What is the difference between fail-safe operation and the supervision requirements of the fire codes and NFPA 72?
The underlying principle of fail-safe design assumes that a process or item of equipment can be designed to take the process to a safe status on equipment failure or power interruption. This approach requires that the switch to “safe state” be possible without power and that the “normal operating state” of the equipment utilize energized control circuits. Almost all detection, extinguishing and notification circuits of a Fire Alarm system are not normally energized and are not “fail-safe”. In order to be sure these fire circuits are intact and ready for use when needed these circuits are “supervised”. Supervision is normally done using a small current or voltage passed through a field circuit device called an “end of line device”. This small current or voltage is continuously monitored to verify that the circuit is intact and ready for operation. Fire Alarm systems in many cases need to activate suppression or notification equipment in the event of a hazardous condition and these syste
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