Switching on UPS, bat or mains 1st?

[domi]

Hi,
First time switching a UPS on problem. I have not blown it up so far by switching the batteries in followed by the mains and without load but am worried as the relay click is rather strong.

From this forum I learned switching mains in first and then the batts will blow the UPS. If this is correct, I would like to know why this is so.

Thanks.

[joestue]

what kind of a ups is this?

if it is intended to be an "uninteruptible power supply" then it should not care if switches on under load.

however you mention switching the battery under load? that may be a problem.