying in that the fuse in the PSUs plug didn't blow (so the
current must have been in neutral/earth) and the RCD in the house
didn't trip, so the UPS must completly isolate its output, and any
earth leakage current is hidden from the houses RCD.

 

It wasn't just warm, it was "another minute and it would have melted"
warm.

Plus the 4 way extension that's plugged into the UPS and its cable (13A
trimmed down to 1') was warm.