Should a carbon monoxide condition result in an alarm signal or in a supervisory signal? The answer is: It depends.

Combined, NFPA 721 and NFPA 7202 address a full range of signaling systems, from simple smoke or carbon monoxide alarms, to basic fire alarm systems, to complex emergency communications systems for many types of hazards and risks. System designers and authorities having jurisdiction often question whether a certain event should trigger an alarm or a supervisory signal. Too often, users of NFPA 72and NFPA 720 get caught up in trying to categorize a signal rather than focusing on what needs to be done with that signal.

In the past, fire alarm terminology was fairly simple. An alarm meant a warning of fire danger.3 A trouble signal was an indication that some part of the fire alarm system was broken or off-normal. A supervisory signal was used to indicate that some other protection system, monitored by the fire alarm system, was off-normal. Supervisory signals were used to indicate when sprinkler valves were closed or when a suppression system releasing panel was disabled. At a protected premise, a fire alarm almost always resulted in occupant notification and the intended evacuation of the building or area.

Some fire alarm systems monitor sprinkler systems in large storage occupancies where those systems may have few or no other common fire alarm system features. For a variety of reasons, many of these systems do not require in-building, general occupant notification. They may only signal at an on-site control unit or annunciator and at an off-premises supervising station or fire department dispatcher. This type of signaling was one of the first recognitions that there are degrees of danger conditions requiring different responses.4

In the 2007 edition of NFPA 72, the definition of alarm was changed by dropping the word fire so that it reads: a warning of danger.5 This was done as the code began to branch out to address signaling system use for many different types of hazards. In 2007, a new annex was added to address emergency communications systems, including mass notification systems. In 2010, the emergency communications systems annex evolved into Chapter 24 of NFPA 72.

The development of the first edition of NFPA 720, released in 1998 as a recommended practice, brought about a new type of alarm signal: A signal indicating a concentration of carbon monoxide that could pose a risk to the life safety of the occupants in the family living unit, requiring immediate action. NFPA 720 did not define a carbon monoxide supervisory signal and did not formally define any degrees of response other than immediate action. However, paragraph 1-2.1 did note that the purpose was to provide a warning of the presence of carbon monoxide to allow occupants to either escape or take other appropriate action. The 2009 edition added a definition for supervisory signal: A signal indicating the need for action in connection with a pre-alarm condition, or in connection with the supervision of protected premises carbon monoxide safety functions or equipment, or the maintenance features of related systems.

The changes to NFPA 72 and NFPA 720 are recognitions that there are varying degrees of conditions that require varying degrees of response that are initiated by various types of signals. NFPA 72 and NFPA 720 do not dictate specific responses. Instead, they are standards that facilitate the gathering of information about conditions and the communication of that information in a way that can be used to affect the needed response. The response, action or service that is needed for a given condition must be specified by the system designer in consultation with all stakeholders. Under the simplest circumstances, the response may be pre-defined by other governing laws, codes or standards.

Proposals have been processed for the 2013 edition of NFPA 72 that would formally define condition, signal and response, along with several sub-definitions. Though still in the development process, Figure 1 shows a working model.6

In addition to the sub-definitions shown in Figure 1, there might be additional defined terms such as Fire Alarm Signal, Evacuation Signal, Guards Tour Signal, Delinquency Signal and Carbon Monoxide Alarm Signal. Introduction of this model into NFPA 72 is intended to help focus designers and users on the response that is needed for a given condition and to demonstrate that there are a variety of ways that signaling systems can be used to convey meaningful information to the correct target audiences. In fact, NFPA 72 will not define all possible conditions, signals and responses that a signaling system might manage. For example, a runaway chemical reaction might produce a detectable overheat condition. There is no need to define an overheat condition, signal and response. Similarly, is it necessary to define a sprinkler water flow condition, signal and response?

The need for this model, and the need for several discrete categories of condition, signal and response, also is driven by product ergonomics, development and listing. Currently, system control units are designed and listed to produce alarm, trouble and supervisory signals. Every attached initiating device must trigger one of these three standard responses at the control unit interface. If an initiating device is intended to result in an alarm signal, the panel response must include a unique audible alert signal.

As of the 2010 edition of NFPA 72, the audible alert signal can be the same used to indicate trouble conditions and supervisory conditions, provided that the signals are distinguishable by some other means, such as visual indicators (LEDs). (Prior to the 2010 edition, the panel would have to have an audible alarm alert signal that was distinct, while trouble and supervisory conditions could trigger a different, but common audible signal.) The code was changed, in part, because the intent is to get someone to pay attention to the control unit and to investigate the signal.

As different types of alarm, trouble and supervisory conditions, signals and responses are being introduced, it is not practical or ergonomic to require unique audible alerts for each. If a control unit experiences a fault, the control unit may signal trouble by sounding the common local audible attention signal and by lighting a yellow LED labeled trouble. A separate LED (yellow or amber) is commonly used for supervisory signals along with the common audible alert signal.

While most control units use the three LED method for signal distinction - along with green for normal - it is not required. Some have a pre-alarm LED or two or more alarm level LEDs. A unit could use a text display to provide the visual information in lieu of LEDs. The use of text displays creates greater opportunity for distinction of the various types of conditions, signals and responses. However, the use of three to five LEDs on a display has the advantage of providing a quick visual indication of the potential importance of the incoming signal. This can be very important during the management of actual emergencies.

For example, during a fire, as smoke and heat spread (conditions), additional initiating devices activate and cause the audible alert on the panel to resound, even if it had been silenced. It will also resound as new faults are introduced by circuits being attacked by the fire and for incoming supervisory signals from fire pumps starting and valves being operated to control water. In a fire emergency, this resound might happen hundreds of times, which causes the person at the panel to be nearly continuously assaulted by the audible alert signal. While a text display can convey a richer message, having a simple three to five LED display indicating the level of an incoming signal can be an important ergonomic simplification.

So, should a carbon monoxide condition result in an alarm signal or in a supervisory signal? The answer still is: it depends. For example, in a residential home or apartment where occupants might be sleeping, if a carbon monoxide condition is detected, the desired response is to immediately alert the occupants so that they can take appropriate actions - evacuate or ventilate the space and correct whatever caused the condition. In that case, the signal should be classified as an alarm signal as it is intended to indicate that a dangerous condition exists. If a carbon monoxide condition is detected in a parking garage, the required response might only be to activate a ventilation system. There might be no other signal.

If the condition is not corrected or worsens, a signal might be used to alert maintenance staff that repairs are needed to the ventilation system or to investigate why the CO levels were not reduced by the ventilation system. The conditions being used to trigger these signals and responses are levels of CO that are not an immediate threat to occupants. Therefore, these signals might be categorized as either supervisory or as pre-alarm. Less emphasis needs to be placed on the category and more emphasis needs to be placed on ensuring the correct response.

There have been cases where signals have been ignored or missed because of complex and confusing interface ergonomics. The proposed condition, signal or response model does not dictate any new control unit interface or means for alerting users or occupants. However, it does define a framework for panel designers, system designers and even for the NFPA 72 technical committees to use as systems evolve and increase in complexity.


  1. NFPA 72, National Fire Alarm and Signaling Code, National Fire Protection Association, Quincy, MA, 2010.
  2. NFPA 720, Standard for the Installation of Carbon Monoxide (CO) Detection and Warning Equipment, National Fire Protection Association, Quincy, MA, 2009.
  3. NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 2002.
  4. A Partial Record of the Transaction of the Third Annual Meeting, Boston, June 13-15, 1899, reprinted courtesy of Carmel Fire Protection Associates, Carmel-by-the-Sea, CA.
  5. NFPA 72, National Fire Alarm and Signaling Code, National Fire Protection Association, Quincy, MA, 2007.
  6. 2012 Annual Revision Cycle Report on Proposals, National Fire Protection Association, Quincy, MA, 2011.