72–204 NATIONAL FIRE ALARM AND SIGNALING CODE
A.17.6.1.1 The performance objective statement should de- it is positioned at the farthest possible distance from the fire
scribe the purpose of the detector placement and the in- while remaining within the square. Thus, the distance from
tended response of the fire alarm control unit to the detector the detector to the fire is always the test spacing multiplied by
activation. This statement can include a narrative description 0.7 and can be calculated as shown in Table A.17.6.3.1.1. Fig-
of the required response time of the detectors, a narrative of ure A.17.6.3.1.1(b) illustrates the smooth ceiling spacing lay-
the sequence of operations, a tabular list of programming re- out for line-type heat detectors.
quirements or some other method.
The performance objective of a fire detection system is
usually expressed in terms of time and the size fire the sys-
tem is intended to detect, measured in British thermal units Table A.17.6.3.1.1 Test Spacing for Spot-Type Heat
per second (Btu/sec) or kilowatts (kW). Typically, the fire Detectors
alarm system designer does not establish this criterion. It is
usually obtained from the design documentation prepared Maximum Test
by the designer responsible for the strategy of the structure Distance from Fire to
as a whole. Where a prescriptive design is being provided, Test Spacing Detector (0.7D)
this requirement is fulfilled by stating in the design docu-
mentation that the design conforms to the prescriptive pro- ft m ft m
visions of this Code.
A.17.6.1.3 In a performance-based design environment, the 50 × 50 15.2 × 15.2 35.0 10.7
performance objectives for the fire alarm system are not estab- 40 × 40 12.2 × 12.2 28.0 8.5
lished by the fire alarm system designer. 30 × 30 9.1 × 9.1 21.0 6.4
25 × 25 7.6 × 7.6 17.5 5.3
A fire protection strategy is developed to achieve those 20 × 20 6.1 × 6.1 14.0 4.3
goals. General performance objectives are developed for the 15 × 15 4.6 × 4.6 10.5 3.2
facility. These general objectives give rise to specific perfor-
mance objectives for each fire protection system being em-
ployed in the facility. Consequently, the performance objec-
tives and criteria for the fire alarm system are part of a much
larger strategy that often relies on other fire protection fea- Once the correct maximum test distance has been deter-
tures, working in concert with the fire alarm system to attain
mined, it is valid to interchange the positions of the fire and
the overall fire protection goals for the facility.
the detector. The detector is now in the middle of the square,
In the performance-based design environment, the de- and the listing specifies that the detector is adequate to detect
signer uses computational models to demonstrate that the a fire that occurs anywhere within that square — even out to
spacing used for automatic fire detectors connected to the fire
the farthest corner.
alarm system will achieve the objectives established by the sys-
tem, by showing that the system meets the performance crite- In laying out detector installations, designers work in terms
ria established for the system in the design documentation. of rectangles, as building areas are generally rectangular in
Consequently, it is imperative that the design objectives and shape. The pattern of heat spread from a fire source, however,
performance criteria to which the system has been designed is not rectangular in shape. On a smooth ceiling, heat spreads
are clearly stated in the system documentation. out in all directions in an ever-expanding circle. Thus, the
coverage of a detector is not, in fact, a square, but rather a
A.17.6.1.4 In order to predict the response of a heat detector
circle whose radius is the linear spacing multiplied by 0.7.
using current fire modeling programs and currently pub-
lished equations describing plume dynamics, two parameters This is graphically illustrated in Figure A.17.6.3.1.1(d).
must be known: operating temperature and response time in- With the detector at the center, by rotating the square, an
dex (RTI). The RTI is the quantification of the rate of heat infinite number of squares can be laid out, the corners of
transfer from the ceiling jet to the detector sensing element which create the plot of a circle whose radius is 0.7 times the
per unit of time, expressed as a function of ceiling jet tempera- listed spacing. The detector will cover any of these squares
ture, ceiling jet velocity, and time. Spot-type heat detectors and, consequently, any point within the confines of the circle.
manufactured prior to July 1, 2008, were not required to be So far this explanation has considered squares and circles.
marked with an RTI. In practical applications, very few areas turn out to be exactly
A.17.6.2.3 Detectors should be selected to minimize this tem- square, and circular areas are extremely rare. Designers deal
perature difference in order to minimize response time. How- generally with rectangles of odd dimensions and corners of
ever, a heat detector with a temperature rating that is somewhat rooms or areas formed by wall intercepts, where spacing to
in excess of the highest normally expected ambient temperature one wall is less than one-half the listed spacing. To simplify the
is specified in order to avoid the possibility of premature opera- rest of this explanation, the use of a detector with a listed
tion of the heat detector to non-fire conditions. spacing of 30 ft × 30 ft (9.1 m × 9.1 m) should be considered.
The principles derived are equally applicable to other types.
A.17.6.3.1.1 Maximum linear spacings on smooth ceilings for
spot-type heat detectors are determined by full-scale fire tests. Figure A.17.6.3.1.1(g) illustrates the derivation of this con-
[See Figure A.17.6.3.1.1(c).] These tests assume that the detec- cept. In Figure A.17.6.3.1.1(g), a detector is placed in the center
tors are to be installed in a pattern of one or more squares, of a circle with a radius of 21 ft (0.7 × 30 ft) [6.4 m (0.7 × 9.1 m)].
each side of which equals the maximum spacing as deter- A series of rectangles with one dimension less than the permitted
mined in the test, as illustrated in Figure A.17.6.3.1.1(a). The maximum of 30 ft (9.1 m) is constructed within the circle. The
detector to be tested is placed at a corner of the square so that following conclusions can be drawn:
2013 Edition
C18EE618-F3E9-4425-9C01-5EFE9A901BB3
ANNEX A 72–205
(1) As the smaller dimension decreases, the longer dimen-
sion can be increased beyond the linear maximum spac- 0.7S ¹⁄₂ S 0.7S 0.7S ¹⁄₂ S 0.7S
ing of the detector with no loss in detection efficiency.
(2) A single detector covers any area that fits within the circle.
For a rectangle, a single, properly located detector may be
permitted, provided the diagonal of the rectangle does
not exceed the diameter of the circle.
(3) Relative detector efficiency actually is increased, because
the area coverage in square meters is always less than the
900 ft2 (84 m2) permitted if the full 30 ft × 30 ft (9.1 m ¹⁄₂ S S S S ¹⁄₂ S
× 9.1 m) square were to be utilized. The principle illus-
trated here allows equal linear spacing between the detec- Line-type
tor and the fire, with no recognition for the effect of re- detector
flection from walls or partitions, which in narrow rooms
or corridors is of additional benefit. For detectors that are
not centered, the longer dimension should always be used
in laying out the radius of coverage.
¹⁄₂ S
Areas so large that they exceed the rectangular dimen-
sions given in Figure A.17.6.3.1.1(g) require additional de-
S = Space between detectors
tectors. Often proper placement of detectors can be facili-
tated by breaking down the area into multiple rectangles of
the dimensions that fit most appropriately [see Fig- FIGURE A.17.6.3.1.1(b) Line-Type Detectors — Spacing Lay-
ure A.17.6.3.1.1(e) and Figure A.17.6.3.1.1(f)]. For example, outs, Smooth Ceiling.
refer to Figure A.17.6.3.1.1(h). A corridor 10 ft (3.0 m) wide
and up to 82 ft (25.0 m) long can be covered with two 30 ft
(9.1 m) spot-type detectors. An area 40 ft (12.2 m) wide and
up to 74 ft (22.6 m) long can be covered with four spot-type
detectors. Irregular areas need more careful planning to
make certain that no spot on the ceiling is more than 21 ft
50 ft (15.2 m)
(6.4 m) away from a detector. These points can be deter-
mined by striking arcs from the remote corner. Where any 40 ft (12.2 m)
part of the area lies beyond the circle with a radius of
0.7 times the listed spacings, additional detectors are re- 30 ft (9.1 m)
quired.
20 ft (6.1 m)
Figure A.17.6.3.1.1(h) illustrates smoke or heat detector
spacing layouts in irregular areas. 10 ft
(3.1 m)
F
0.7S ¹⁄₂ S 0.7S
S S S
¹⁄₂ S ¹⁄₂ S
F = Test fire, denatured alcohol, 190 proof. Pan located
approximately 36 in. (0.9 m) above floor.
S = Indicates normal sprinkler spacings on 10 ft (3.1 m)
schedules.
= Indicates normal heat detector spacing on various
spacing schedules.
S FIGURE A.17.6.3.1.1(c) Fire Test Layout.
¹⁄₂ S
= Heat detector
S = Space between detectors
FIGURE A.17.6.3.1.1(a) Spot-Type Heat Detectors.
2013 Edition
C18EE618-F3E9-4425-9C01-5EFE9A901BB3
