conventional system operation detection line operation
detection line fault monitoring
detector to limit the current draw in alarm. Resistors fitted into the detector base are also used in some cases to distinguish between a short circuit fault and an alarm. The value of the base resistor is
Standard conventional systems are able to monitor the zone for short circuit, open circuit and detector head removal.
dependant on the control panel,
Conventional detection systems
however a typical value is 470
When a short circuit occurs on a
normally operate on a 24VDC line.
Ohms. If in any doubt, contact the
zone, a high current will be drawn,
In the standby condition, the
control panel manufacturer who
and the line voltage will be pulled
detectors will draw a low current,
should be able to specify which
towards zero volts. The panel
typically less than 100µA. When the
detector bases should be used
detects the low voltage /high
detector senses a fire, it will switch
with different detector brands.
current and a fault is signalled.
LED illuminated, and will collapse
A m anual call -poin t consists of a
In order to detect an open circuit, or
the line voltage by drawing a larger
simple switch with a resistor in
detector head removal, a device is
current - dependant on the
series with it, usually 470 Ohms or
connected across the end of the
detectors and control panel, but
680 Ohms. When the call point is
zone, which can be monitored.
typically 50-80mA. The control
activated, the resistor is switched
This device can take various forms
panel can sense this, and activate
across the line, and a current of
dependant on the control panel.
the appropriate alarms. The
50-80mA, dependant on the
detector will remain latched in the
control panel, is drawn.
into the alarm condition with it's
alarm state with its LEDs illuminated, even if the smoke or heat is removed until it has been reset from the panel by momentarily
+24V
removing power from the line. This allows the fire to be located even if the signal is intermittent, or to locate possible sources of
Contact Closed
nuisance alarms.
when detector inserted in base
For some control panel - detector combinations, when a standard base is used, there is an incompatibility between the current
r o t s i s e R
specifications of the detector and panel, leading to incorrect reporting by the control panel, for example signalling a fault in place of an
0V
alarm, and in some cases damage to the detector due to over current
Figure 2: Resistive end of line operation
in the alarm state. In these cases it is necessary to use a base fitted with a resistor in series with the
5
The simplest end of line device is
removed detector, it will no longer
head is removed, the diode is
a resistor, which will draw a current
work, which contravenes the
connected across the contact,
distinct from the quiescent and
requirements of BS 5839 part 1. To
allowing power to continue to be
alarm currents drawn by the
overcome this either all call points
supplied to the remainder of the
detectors. Installation of detectors
must be mounted at the start of
zone, whilst still permitting the
into their bases closes a contact in
each zone, or in completely
removed detector to be monitored.
the base supplying the remainder of
separate zones (both of these
This is achieved in a number
the zone. Thus if the line is broken,
solutions are often impractical and
of ways.
or if a detector head is removed,
too costly), or head removal
the current drawn by the zone will
monitoring can be employed.
An active end of line device uses a switched resistor at the end of line
fall, and a fault will be signalled (See figure 2). Example zone current
Acti ve monitoring uses bases fitted
and can thus be used with a
and voltage figures are
with a diode across the contact in
standard control panel. It sends a
given in figure 3.
the base (fig 4). Whilst the detector
periodic signal back along the detection line, which is normally quenched by the control panel.
Monitoring of detection line (example only)
When a head is removed, the base diode is switched into the line, and
Co n d i t i o n
Cur r en t
Vo lt ag e
pulse can be seen. The Active end
Op en Ci r c u it
<3m A
24V
of line then switches the resistor out
Normal
5mA (dependant on EOL device)
18V
Fire
50mA (dependant on control panel)
4-15V
If a capacitive end of line is used,
0V
the panel periodically drops the line
Short Circuit
High (dependant on control panel)
Figure 3: Example resistive conventional system current and voltage figures
of the line, and a fault is signalled.
voltage for a few milliseconds, and looks for the line voltage being held up by the capacitor. When a head is
The problem with a simple resistive
is mounted in the base, the base
removed, the panel will see the line
end of line is that should a detector
contact connects directly across the
voltage drop immediately as the
head be removed, the remainder of
diode, and links it out. There is
capacitor's discharge will be
the zone beyond that detector is
usually provision for manually
inhibited by the diode, and thereby
lost and no alarm can be signalled
linking the diode out to permit
a fault can be signalled.
beyond this point. Should a call
continuity testing during
point be mounted beyond the
commissioning. When the detector
A t hird type of end of line device is a diode. With this the panel
+24V
periodically reverses the line voltage for a few milliseconds: If the line is broken by the diode in the detector . L . O . E
base, then no current can flow in the reverse direction. The type of end of line monitoring used on a system will depend
0V
Contact Closes when detector inserted in base
Figure 4: Active end of line monitoring circuit
6
on the control panel. However it is important, particularly when using active end of line monitoring
to ensure that the detectors
wires, see figure 5. In this instance,
relay contacts are wired in series
are compatible with the type
a base incorporating a change over
with the normally closed contacts of
of monitoring being used.
relay is used. This configuration is
the detector relay base(s). Thus in
Reference must be made to
known as a four-wir e system, and is
the normal state the detection
the panel manufacturer to
often seen when a fire zone is
circuit is closed; in the case of fire
ensure compatibility.
integrated into a security panel.
or power failure the relevant relay contacts will open.
Figure 5 shows the simplest form of
remote LEDs
four-wire system, as used with most
Normally after an alarm, the
Most system smoke detectors are
security panels. This is used where
detectors are reset by disconnecting
equipped with a terminal to allow
the monitor line is able only to
the power to the relevant zone for a
the connection of a remote LED.
register an open or closed circuit -
short period by pressing a central
Remote LEDs are often used
there is no distinction between a
panel reset button. Fire panels have
outside bedroom doors in hotels so
fault and a fire. By using a normally
this facility built in, however many
that in case of a fire, it is easy for
closed relay at the end of the power
security panels are unable to do this
the fire brigade to identify the
line, it is possible to monitor for a
without turning the entire panel off.
location of the fire without needing
power failure to the detectors. The
Therefore to allow the use of
to enter every room in the building. They may also be used where a
- 24VDC
POWER SUPPLY END OF LINE RELAY COIL
+ 24VDC
detector is located in a hidden position, such as a floor or roof void
N I C D V 4 2 +
or cable tunnel, for example, to provide a visual indication that the
T T N I U U C O O D C C V 4 D D 2 V V 4 4 2 2 +
N I C D V 4 2 +
B E A M DETECTOR #1 E C N ) / A C N N E ( T Y A N I L A E M R
detector is in an alarm state.
) C / N ( T Y L A U L A E F R
T T N I U U C O O D C C V 4 D D 2 V V 4 4 2 2 +
B E A M DETECTOR #2 E C N ) / A C N N E ( T Y A N I L A E M R
) O / N ( M Y A R A L L E A R
) C / N ( T Y L A U L A E F R
) O / N ( M Y R A A L L E A R
four-wire system operation In some cases it is necessary that the power to the detectors and the fire detection signal be on separate
DETECTION CIRCUIT
r o t s i s e R
r o t s i s e R
L O E
Figure 6: 4-wire system with full monitoring
detectors with security panels, non-latching versions of the relay bases are usually made available,
Detection Line (Open Circuit for Alarm and Fault)
which automatically isolate the detector from the supply every few
FIRE ALARM CONTROL PANEL Power Supply (Nominal 12V or 24V System)
seconds. Thus once the fire End of Line Monitoring Relay C NO NC
C NO NC
condition has passed the detector will automatically reset (note that the alarm condition should be latched at the control panel.) Four-wire type systems are also often used with devices such as
Figure 5: Typical 4 wire system wiring
beam detectors where an auxiliary
7
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