Smoke alarm wirng.

[javaman]

Doing some redecorating and decided to replace 15 year old dicon alarms with BRK 670MBX.
1 in kitchen one on landing.
Disconnected kitchen alarm and had to change over connection.
No problems there only three wires. Cable supplying power colours blue , yellow red(EARTHcutback inside sheathing.) Connected new alarm as old. Blue to blue on connector Yellow to white and red to brown.
Turned on power and both alarms started sounding.
Disconnected upstairs alarm.
Checked the wiring for upstairs and I have 2 cables running to it.
First cable red , black and earth. 2nd cable red , blue and yellow..Both have earth wires joined together.
Both reds are joined together and black and blue joined together.
Connections to alarm the 2 reds are connected to the brown wire on alarm , white is connected to yellow.Blue is not connected. When removing original alarm this was either not connected or came loose.Looking at the wire it doesn't appear crimped and the connector block screw is fully in.
Any ideas to get both alarms working properly.

Garry

 

[PBC_1966]

Blue is not connected. When removing original alarm this was either not connected or came loose.

The blue is the neutral - The black from the supply cable, blue on the alarm, and blue on the 3-core to the other alarm should be connected together.

 

[javaman]

Blue is not connected. When removing original alarm this was either not connected or came loose.

The blue is the neutral - The black from the supply cable, blue on the alarm, and blue on the 3-core to the other alarm should be connected together.

So black from cable and blue from separate cable should be connected together and connected to blue wire on alarm.
Is this right.
Thanks for the quick reply by the way
Garry

 

[PBC_1966]

Based on what you've said about the rest of the wiring, yes. You need to bring live & neutral from the supply to both alarms, which you're doing using the red & blue of the 3-core. The yellow is then used as the interconnection so that both alarms sound when either one is triggered.

 

[Echo the husky]

Did you replace both alarms at the same time? Mixing manufacturers alarms on the same interconnected circuit could cause problems, like them sounding all the time.

 

[javaman]

Did you replace both alarms at the same time? Mixing manufacturers alarms on the same interconnected circuit could cause problems, like them sounding all the time.

Yes I did.
Will have a look again in the morning.
Cant think why the blue wire from the alarm to the cable has come disconnected though.
Garry