Overview of NPFA 72, Chapter 17: Initiating Devices

Today, we're continuing our overview of the NFPA 72: National Fire Alarm and Signaling Code. This document sets the standard for fire alarm systems throughout the United States (subject to modification by the local AHJ).

I last left off with Chapters 1-10 and Chapters 11-14, so let's pick up with Chapter 15 and go from there...

Chapter 15: Reserved

Like many other chapters in the 2022 Edition of NFPA 72, this one is unused. This will allow the writing of chapters later without disrupting the numbering of later chapters in future editions.

Chapter 16: Reserved

Chapter 16 is also just a placeholder in the 2022 Edition.

Chapter 17: Initiating Devices

Right out of the gate here, we get the most fundamental division in the science of initiating devices:

17.1.1 The performance, selection, use, and location of automatic or manual initiating devices shall comply with the minimum requirements of this chapter.

As you may have read in my previous writing, there are two major subgroups of "initiating devices":

• Manual
• Automatic

Manual initiating devices are those that transmit a signal only after the manual intervention of a person. Every schoolkid and many urban residents will recognize the bright red pull handles of a fire alarm. These are your connection between seeing a fire and reporting it to authorities quickly.

Automatic initiating devices, by contrast, require no human intervention at all. Because they cannot rely on the five senses of a human, there is much more variety in automatic devices. Common examples include smoke detectors, heat detectors, and flow sensors inside of fire-sprinkler plumbing.

Beyond this basic division, we obviously see many specific regulations in this chapter. Our old friend, "Performance-Based Design", makes an appearance again here. This is the idea that, regardless of the specific rules and regulations in this chapter, you can be compliant if you prove that you achieve the basic purpose:

17.2 Purpose. Automatic and manual initiating devices shall contribute to life safety, fire protection, and property conservation by providing a reliable means to signal other equipment arranged to monitor the initiating devices and to initiate a response to those signals.

Although it's beyond the scope of this article to discuss everything inside this NFPA 72 chapter, let me pull some highlights to give you an overview.

Section 17.3.5.1 is "Total (Complete) Coverage" and focuses on the requirement that every room, hall, storage area, etc. contains at least some initiating devices.

For heat detectors, there's an interesting compensation for ceiling height when it comes to detector spacing. Table 17.6.3.5.1 (a bit of a mouthful, isn't it?) lists "Heat Detector Spacing Reduction Based on Ceiling Height".

It's quite a large table. The entries range from ceilings less than 10 feet in height (use the sensor spacing as listed by the manufacturer) to ceilings greater than 28 feet in height (multiply the listed spacing by only 0.34).

Think about that for a second. If, for example, a heat detector was listed with a 30-foot spacing for low ceiling heights, the required spacing would only be 10 feet on a 28-foot-tall ceiling. Over any given area, you have 3x3 grids of sensors when you'd have just 1 on a lower ceiling. Why such a dramatic difference?

I didn't write NFPA 72, but the closer spacing makes sense when you consider simple physics. If we're trying to detect a fire, seconds save lives. When heat detectors are close to the ground on a low/normal ceiling, rising heat can "splash" on the ceiling, spread out, and still be hot enough to trigger an alarm when it hits the sensor.

If you imagine the same fire under a 30-foot ceiling, hot rising air has much farther to travel. It's going to cool off during the trip as it mixes with room-temperature air. If it has to travel another 15 feet laterally (if it strikes the ceiling between two 30-foot-spaced detectors), that delays an alarm.

Therefore, NFPA 72 Chapter 17 dictates that we must cluster more heat detectors tighter together when we have taller ceilings in order to maintain the same level of protection.

In 17.13.2, we see a similar consideration of alarm activation time for water flow from a fire sprinkler system. Although movies portray things differently, fire sprinklers don't all activate automatically when a small fire is detected. Instead, they trigger individually in response to heat from a fire.

That's why NFPA 72 says:

17.13.2* Activation of the initiating device shall occur within 90 seconds of waterflow at the alarm-initiating device when flow occurs that is equal to or greater than that from a single sprinkler of the smallest orifice size installed in the system.

As you can see, we measure a worst-case scenario for sprinkler-activation detection by requiring a 90-seconds-or-less alarm trigger if just one sprinkler is activated.

Interestingly, this means that the required reaction time to a fire scales in nearly direct proportion to the size of that fire. I don't know that the NFPA intended this from the beginning. You have to draw a minimum requirement somewhere, and water flow is proportional to how many sprinklers are open. Still, I found it rather beautiful that we get this sort of proportionality written into the laws of physics and, therefore, included in NFPA 72.

Digitize Will Help You Understand and Comply with NFPA 72

We're manufacturers of fire alarm monitoring devices, so NFPA 72 is something we necessarily have had to learn very well. While some companies hoard knowledge and simply don't like to spend time on customer education, that's not the approach we take at Digitize.

You're a client when you work with us, even if it's just an inbound call to ask an NFPA code questions. There's a decent chance you might use Digitize equipment someday, so we're happy to speak with you about your fire code questions. We have a large team of engineers, and all of them have familiarity with fire alarm monitoring equipment and the fire codes that drive its design.

When you call Digitize, we'll help you sort out your obligations under local regulations. We can't spend hours and hours with you, but it's often that a clarifying 20-minute conversation will really shorten your overall research time required.

To get started, just give us a call at 1-800-523-7232 or email info@digitize-inc.com