Heat Detector Operation

The heat detector senses the heat or the Rate-of-Rise (ROR) in the air temperature of the
environment in which it is located. The heat detector is comprised of electronic circuitry and a
mechanical package that is designed to sense the ROR of the air temperature in an expedient and
reliable fashion. Upon detection of an abnormal increase in air temperature, or ROR in air
temperature, the electronics indicate an alarm by increasing the amount of current draw from the
monitor zone it is connected to. The monitor zone is a supervised detection circuit that is tied
back to a main control panel that takes appropriate action to indicate an alarm has been reported,
if the zone current is substantially increased.

Being of an electronic design, the temperature of the air is sensed by using two negative
temperature coefficient thermistors. The resistance of the thermistors goes down with an increase
in temperature. One thermistor is placed in a position such as to sense the open air temperature
very rapidly (RT1). The second thermistor is positioned in a small cavity that protrudes out from
the main body of the detector (RT2). The location of RT2 allows for fast detection of a quick
change in the air temperature, but yet for a slow or medium rate of temperature change, the
detector does not trip due to the ROR feature. For a slower change in temperature, the detector
trips into alarm due to a set fixed temperature that is sensed by RT1. For a fast temperature rise,
when a difference in temperature sensed by RT1 and RT2 has reached a predetermined amount,
the detector trips into alarm.

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