Despatch MIC1462 Controller Manual - Manual (Page 65)

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8.1 Alarm Inhibit Facility
On power-up, an "alarm" condition may occur, based on the alarm value, the process
value and, if appropriate to the alarm type, the setpoint value. This would normally
activate an alarm; however, if the pertinent alarm is inhibited, the alarm indication is
suppressed and the alarm will remain inactive. This will prevail until the "alarm"
condition returns to the "inactive" state, whereafter the alarm will operate normally.
8.2 Loop Alarm And Loop Alarm Time
The Loop Alarm is a special alarm which detects faults in the control feedback loop by
continuously monitoring process variable response to the control output(s).
The Loop Alarm facility, when enabled, repeatedly checks the control output(s) for
saturation i.e. either or both outputs being at the maximum or minimum limit. If an
output is found to be in saturation, the Loop Alarm facility starts a timer; thereafter, if
the saturated output has not caused the process variable to be corrected by a pre-
determined amount V after a time T has elapsed, the Loop Alarm goes active.
Subsequently, the Loop Alarm facility repeatedly checks the process variable and the
control output(s). When the process variable starts to change value in the correct
sense or when the saturated output comes out of saturation, the Loop Alarm is
deactivated.
For PID control, the Loop Alarm Time T is always set to twice the value of the Reset
(Integral Time Constant) parameter. For On/Off control, the user defined value of the
Loop Alarm Time parameter is used.
The value of V is dependent upon the input type:
°C ranges: 2° C or 2.0° C
°F ranges: 3° F or 3.0° F
Linear ranges: 10 least significant display units
For single output controllers, the saturation limits are 0% and Output Power Limit. For
dual output controllers, the saturation limits are -100% and Output Power Limit.