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Peter,
It's a little technical but there are 3 main possibilities
1) Output is isolated with no earth. No output RCD will trip. (very cheap portable type inverters I have seen - like a double insulated device)
2) Output is isolated and an earth is provided which links back to the main input earth. No output RCD will trip (really no better than 1)
3) Output is isolated and an earth is provided which links back to the main input earth AND the output of the UPS neutral is bonded to the earth (ie an MEN link). Output RCD will trip
If your UPS has an output RCD (eg plug in type) and it trips, then this means you almost certainly have (3) above. But to be 100% sure, the earth on the output needs to be bonded to the input earth.
Why. Well the RCD is tested by diverting a current to the earth most likely in the cable. If this goes back to the inverter without being bonded to the input, then this is no good. You are most likely to get zapped through common earth or some other conductive path other than the earth in the cable. The return current can only come back via the input's 240V cable from your switchboard to complete the cct. Therefore it is important the earth on the output is bonded the input's earth. Unplug the UPS from the 240V input supply, ensure it is OFF and test this with a multimeter from the input earth terminal to the output earth and ensure it is continuous.
Hope this helps. If in doubt, engage an electrician always. I have an APC inverter and this is wired like (3) above with the input earth bonded to the battery (-) and output earth and a RCD works satisfactorily on the output without affecting the main RCD in the switchboard.
Darrin...
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