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SD-521
Issue: 5.8
Date Of Issue: 02/06/2010
© 2010 Sontay Limited. All rights reserved.
Technical Overview
Recommendations (continued)
The SD-521 provides an auto or manual reset relay output
on detection of smoke, with a user adjustable threshold. An
analogue output can transmit smoke and service alarms to a
BEMS system.
Description
The SD-521 duct smoke detector has been developed from a
proven design using modern devices to provide effective and
reliable detection of smoke in ventilation systems.
Detection of smoke is achieved by monitoring a carefully
controlled infra red beam within a perforated tube which is
inserted into the ductwork. This method senses smoke
directly within the duct eliminating the problems associated
with conventional detectors mounted in sampling boxes.
Circuitry incorporated in the design of the detector controls
the infra red beam continuously. This ensures that the
detectors response will remain constant throughout
operation of the system. External influences including
background pollution, airborne dust and low level electrical
interference frequently found in commercial and industrial
applications, a common source of problems with some other
types of detector, are selectively filtered by the electronics
thereby providing the highest sensitivity to smoke combined
with effective rejection of short term disturbances that
would otherwise give a false alarm.
The detector features variable sensitivity and an advanced
signal damping circuit which can be bypassed for testing
purposes. And this signal is available as an analogue output
to communicate with a building management system.
Recommendations
The SD-521 has a probe length of 300mm and should NOT
be used in ducts with a diameter or square size greater than
450mm, as it will not monitor the whole of the duct and
smoke may pass along the duct without being detected.
Units should NOT be mounted where ambient sunlight will
fall on the photocell.
Units should never be mounted at or near sharp bends in
ductwork, as turbulence and side currents can carry the
smoke away from the probe.
The SD-521 is an optical beam detector and therefore, air
velocity has little effect on performance. However, careful
positioning of the detector is required if optimum detection
is to be achieved. The detector should always be fitted with
the holes in the sensing tube parallel to the airflow.
Units should to be mounted close to supply or extract fans.
With large installations, where several ducts collect into a
common duct, consideration must be given to dilution of
smoke and rate of airflow. For example, 10 small ducts may
feed at different points into one large main extract duct, and
if only one smoke detector is fitted in the main extract duct,
the position can arise where one duct is feeding smoke and
nine are feeding clean air. The amount of smoke is therefore
diluted by 90%, and if this is coupled with a high flow rate,
the smoke may not be detected for the minimum operating
period of 1 second, resulting in no alarm being generated. It
may be necessary to mount 10 detectors in each of the
small ducts to guarantee correct operation.
Installation Notes
Removable links are provided for signal damping and
internal latching of the output relay.
It is recommended that the damping be used (LK1) for
normal use as this will give the best false alarm rejection.
The link should be removed for testing purposes.
The internal latching circuit is enabled with the link LK2 in
place, in this condition any alarm will result in the detector
signal remaining on until locally reset. If the latching link is
removed and conditions return to normal for this reason the
electrical circuit to which the detector is connected should
have self latch with reset facilities.
Due to the self calibrating design of the detector as dust
builds up on the optics the internal signal is increased to
compensate, this results in the detector maintaining
sensitivity during operation. Failure to clean the detector will
ultimately result in an alarm signal occurring. At the final
signal level, either from dust accumulation or dense smoke
the detector will remain in alarm state for a minimum of 1
minute, following this final stage the detector will not be
able to recalibrate and will no longer function correctly.
As with all detectors it is not recommended to switch fan or
damper loads directly as any fire in the vicinity may damage
the detector or the wiring resulting in loss of control.
