smoke alarm interconnect problem

Hi,
I have a pair of new Kidde 4870 smoke alarms interconnected but there seems to be a problem. I got these to replace my old ones which failed after long service.
I have live (red) and neutral (black) connected from a lighting circuit. When I test these with a voltage tester and no smoke alarms connected I get 240v from the red, zero from the interconnect and the netral.

my interconnect is a single core white/black stripe which I have connected to the middle (orange) interconnect wire on each connector. The neutral (black) is connected to each blue wire in the connector.

Both alarms have the green light on, but when testing, only the alarm on the unit I am pushing goes off, the conected alarm does not sound. I am confident that the interconnect wire is connected properly. Any ideas please?

If i read this right, you have 4 wires between detectors. Normaly there will only be three, Line/neutral and interconnect. There is usually three core and earth used.

Drop the power from the feed. We know that has power, test between 1st and next smoke, you need a test meter set on Ohms or bell.