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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).

www.acornfiresecurity.com

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:

Sampling frequency:

ELECTRICAL

Supply voltage:

Supply wiring:

Maximum polarity reversal:

Power-up time:

Minimum 'detector active' voltage: 6V

Switch-on surge current at 24V: 95µA

Average quiescent current at 24V: 95µA

Alarm current:

Alarm load:

Holding voltage:

Minimum holding current:

Minimum voltage to light

alarm LED:

Alarm reset voltage:

Alarm reset time:

Remote output LED

(–) characteristic:

MECHANICAL

Material:

Alarm Indicator:

Dimensions:

Weight:

ENVIRONMENTAL

Temperature:

Humidity:

Wind speed:

Atmospheric pressure:

IP rating to EN 60529: 1992*: 23D

Electromagnetic Compatibility: The detector meets the requirements of EN 61 000-6-3

*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.

www.acornfiresecurity.com

CONVENTIONAL DETECTORS

TECHNICAL DATA

Measurement of heat by means of a thermistor.

Once every 4 seconds

8.5—33V DC

2 wires, polarity sensitive

200ms

<20 seconds

At 12 volts

At 24 volts

600Ω

5–33V

8mA

<1V

1 second

1.2kΩ connected to negative supply

Detector and base moulded in white polycarbonate.

Integral indicator with 360° visibility

(See Table 3 on page 13 for details of flash rate)

97mm diameter x 36mm height

100mm diameter x 51mm height (in base)

Detector

Detector in base

Operating and storage (see table 1)

0% to 98% relative humidity (no condensation)

Unaffected by wind

Insensitive to pressure

for emissions and BS EN50 130-4 for susceptibility.

20mA

40mA

70g

130g

–40°C to +70°C

(no condensation or icing)