Which fire alarm signal should be used?
Fire alarm signals should be based on response, not on the type of device
- Key in on the basics of NFPA 72, and how it relates to fire alarm signals.
- Learn about the types of notification signals.
- Understand how to prioritize these fire alarm signals.
When designing a fire alarm system, many engineers and designers will select a type of signal based upon the initiating device. For example, if the initiating device is a waterflow switch, the sequence of operations will indicate that the device should initiate an alarm signal. Whereas codes often give the option for duct detectors to initiate an alarm or supervisory signal, duct detectors may require some additional consideration.
However, in accordance with NFPA 72: National Fire Alarm and Signaling Code, the type of signal to be initiated should not be based on the type of device, but rather the desired response to the identified condition.
In addition to defining normal condition and abnormal condition, NFPA 72 Section 3.3.61 defines four other types of conditions. These four types of conditions relate to the four types of responses that should happen if a signal of a certain type is received.
- Alarm condition: An abnormal condition that poses an immediate threat to life, property or mission.
- Pre-alarm condition: An abnormal condition that poses a potential threat to life, property or mission and time is available for investigation.
- Supervisory condition: An abnormal condition in connection with the supervision of other systems, processes or equipment.
- Trouble condition: An abnormal condition in a system due to a fault.
The response to each of these conditions is different and therefore warrants a different type of signal. Because an alarm condition indicates an immediate threat to life, property or mission, one would normally expect the response to involve implementing the emergency plan for the condition identified. The emergency plan may call for total evacuation, partial evacuation, staged evacuation or relocation of some of the occupants. While evacuation may not be immediate, positive alarm sequence and alarm verification are still considered alarm conditions that warrant an alarm response.
Pre-alarm conditions involve an abnormal condition that typically precedes an alarm condition. For example, one could establish a lower sensitivity point in a smoke detector as a point in which the receipt of such a signal results in an investigation of the condition. If the condition continues to worsen, the signal could become an alarm signal.
In this case, in areas in which sensitive electrical equipment is located, a pre-alarm signal could be used to advise personnel that the source of smoke or particles should be investigated. If the condition does not stabilize, the concentration of smoke/particles can continue increasing until reaching the alarm condition sensitivity level, which would then lead to an alarm signal.
This approach has also been used in patient sleeping areas of health care facilities by establishing a pre-alarm signal for the staff of that unit to investigate a potential fire condition. Should there actually be a fire and the alarm threshold is reached, the smoke detector would then initiate an alarm condition to alert staff to implement the emergency plan.
A supervisory condition occurs when the fire alarm system is monitoring another system, process or piece of equipment. A type of supervisory device that is common to most people is a valve supervisory switch on the control valve of a water-based extinguishing system. The valve supervisory switch monitors the position of the control valve and if that position changes, a supervisory alarm should be initiated.
Fire alarm signals
As previously mentioned, codes often allow the designer of the system to choose between an alarm signal and a supervisory signal when the initiating device is a duct detector. Duct detectors are not considered to be a substitute for area smoke detection. Area smoke detection would typically result in an alarm signal in that an immediate threat has been identified.
That may not be the case with a duct detector that is sensing smoke (or particles) in the air handling system, potentially resulting in smoke being brought into the building through the outside air intake. In this instance, there may be no need to implement the emergency plan within the building containing the duct detector, but rather someone should investigate the situation and possibly verify that the supply fan has shutdown.
As indicated in the NFPA 72 definition, a trouble condition is an abnormal condition within the fire alarm system itself, such as a ground fault. With the advent of remote testing devices, there has been confusion as to whether a negative result from a remote testing device should indicate as a trouble condition or a supervisory condition. The appropriate signal is more obvious when the remote test device is part of another system.
For example, if a smoke damper is equipped with a remote test feature and the test indicates that the damper is not operating properly, the signal on the fire alarm control unit should be a supervisory signal.
However, if a fire alarm device, such as an addressable alarm notification appliance, is being tested remotely or automatically, the failure of the device being tested should result in an audible and visual trouble signal, according to NFPA 72-2019 Section 188.8.131.52.
Before including carbon monoxide detectors and mass notification systems in NFPA 72, the priority of signals received was apparent to most users of NFPA 72:
- Supervisory signals take precedence over trouble signals.
- Pre-alarm signals take precedence over supervisory signals.
- Alarm signals take precedence over pre-alarm signals.
Table 1: risk analysis
Where a fire alarm system is also being used for mass notification or carbon monoxide detection, the designer now needs to select the prioritization of those signals with respect to fire alarm signals. NFPA 72 indicates that carbon monoxide detectors are permitted to take precedence over supervisory and trouble signals.
However, as noted above, it is not the type of initiating device, but rather the intended response that should dictate the type of signal as well as the priority of the signal. With duct detectors, there may be instances in which a carbon monoxide signal should be treated the same as a supervisory signal.
Regarding mass notification systems, the answer is not always the same either. NFPA 72 requires that mass notification signals take precedence over supervisory and trouble signals. However, when it comes to a fire alarm signal, mass notification signals are permitted to take precedence over a fire alarm signal.
The risk analysis associated with the mass notification system should identify the events for which the mass notification system should take precedence over the fire alarm signal and the events for which the fire alarm signal should take precedence over the mass notification signal (see Table 1).
The allowance for mass notification signals to take precedence over a fire alarm signal is one way to address active shooter incidents. Because fire alarm systems have been used in several active shooter incidents to lure the occupants out of the building, there has been some public and legislative action to eliminate the requirement of fire alarm systems in schools. Some have suggested that fire alarm systems in schools should be pre-signal alarm systems. However, most codes would not permit a new fire alarm system in a school to be a pre-signal type fire alarm system.
Proper prioritization of signals is another way to address the use of a fire alarm system during an active shooter incident concern in a manner that is permitted by current codes and standards where the solution does not require reducing the current level of protection required in educational occupancies. If the educational facility has a mass notification system, the mass notification system signal could take precedence over a fire alarm signal during an active shooter incident.
More specifically, if the fire alarm system is actuated early in the incident, the message to the occupants and the anticipated response would be to initiate evacuation of the facility. If it was determined that the event is an active shooter incident, with or without a fire event, the mass notification system could be used to deliver a different message to the building occupants.
When designing a fire alarm system, the fire protection engineer needs to identify the condition being detected and the desired response to the condition to determine the type of signal to be processed by the fire alarm control unit. It is not as simple as saying all initiating devices of a certain type shall initiate a certain type of signal.
In addition, the design should consider the prioritization of the anticipated type of signals. While in some instances NFPA 72 will dictate that a certain type of signal takes precedence over another type of signal, there are instances in which the prioritization of the signals needs to be determine by the designer of the system. Mass notification systems and carbon monoxide detection are two examples in which the prioritization of the signals may not always be the same or consistent for all types of conditions/events that have been identified/detected.