Townhouse SA replacement where flow switch triggers the SA

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Townhouse SA replacement where flow switch triggers the SA

Post by firetech1 » Thu Apr 11, 2019 5:44 am

In case you bump into Firex smoke alarms in a townhouse you have a good chance that 120 Volt is used for triggering the other smoke alarms. In this case the electrician run 120 Volt to the normally open contact of the flow switch. When the sprinkler head pops the normally open contact gets closed and the 120 Volt appears on the signal line of the old school smoke alarms, sounding all of them in the building. So far so good.

During the inspection you write up these smoke alarms as 10+, therefore their replacement will be quoted. The problem is that the new smoke alarms use 9 Volt DC for signaling and not 120 Volt AC. Therefore you cannot just replace the old smoke alarms with the new. They will be partially or fully toasted once you test the flow switch.

I don't promote any electrical work to be done other by qualified electrician but let me share here few thoughts that you should take into consideration in case you cannot resist to alter an electrical installation:

You need to find a junction box where both, the switched wire of the flow switch and the signaling wire of the smoke alarms are present. Then you need to separate them and insert your SM120X Kidde relay/power supply module into the circuit (assuming the box big enough to accommodate the module). When the flow switch gets activated, energizes the SM120X, and that will put 9 Volt on the signal line, triggering the interconnected smoke alarms. Now you are safe to replace your smoke alarms, you will not fry them.

Easy-peasy, except when it is not. Why?

Because you sometimes have 9 or more wires in that junction box, and you need to find that wire that will deliver the 9 Volt towards the smoke alarms and isolate from the 120 V wire that comes from the flow switch. And the flow switch might also triggers other devices such as strobe that still has to receive the 120 Volt (strobe might have a conversion plate for 24V applications).

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