Approved methods of wiring duct detector shutdown

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Mike Meredith

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This has been touched on in earlier threads, but I didn't see a specific response. There is a practice currently in use of wiring duct dector shutdown contacts to a PLC or directly to a VFD circuit. Most OEM Cut Sheets for their duct detectors show the shutdown hardwired directly in the motor control start/stop circuit, and this seems to be the best practice.

However, I've found nothing in NFPA 90A or NFPA 72 that prevents landing the shutdown to a PLC, VFD, or other Automation Control. In my (poor) thoughts, if the device (PLC or VFD) is not listed for fire alarm service, then is it acceptable to interlock the Duct Detector to it?

Thanks...
 
If you wire the shutdown contacts directly into the starter control circuit (sometimes that is an EPO on the VFD if there is no formal starter), it will definitely shut down the fan.
If you wire it to a BMS or other PLC that is not listed for fire or smoke control service, then you don't know that the fan will definitely shutdown.
However, I don't have a code section to quote for you.
 
I found one wired to the 24 volt transformer that kills the 24 volts to the thermostat, this was a combo gas heat/air unit, and with it wired like this in heat mode the line voltage fire box control will keep the fan going, we had to install a relay to kill the power to the unit to shut both gas, and fan off.
 
Where I used to work, you had to provide both hardware and software interlocks...

Where I used to work, you had to provide both hardware and software interlocks...

Conventional duct smoke detector:

1.) Main alarm contacts to alarm zone or addressable alarm input of fire alarm.

2.) Aux. alarm contact #1 to motor starter (wired as a safety in the ladder diagram).

3.) Aux. alarm contact #2 to PLC or BMS input.


Addressable duct smoke detector:

1.) Addressable control relay #1 wired to power the coil of an interposing relay. Interposing relay wired to motor starter (wired as a safety in the ladder diagram).

3.) Addressable control relay #2 wired to PLC or BMS input.


In this manner, regardless of any problems with the BMS software or hardware, we had fan shutdown.

Speaking of VFD's, the literature for every model I've worked on showed where the dry contacts from a duct smoke detector should be landed.
 
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