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 expressed as a percentage of the attribute’s full value range. The deadband value sets the percentage of the total range that the attribute value must change to reset a limit alarm to the inactive state.

For example:

• An attribute's full value range is 0 to 50, and the HiHi alarm limit is 40.

• The alarm deadband is set to 5. Since the attribute's range is 50, the actual alarm deadband value is 2.5 (50 * 5%).

• The attribute value reaches the HiHi alarm limit (40).

• To reset the HiHi alarm to an inactive state, the attribute value must fall below 37.5 (40 - 2.5).

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.