Rockwell Automation FactoryTalk View Site Edition Users Guide - Manual (Page 264)

F ACTORY T ALK V IEW S ITE E DITION U SER ’ S G UIDE
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In this example, a tag’s value changes as it monitors a motor’s revolutions per minute
(rpm). An X in the illustration shows when the alarm condition goes into alarm, and an O
shows when the alarm condition returns to normal.
With the given limit settings, the motor must run between 3000 rpm and 4000 rpm (the
safe zone), otherwise, an alarm will be triggered.
Variable limits and alarm faults
Limits can be constant or variable. Variable limits are derived from the value of another
tag. As the value of the specified tag changes, the limit changes.
A variable limit must not become higher than the limit above it, or lower than the limit
below it. If this happens, an alarm fault is generated for the tag being monitored.
To correct an alarm fault, you must change the variable limit so it does not overlap either
of its neighbors. This can become complex when the neighboring limits are also variable,
because these boundaries are determined dynamically at run time.
When the faulty limits return to their normal operating range, the alarm fault is cleared,
the out-of-alarm-fault status is generated and logged, and normal alarm monitoring for the
alarm condition resumes.
Deadband
With some types of measured values, such as line pressure, tag values can fluctuate
rapidly above and below a critical limit.
Where such conditions exist, you can create a deadband as a buffer zone to prevent the
fluctuations from re-triggering unnecessary alarms.
For the High and High High alarm conditions, the tag value must drop below the alarm
limit minus the deadband, before the alarm condition goes Normal (Out of Alarm). For
the Low and Low Low alarm conditions, the tag value must go above the alarm limit plus
the deadband, before the alarm condition goes Normal.
If the motor speed It triggers an alarm of this severity
Exceeds 4000 rpm 750 (High limit)
Exceeds 6000 rpm 1000 (High High limit)
Falls below 3000 rpm 500 (Low limit)
Falls below 1000 rpm 250 (Low Low limit)