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10.4 Alarms
The BA474ND can be supplied with two solid state
single pole alarm outputs that may be independently
programmed as high or low alarms with normally
open or normally closed outputs. Fig 9 illustrates the
conditions available and shows which are fail safe,
i.e. in the alarm condition the output is open.
WARNING
Alarms should not be used for critical
safety applications such as a shut down
system.
When an alarm is activated an annunciator appears
on the left hand side of the BA474ND display. This
shows which alarm has been activated and will flash
if the alarm is delayed or silenced.
Fig 9 Alarm outputs
Programmable functions for each alarm include
adjustable setpoint, hysteresis, alarm delay and
alarm accept.
10.4.1 Solid state output
Each alarm has a galvanically isolated single pole
solid state switch output as shown in Fig 12. The
outputs are polarised and current will only flow in one
direction. Terminals 8 and 10 should be connected
to the positive side of the supply.
Ron
=
8ohms + 1.2V
Roff
=
greater than 180k
Note:
Because of the series protection diodes
some test meters may not detect a closed alarm
output.
Fig 10 Equivalent circuit of each alarm output
WARNING
Connection of alarm output to a supply
greater than 30V may rupture the internal
encapsulated fuse.
10.4.2 Type nA certification
Each alarm output is a galvanically isolated single
pole solid state switch that has been certified as a Ex
nA Group II, Category 3G circuit.
The input safety parameters for each alarm are:
Ui
=
30V dc
Ii
=
100mA
Pi
=
0.5W
This allows each alarm output to switch any dc circuit
providing that in normal operation the maximum
supply voltage is not greater than 30V and the
current is not greater than 100mA.
Fig 11 shows a typical application in which alarm 1 is
switching a Type n certified solenoid valve and alarm
2 is switching a safe area mounted sounder.
Fig 11 Typical alarm application