In either case, knowledge of the applicable codes is important. But, knowing how to apply the detection devices available is equally important. The most common detection device in fire alarm systems is the smoke detector. Of course, if you have been installing fire alarm systems for any period of time, you also know that smoke detectors can be the cause of unnecessary alarms and numerous (and costly) callbacks to an installation.

A professional contractor knows the electrical products they sell to clients. They use this knowledge to ensure what is installed is the right product for the job, is properly rated for the application and is reliable. Knowledge and application of smoke detection devices is no different.

The National Fire Alarm Code does not require a fire alarm system to be installed. The building and fire codes dictate when systems are to be installed. But, once a contractor plans to perform an installation, the code provides the requirements to follow.

Starting with the basics, answer the following question: “What is the listed spacing of a smoke detector?”

Many of you just said with confidence: “30-foot on-center!”

It may surprise you to find out that there is no listed spacing for smoke detectors. NFPA 72-2002 actually states in section 5.7.3.2.3 that, “On smooth ceilings, spacing for spot-type smoke detectors shall be in accordance with 5.7.3.2.3(A) through 5.7.3.2.3(E). [Below]

(A) Spacing of 9.1 m (30 ft.) shall be permitted to be used as a guide. [Emphasis added.]

(B) In all cases, the manufacturer’s documented instructions shall be followed.

(C) Other spacing shall be permitted to be used depending on ceiling height, different conditions, or response requirements.

(D) For the detection of flaming fires, the guidelines in Annex B shall be permitted to be used.

(E) For smooth ceilings, all points on the ceiling shall have a detector within a distance equal to 0.7 times the selected spacing.”

The Annex of the code explains that the 30-foot spacing is a guide for prescriptive designs and that the use of such spacing is based on customary practice in the fire alarm community. Those of you who answered, “Follow the manufacturer’s instructions” are only partially right.

The manufacturer inevitably requires that either Underwriters Laboratories or National Fire Protection Association requirements be followed, which brings everyone back to the 30-foot spacing as a guide. Having revealed the listed spacing myth, you still must understand that regardless of its origin, the spacing guideline has provided sufficient detection and early warning capability for our fire alarm system installations.

The reason this information is important is to ensure everyone involved in the design, installation and inspection understand that the measurement of 30 feet between detectors is not absolute. If the measurement extends to 32 or 33 feet, no measurable difference for detector operation can be expected.

One of the mistakes made most often in smoke detector installations is allowing the general contractor to force you into installing smoke detectors too soon. This is often a result of promises made to the owner, and the general contractor wants to show as much finished work as possible. Many electrical contractors fall into this trap not knowing that if the smoke detectors are installed before the building is completely finished, he has just guaranteed callbacks due to unnecessary alarms.

NFPA 72-2002 provides guidance in this area as well. The code states in section 5.7.1.11 that “Detectors shall not be installed until after the construction cleanup of all trades is complete and final.”

There is an exception that allows the authority having jurisdiction to require detection installed to protect the building during construction. The exception goes on to state that, “Detectors that have been installed during construction and found to have a sensitivity outside the listed and marked sensitivity range shall be cleaned or replaced in accordance with Chapter 10 at completion of construction.”

Therefore, explain code requirements to the general contractor and inform him that if you do install the smoke detectors prematurely, you are required by the code to check the sensitivity of all of the detectors. Those found outside the listed and marked sensitivity will have to be cleaned or replaced. Obviously this extra effort will cost money and you will be happy to provide a quote to do as requested.

Knowing the code will keep you from falling into these common pitfalls and add profits to your bottom line. EC

MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a co-editor of the current National Fire Alarm Code Handbook. Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office.