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I.S. Field Display User’s Manual
Michell Instruments 17
OPERATION
3.3 Alarms (Optional)
!
CAUTION:
These alarms outputs should NOT be used for critical safety
applications such as an emergency shut down system.
The I.S. Field Display can be supplied with factory- fi tted dual solid state, single pole
alarm outputs. Each alarm output may be independently conditioned as a high or low
alarm with a normally open or normally closed output in the non-alarm condition.
When the 4-20 mA current powering the I.S. Field Display is removed, both alarm outputs
will open irrespective of con fi guration. The open circuit condition should therefore be
chosen as the alarm condition when designing an alarm system. Figure 8 illustrates the
conditions available and shows which are fail safe.
When an alarm occurs, an alarm annunciator on the front panel is activated and, if
required, the numerical display can alternate between the measured value and the
alarm channel identi
fi cation
ALr1
or
ALr2
.
!
The alarms are activated by the I.S. Field Display’s
numerical display. Use of the Tare Function
tArE
will change
the numerical display - the alarms will continue to function
at the original displayed value, but this will correspond to a
different input current.
Output
Closed
Output
Open
Alarm
setpoint
4-20 mA Input
HIGH
ALARM
Output
Open
Output
Closed
Alarm
setpoint
Alarm
setpoint
4-20 mA Input
LOW
ALARM
Normally closed output ‘nc’
in the non-alarm condition
FAIL SAFE
Alarm
setpoint
Normally open output ‘no’
in the non-alarm condition
NOT FAIL SAFE
Output
Open
Output
Closed
4-20 mA Input
Output
Closed
Output
Open
4-20 mA Input
Figure 8 Alarm Outputs
Con fi gurable functions for each alarm include adjustable setpoint, hysteresis, alarm
delay and alarm accept.