First off, remember that the
Signaling Line Circuit (SLC) is basically an extension of the Central Processing Unit in the Fire Alarm Control Panel. By itself, it does not detect fires or switch anything on or off. Instead, input modules on the SLC interpret the signals from a detector into the data sent on the SLC, and output modules interpret the data on the SLC to open or close control relay contacts.
Control relays, by themselves, do not provide any
voltage or power. However, the relays can be used to switch on or off power like a light switch turns on and off power to a light bulb.
Source of Power
To supply the 2 to 24 Volts DC for the sound board, you need power from somewhere. The trouble is this is a Life Safety Fire Alarm System being used to turn off the sound in the auditorium.
That means that the source of this 2 to 24 VDC has to be provided by a Fire Rated power supply that is monitored for trouble by the Fire Alarm Control Panel (FACP). This could be power from the FACP itself, a horn and strobe power supply for a Notification Appliance Circuit (NAC), or a Fire Rated Auxiliary Power Supply.
Using an input module, the voltage at the control relay that is installed by the sound board for sound shutdown also has to be monitored to make sure the power doesn't get disconnected or turned off.
As a power source for the shutdown, the best thing to use is the auxiliary power supply output terminals on the Fire Alarm Control Panel. Second choice would be the auxiliary power supply output terminals on the Booster Power Supply (or Signal Power Expander, or whatever the manufacturer calls the box that provides extra power to the Notification Appliance Circuit of NAC).
However, if the wires are already buried in the walls and ceilings, and an extra power supply wire can't be run all the way from the Fire Alarm Control Panel, a fire rated 12 or 24 volt, battery backed up power supply will have to be installed.
One thing about the added fire rated power supply. Don't forget to us an input module on the SLC to monitor the trouble contacts on the added power supply. That way if anything happens like a
battery going bad, the troubles on the power supply can be sent back to the Fire Alarm Control Panel, and then the problem can be fixed.
Notification Appliance Circuit (NAC)
The NAC is a bad choice for the sound shutdown signal because connecting the sound system shutdown to the Notification Appliance Circuit (NAC) destroys the Class B or Class A status of the horn and strobe (NAC) circuit.
If there is a fire and the fire alarm panel is silenced but not reset, another problem is that using the NAC as a signal source will allow the auditorium sound control board to automatically reactivate the normal sound reinforcement. This could indicate to those on stage and those in the audience that everything is OK, when it's not.
Service After the Sale
Also keep in mind, to the technicians working on the system at a later time, your using
the NAC as sound shutdown signal source won't be intuitive. For many years to come, fire alarm service technicians may be calling on you to help them figure out problems with sound shutdown.
Douglas Krantz
