11
CONVENTIONAL DETECTORS
All data is supplied subject to change without notice.
Specifications are given at 23°C and 50% relative humidity unless otherwise stated.
DETECTOR OPERATING PRINCIPLES
Principle of detection:
Measurement of heat by means of a thermistor.
Sampling frequency:
Once every 4 seconds
ELECTRICAL
Supply voltage:
8.5—33V DC
Supply wiring:
2 wires, polarity sensitive
Maximum polarity reversal:
200ms
Power-up time:
<20 seconds
Minimum ‘detector active’ voltage:
6V
Switch-on surge current at 24V:
95µA
Average quiescent current at 24V:
95µA
Alarm current:
At 12 volts
20mA
At 24 volts
40mA
Alarm load:
600Ω
Holding voltage:
5–33V
Minimum holding current:
8mA
Minimum voltage to light
5V
alarm LED:
Alarm reset voltage:
<1V
Alarm reset time:
1 second
Remote output LED
1.2kΩ connected to negative supply
(–) characteristic:
MECHANICAL
Material:
Detector and base moulded in white polycarbonate.
Alarm Indicator:
Integral indicator with 360° visibility
(See Table 3 on page 13 for details of flash rate)
Dimensions:
97mm diameter x 36mm height
100mm diameter x 51mm height (in base)
Weight:
Detector
70g
Detector in base
130g
ENVIRONMENTAL
Temperature:
Operating and storage (see table 1)
–40°C to +70°C
(no condensation or icing)
Humidity:
0% to 98% relative humidity (no condensation)
Wind speed:
Unaffected by wind
Atmospheric pressure:
Insensitive to pressure
IP rating to EN 60529: 1992*:
23D
Electromagnetic Compatibility:
The detector meets the requirements of EN 61 000-6-3
for emissions and BS EN50 130-4 for susceptibility.
*The IP rating is not a requirement of EN 54 since smoke detectors have to be open in order
to function. An IP rating is therefore not as significant as with other electrical products.
TECHNICAL DATA
CHOOSING THE
CORRECT CLASS OF
HEAT DETECTOR
Heat detectors have a wide range of
response characteristics and the choice of
the right type for a particular application
may not always seem straightforward.
It is helpful to understand the way that
heat detectors are classified as explained
earlier and to memorise a simple rule: use
the most sensitive heat detector available
consistent with avoiding false alarms.
In the case of heat detectors it may be
necessary to take an heuristic approach,
ie, trial and error, until the best solution
for a particular site has been found. The
flowchart (Fig. 3) will assist in choosing
the right class of heat detector.
If the fire detection system is being
designed to comply with BS 5839–1:
2002 heat detectors should be installed
at heights of less than 12 metres with the
exception of class A1 detectors, which can
be installed at heights up to 13.5 metres.
HOW DO ORBIS HEAT
DETECTORS WORK?
Orbis heat detectors have an open-web
casing which allows air to flow freely
across a thermistor which measures
the air temperature every 2 seconds. A
microprocessor stores the temperatures
and compares them with pre-set values to
determine whether a fixed upper limit–the
alarm level–has been reached.
In the case of rate-of-rise detectors
the microprocessor uses algorithms to
determine how fast the temperature is
increasing.
Static heat detectors respond only when
a fixed temperature has been reached.
Rate-of-rise detectors also have a fixed
upper limit but they also measure the rate
of increase in temperature. A fire might
thus be detected at an earlier stage than
with a static detector so that a rate-of-rise
detector is to be preferred to a static heat
detector unless sharp increases of heat are
part of the normal environment in the area
protected by the heat detector.
ENVIRONMENTAL
PERFORMANCE
The environmental performance is similar
to that of the Orbis optical smoke detector
but it should be noted that heat detectors
are designed to work at particular ambient
temperatures (see Fig 3).
