I've purchased a Line Interactive UPS 1200VA. It's supposed to give 8~20min backup (as written in the manual). This UPS has transfer time < 10ms. It works fine when the mains is active.

But the problem is that when the power went off, the PC also went off for a fraction of a second and started rebooting.

The strangest thing though is that it has to be a proper power failure by the service supplier. If I just turn off the power at the wall plug, the UPS kicks in immediately and the PC carries on working fine.

What's the solution of that? What should I do? My hard drive was physically damaged because of improper shut down & restart. I've 5~8 times power outage in my country so I badly need an UPS.