Protectowire 2000 FireSystem Bedienungsanleitung PDF herunterladen (Seite 10)

travels freely down the joist channels. However,

heat has trouble spreading across the joists, so

the distance in this direction must be reduced.

The Protectowire shall be mounted on the bot-

tom of the joists, and the spacing of the detector

on all runs measured at right angles parallel to

the joists shall not exceed 50% of the smooth ceil-

ing spacing. On runs that parallel both joists and

walls, the spacing shall not exceed 25%.

Beam Construction

In the case of beam construction where the

beams are 4 inches (100mm) or less in depth,

the ceiling shall be treated the same as a

smooth ceiling. If the beams project more

than 4 inches (100mm) below the ceiling, the

spacing at right angles to the direction of

beam travel shall not exceed two-thirds of the

smooth ceiling spacing. Where the beams

project more than 18 inches (46 cm) below

the ceiling and are spaced more than 8 feet

(2.4 m) on center, each bay formed by the

beams shall be treated as a separate area

requiring detector coverage.

Detector Mounting & Dead Air Space

Heat from a fire rises to the ceiling in a plume

and spreads radially from the fire source. As

the hot gases cool, they begin to bank down

from the ceiling. The corner where the ceiling

and wall meet forms an area known as “dead

air space” as shown in Figure 4. In most fires,

this dead air space is the triangular area

formed along the ceiling by measuring 4 inch-

es (10 cm) out from the corner, and 4 inches

(10 cm) down the side wall. Protectowire

should not be placed in this area.

High Ceiling Considerations

The purpose of this section is to discuss the

effects of ceiling height on the installed spacing

of Protectowire Linear Heat Detector and to

explain the importance of modifying the listed

spacing in order to achieve detector response.

Logic tells us that a detector will ordinarily

operate sooner in detecting a fire if it is clos-

er to the fire source. This is because as smoke

and heat rise from a fire, they do so in a

plume which has the general shape of an

inverted cone. The concentration of smoke

and heat within the cone varies inversely as a

variable function of the distance from the

source. This characteristic is amplified during

the early stages of a fire because the angle of

the plume is wide. As the fire grows in size,

the angle of the plume narrows, which tends

to reduce the effect of ceiling height.

As the ceiling height increases, a larger-size fire

is necessary to actuate the same detector in the

same length of time. As a result of this, system

design practice mandates that the listed spacing

of heat detectors be reduced for the purposes

of providing faster detector response to a fire,

and to increase the likelihood that the detector

will respond to a smaller fire.

To compensate for the effect that high ceiling

installations have on detector performance,

The National Fire Alarm and Signaling Code,

NFPA 72, provides derating factors which can

be applied to the listed spacing of any heat

detector when installed on ceilings 10 to 30

feet (3m to 9.1m) high. These derating factors

are shown as a percentage of the detector’s list-

ed spacing, and have been developed to pro-

vide detector performance on higher ceilings

[to 30 feet (9.1m)] that is equivalent to that of

Figure 3

Figure 4

—

= Max. Listed Spacing

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 32 33

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