Building Design and Construction Handbook - Merritt - Ventech!

PROTECTION AGAINST HAZARDS 3.39
variety of operations can be performed by the detection system. It can initiate an
alarm, local or remote, visual or audible; notify a central station; actuate an extinguishing
system; start or stop fans or processes, or perform any other operation
capable of automatic control.
There are five general types of detectors, each employing a different physical
means of operation. The types are designated fixed-temperature, rate-of-rise, photoelectric,
combustion-products, and ultraviolet or infrared detectors.
A wide variety of detectors has been tested and reported on by Underwriters
Laboratories, Inc. See Art. 3.5.1.
Fixed-Temperature Detectors. In its approval of any detection device, UL specifies
the maximum distance between detectors to be used for area coverage. This
spacing should not be used without competent judgment. In arriving at the permitted
spacing for any device, UL judges the response time in comparison with that of
automatic sprinkler heads spaced at 10-ft intervals. Thus, if a device is more sensitive
than a sprinkler head, the permitted spacing is increased until the response
times are nearly equal. If greater sensitivity is desired, the spacing must be reduced.
With fixed-temperature devices, there is a thermal lag between the time the
ambient temperature reaches rated temperature and the device itself reaches that
temperature. For thermostats having a rating of 135�F, the ambient temperature can
reach 206�F.
Disk thermostats are the cheapest and most widely used detectors. The most
common type employs the principle of unequal thermal expansion in a bimetallic
assembly to operate a snap-action disk at a preset temperature, to close electrical
contacts. These thermostats are compact. The disk, 1 ⁄2 in in diameter, is mounted
on a plastic base 1 3 ⁄4 in in diameter. The thermostats are self-resetting, the contacts
being disconnected when normal temperature is restored.
Thermostatic cable consists of two sheathed wires separated by a heat-sensitive
coating which melts at high temperature, allowing the wires to contact each other.
The assembly is covered by a protective sheath. When any section has functioned,
it must be replaced.
Continuous detector tubing is a more versatile assembly. This detector consists
of a small-diameter Inconel tube, of almost any length, containing a central wire,
separated from the tube by a thermistor element. At elevated temperatures, the
resistance of the thermistor drops to a point where a current passes between the
wire and the tube. The current can be monitored, and in this way temperature
changes over a wide range, up to 1000�F, can be detected. The detector can be
assembled to locate temperature changes of different magnitudes over the same
length of detector. It is self-restoring when normal temperature is restored. This
detector is useful for industrial applications, as well as for fire detection.
Fusible links are the same devices used in sprinkler heads and are made to
operate in the same temperature range. Melting or breaking at a specific temperature,
they are used to restrain operation of a fire door, electrical switch, or similar
mechanical function, such as operation of dampers. Their sensitivity is substantially
reduced when installed at a distance below a ceiling or other heat-collecting obstruction.
Rate-of-Rise Detectors. Detectors and detector systems are said to operate on the
rate-of-rise principle when they function on a rapid increase in temperature, whether
the initial temperature is high or low. The devices are designed to operate when
temperature rises at a specified number of degrees, usually 10 or 15�F, per minute.
They are not affected by normal temperature increases and are not subject to thermal
lag, as are fixed-temperature devices.