detection circuit that is connected to a main control panel that takes appropriate action to indicate an alarm has been reported, if the zone
current is substantially increased.
Heat detectors are an electronic design, the temperature of the air is sensed 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 that senses 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 detects a quick change in the air temperature, but for a slow or medium rate of temperature change,
the detector does not signal an alarm due to the ROR feature. For a slower change in temperature, the detector signals an alarm due to a
set fixed temperature that is sensed by RT1. For a fast temperature rise, when a difference in temperature is sensed by RT1 and RT2 has
reached a predetermined amount, the detector signals an alarm.
2.4.3 Photoelectric smoke detectors or sensors
These devices operate on a light scattering principle. The smoke sensing chamber contains an infrared LED source with a peak spectral
emission of 880 nanometers. This source is placed at an angle from a spectrally matched photo-diode receiver. During a NO SMOKE
condition, only light reflected from the chamber walls enters the receiver and shows up as a small photo-current. As smoke particles enter
the sensing chamber and cross the light beam of the LED, more light reaches the receiver due to scattering. The receiver circuitry converts
this photo-current into a signal voltage. In a detector, when this voltage reaches a preset level, an alarm is signaled. In a sensor, this signal
voltage goes into an 8-bit, analog to digital (A to D) converter. A digital representation of this signal voltage is then transmitted to the fire
alarm panel for further processing.
2.4.4 Carbon monoxide gas detectors or sensors
The CO Sensor and Sounder Bases with a CO Replacement Cartridge (CORC) modules work in conjunction with existing detector heads.
The CO Sensor Base enhances fire detection and combines fire and toxic gas leakage detection in a single device. The CO sensor measures
the concentration of carbon monoxide gas in the air in part per million (PPM). When toxic gas operation is selected, the FACP panel can
generate either a supervisory, utility, or priority 2 alarm. The alarm level is calculated within the panel based on the time integrated CO
levels that are measured at the detector.
page 9 574-709 Rev. AP
4098 Detectors, Sensors, and Bases Application Manual
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