What causes the Fire-Link system to become active and sound the ISDs?
When the main Fire Alarm Control Unit goes into alarm, the interconnect will cause the NMC to go into alarm as well. The NMC will then send a broadcast signal throughout the power lines to cause all ISDs to sound. One method is by utilizing one NMC initiating zone, which will be tied into the common alarm relay of the main fire alarm control unit. Another method is by utilizing the NMC on-board input bell reversal relay. One spare bell circuit of the main fire control unit can be utilized to wire into this NMC relay. When the main fire alarm control unit alarms, and its signal circuits reverse polarity, the NMC bell relay will change status and thereby trigger the ISDs to sound. This latter method is a ‘slave’ whereby silencing the main fire panel will also silence and reset the NMC.
