Bit odd to answer my own question but looking through the 2100 other related Qs I managed to determine that the alarm sounds because the internal rechargeable battery in the control panel is spent -so when the power is off, if the battery doesn't kick in, the alarm senses a tamper attack and sounds (from a reserve battery in the exterior box probably)-anyway replaced the battery and I can turn off the power without a sound (and now I can try and test the wiring in the landing lights that keep blowing)
jimmyfed, October 2009
