ml wrote in :

i got a 6kv liebert ups, trips the dc breaker on testing fail to
backup

never worked on a ups before, i can say the ac side seems to be ok,
and the charger seems ok as the batteries stay floated /charged front
pannel lcd seems to work

any clues what might be a good path to start on or perhaps someone
might
know what these ups typically blow?

Most UPS's are pretty simple.
1. AC line voltage flows into the UPS and straight to the load.
2. AC line voltage also charges storage batteries.
3. When AC line voltage fails, an AC inverter starts up
4. AC inverter draws DC power from the storage batteries
5. AC inverter delivers AC power to the load
6. When AC line voltage comes back on, go back to 1.

So you have some pretty simple systems to analyze and troubleshoot.

You say the batteries seem to be fully charged, which probably means the
charging circuit is working and maybe even the batteries are OK.

You say the DC breaker trips and it doesn't put out any power under
backup mode. So you've isolated the problem to the DC wiring and the
input of the inverter circuit. Try disconnecting some cables and see if
you can isolate the short circuit or low-resistance circuit that's
drawing too much current. If it turns out to be the inverter, take it
out, open it up, and start looking for shorted components.

Good luck!