Fire alarm systems are custom built; like snowflakes, there are no two fire alarm systems alike. That goes for the myriad requirements added by the AHJs, the differences between manufacturers, and the building wiring.
Sometimes, leaving the strobes on while the horns are silenced is one of the differences in fire alarm systems.
Turning Off the Strobes when the Alarms are Silenced
Some fire alarm systems turn off the strobes when the system is silenced, while other fire alarm systems require the strobes to remain on until the fire alarm system is reset.
Whether the strobes stay on until reset, or turn off when the fire alarm system is silenced is usually decided on before the system is installed. The final decision can be made by the fire marshal (AHJ), the insurance agent (AHJ), the design engineer (AHJ), or even the building owner (AHJ).
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Once the decision is made to leave the strobes on even when the horns are silenced, then it's up to the installing company to "make it so". The installing company can sometimes choose which of several manufacturers to use.
The programming inside the fire alarm control panel determines whether of not the strobes will turn off when the silence button is pushed.
Proprietary Fire Alarm System
Every fire alarm system is proprietary; all manufacturers keep the exact method they are using under-wraps to reduce the liability of having some other company's second-rate equipment from being installed on their system.
With many manufacturers of fire alarm systems throughout the world, that makes for many different methods to power and control the strobes.
Synchronization
Synchronization is needed to keep the strobes flashing in exact harmonization with each other. If the strobes didn't flash together, epileptic seizures can be a real life-safety issue in a few individuals.
With synchronization, it is easy for the fire alarm control panel to leave the strobes on while the horns are silenced. But even with synchronization, there are a lot of differences between the fire alarm system in one building and the next.
Synchronization is another one of those proprietary things, so the exact signals used to synchronize the strobes is kept under wraps by each manufacturer.
Old Conventional Notification Appliance Circuits (NACs)
Originally, the NACs throughout the buildings used two-condition steady-state DC circuits; either the NAC circuits backward backward-voltage, and were in a low-powered supervision state, or else they had forward-voltage, and were in a high-powered alarm state. This voltage, backward or forward voltage, was on the NAC circuit in the building. By reversing the voltage, the fire alarm control panel could turn-on or turn-off the power to the horns and strobes.
Because there were no synchronization signals on the NAC circuits at this time, the only way to keep the strobes flashing while the horns were silenced was to use two discrete NAC circuits. One NAC circuit was used to power the horns, and one NAC circuit was used to power the strobes.
The panel could then turn-on both NAC circuits when there was a fire alarm, but only turn-off the horn NAC circuit when silencing only the horns.
Adding Synchronization to Conventional NACs
When the horns and strobes are synchronized, it's easy for the fire alarm panel to turn off the horns by themselves. However, there are dozens of methods used, and each building uses different brands of equipment, and each building is also wired differently, so it is impossible to describe all of the ways that the strobes can continue to flash when the horns are silenced.
Douglas Krantz
