How can I switch off my Optima Compact for house electrical work to be done?

When I switch off mains power to the house, with the alarm in Day mode, my Optima Compact activates with a highly nasty sound level, and code entry has no effect. Switching mains power back on then fully activates the alarm which code entry then shuts off. How can I shut off the alarm to allow electrical work on the house to be done? Looking at other questions on this website suggests that the answer could be back up battery failure? If this is the case does the whole street have to endure the alarm sounding until it is fixed? It also begs an idiot question. If mains is off and back up battery has failed, what on earth is making the alarm sound? I would be grateful for any help.