Configure limit alarms

A limit alarm compares the current value to one or more predetermined alarm limits within the attribute’s full range of values. If the value exceeds a limit, an alarm occurs.

You can individually select and configure values and priorities for the LoLo, Lo, Hi, and HiHi alarm limits. You can set individual messages for each alarm limit.

You can also configure alarm and time deadbands for limit alarms. The alarm deadband is an absolute value, in engineering units, that the attribute value must exceed the configured HiHi, Hi, Lo, or LoLo limit before the alarm is triggered, or that the attribute value must revert to before the limit alarm resets to its inactive state.

For example:

• The HiHi alarm limit is set to 90.

• The alarm deadband is set to 5.

  • The attribute value must exceed 95 (limit plus deadband value) to trigger the HiHi alarm.

  • The attribute value must fall below 85 (limit minus deadband value) to reset the HiHi alarm to an inactive state.

The time deadband sets the length of time that an attribute value must continuously remain in an alarm or unalarmed condition. The process variable must remain above or below the indicated limit for at least the indicated deadband time before the application object updates the status of the alarm CONDITION Boolean. Then, standard Alarm feature logic determines whether to take that updated alarm condition and report changes to the alarm state or not.

The timestamp when a limit alarm becomes active or inactive is the most current timestamp of the corresponding input value. If there is no timestamp associated with the alarmed value, the AppEngine timestamp is used instead.