FHSD700C Series Product Guide
GE Security
2 Principle of Operation
Air samples are drawn through the pipe network from the protected area through wide or small bore
pipe systems. Wide bore tubes generally have sampling holes drilled at intervals within the protected
area while small bore tubes normally sample air at their end points.
The aspirator draws air from the pipe network into the detector inlets where the samples are combined,
filtered, and directed to the laser detection chamber. Systems incorporating a rotary valve have an
Overall position which draws air equally from all sectors. In contrast, the FHSD721C / FHSD721DC has
an inlet manifold which combines samples from up to four inlet pipes while the FHSD720C uses a T-
piece of pipe to combine air sampled from two inlet pipes.
All models, except the FHSD720C, have flow monitoring on each inlet. The FHSD720C has flow
monitoring of the combined air-flow. More information on flow monitoring can be found in Section 2.1.
The detection chamber consists of a laser beam directed across an optical chamber, through which the
air sample flows. A photodetector built into the optical chamber measures the amount of light scattering
from particles in the air. A clean air sample will cause very little scattering but as the smoke density of
the sample increases, the amount of light directed onto the photodetector will also increase. The light
signal is processed to become a direct measurement of the amount of obscuration caused by smoke.
Information about the laser chamber can be found in Appendix C.
If the smoke detected is higher than the preset alarm thresholds in the detector (Alert, Action, Fire 1 and
Fire 2), an alarm will be reported. One or more Alarm relays, preset to activate at an alarm threshold will
signal the host panel after a preset time delay. Alarm states are also shown on the display panel, and
an audible warning is given. An optional alarm beacon can also be fitted for all detectors except the
FHSD720C.
Systems with a rotary valve (FHSD724DC to FHSD7215DC) have an additional preset trigger level,
Trace, usually set below the Alert level. When Trace is activated, the rotary valve will sequentially scan
the sectors, in order to determine the source of the event. While in Trace mode, the default levels for
Alert, Action, Fire 1 and Fire 2 are the same as used for Overall monitoring. If required, different levels
for Alert, Action, Fire 1 and Fire 2 may be set for each sector in the Sector Alarms menu. The Overall
alarm levels may also be changed as can all delay times (FHSD720C and FHSD721C / FHSD721DC
only).
2.1 Flow Monitoring
The control system monitors for blockages or disconnection of tubes for each sector by detecting when
the air flow is above or below acceptable flow thresholds. Flow thresholds are dependant on air flows
measured during normalization.
The normalization process enables the detector to learn typical air flow characteristics of the system
and sets these expected flow readings to 100% for each sector. A normalization sequence must be
performed at installation. The detector has default high and low limits and associated delay times, which
may be changed in the Configure menu.
Air flow characteristics of the system are highly dependent on the detector's pipe network. The design of
the pipe network should be considered carefully prior to installing the system.
2
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