Is Five Minutes Too Much?
NFPA 13 and maximum time limits for audible alarms
NFPA Journal®, March/April 2011
NFPA 13, Installation of Sprinkler Systems, includes a maximum time limit of five minutes for an audible alarm to sound on the premises. The requirement is stated both in Chapter 6, in the rules for system waterflow alarm devices, and Chapter 24, which addresses system acceptance requirements.
Since prompt notification and response is a basic mantra among fire protection professionals, the relative generosity of this rule has been a longstanding source of confusion. After all, there is widespread recognition that room flashover can take place within five minutes of ignition in a typical residential compartment. How can the sprinkler rules be so seemingly lax?
Before its 1980 edition, NFPA 13 contained no maximum time requirement. The standard simply required that a local alarm be provided for systems having more than 20 sprinklers and that the alarm unit be constructed so that any flow from a single sprinkler results in an audible alarm on the premises. Electrically operated alarm attachments were required to meet the requirements of appropriate NFPA signaling system standards. As part of the development of the 1980 edition of NFPA 13, the Committee on Automatic Sprinklers added the five-minute maximum time limit, indicating it was appropriate for consistency with the signaling system standards and the product standards of Underwriters Laboratories.
During the public comment period, the proposed time limit was challenged as being excessive, and it was pointed out that the five-minute limit did not exist in the NFPA signaling system standards. However, the Sprinkler Committee stuck with the proposed requirement, and the committee discussions, at which I was present, focused on the need for a generous maximum due to the potential for “bouncing” of the system due to entrapped air.
It was recognized that the alarm devices were generally equipped with a retard or delay capability such that small water pressure surges did not trigger false alarms. In combination with pockets of trapped air, these delays could result in some cases in a considerable length of time before the audible alarm sounded on premises. The bottom line for the Sprinkler Committee, however, was that the alarm was of secondary importance because the fire itself was being controlled or suppressed by the automatic sprinkler system.
Today, large air pockets in sprinkler systems are discouraged for another reason: corrosion. Annex section A.126.96.36.199.2 in the 2010 edition of NFPA 13 discusses the use of manual or automatic air venting valves as a means of reducing corrosion in steel wet-pipe systems. Another aspect of corrosion involves frequency of testing, especially for system alarm test connections located at a remote point of the system. Experts tell us that it takes 28 days for residual oxygen entrained in fresh water to be used up and corrosion activity to cease each time the water is replenished. For that reason, duplicate testing of sprinkler flow alarms by sprinkler system and alarm system inspectors should be eliminated.
NFPA 72®, National Fire Alarm and Signaling Code, requires that sprinkler system flow switches be tested with the actual flow of water, but requires an alarm within 90 seconds. The difference between the NFPA 72 time limit and NFPA 13’s five-minute limit should not be used to justify unnecessary duplicate testing, especially considering the corrosion potential. Where different parties are responsible for inspecting and testing sprinkler and alarm devices, efforts should be made to coordinate the activities to conserve resources. Where both requirements apply, the 90-second limit takes precedence over the five-minute allowance.
Russell P. Fleming, P.E., is the executive vice-president of the National Fire Sprinkler Association.