On Sunday, 14 June 2015 13:03:03 UTC+1, John Mulrooney wrote:
I'm starting to install some interlinked smoke and heat alarms. Can they
use L & N from different circuits, or sgould they have a seperate dedicated
one.

All alarms must be on the same circuit.

It doesn't have to be a dedicated alarm-only circuit and there are good reasons for it not being so; but the alarms should then have battery or capacitor back-up.

On a mixed use circuit you are likely to notice a supply failure more quickly than if only the alarms are affected.

Owain