SMT3000I output over volt event when switching to battery but only with server switch mode psus attached

[GraemeA]

Hi, hoping someone can shed some light on an issue that has appeared on one of our clients SMT3000I units. which is normally attached to 2 server power supplies & a handful of network switches. The clients other SMT3000I feeds a similar load with no issues.

Unit works ok on 230VAC mains with a load of approximately 10% . However whenever the UPS either self-tests or goes to battery following a mains fail, the unit growls, shudders drops the load & goes into error, reporting OUTPUT OVER VOLT.

Now we have had this unit in our workshop & initially failed to replicate the issue. We had a load of 8% derived from  halogen  lamp & could not get the error to happen. Also tried the load attached to the UPS with an isolation transformer, still could not get the error to happen.

We have however managed to replicate the issue .

If you start the UPS with NO LOAD, it starts OK, self tests ok & then is ready online. If you plug in any load you want with a server powersupply in the mix of equipment, everything remains ok until you self test the unit. At this point when the unit goes on battery, within a second the output voltage shoots up to anywhere between 270 & 335Volts, followed by the unit dropping the load & reporting the OUTPUT OVER VOLT.

Test powersupplies & loads operate perfectly on another SMT3000I.

If you power-up the UPS with the load attached all is well until the startup self test, when the issue reoccurs.

Model & S/No. SMT3000I  IS1130008405. FW UPS 5.0, CO 02.1 UBL 05.0 CBL 5.21.

Any suggestions?