Configure limit alarms

Limit alarms compare the input value with the values set for HiHi, Hi, Lo and LoLo. If the input value remains greater than the Hi or HiHi limit, or lower than the Lo or LoLo limit for at least the engine time period specified by the Time Deadband, then the condition for that alarm is set with the first input value that exceeds the limit. If the next value is normal and arrives before the end of the Time Deadband, then the alarm condition is not set.

Note: A limit alarm is not triggered if the value matches the limit but does not exceed it.

If an Alarm Deadband is set, this value is added to the input value for HiHi and Hi alarms, and subtracted from the input value for Lo and LoLo alarms.

• For HiHi and Hi alarms, an alarm condition is set only if the input value is greater than the HiHi or Hi setting plus the Alarm Deadband.

• For Lo and LoLo alarms, an alarm condition is set only if the input value is less than the Lo or LoLo setting minus the Alarm Deadband.

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.

Alarm deadband

The alarm deadband is an absolute value that is added to the HiHi and Hi limits, and subtracted from the Lo and LoLo limits. The PV value must be more than the configured HiHi and Hi limits plus the deadband, or less than the Lo and LoLo limits minus the deadband in order to trigger an alarm.

• If the input value plus the alarm deadband stays greater than the Hi or HiHi limit for at least the engine time period specified by the Time Deadband, then the condition for that alarm is set.

• If the input value minus the alarm deadband stays less than the Lo or LoLo limit for at least the engine time period specified by the Time Deadband, then the condition for that alarm is set.

Time deadband

The time deadband sets how much time must elapse before an alarm is set. After a PV value is received that exceeds the alarm threshold, and if the next value is not received within the time deadband, then an alarm condition is triggered. If the next value received is within the normal range and is received before the time deadband expires, no alarm occurs.

• If the initial value exceeds the alarm limit, but the next value is within the normal range, and is received before the end of the time deadband, no alarm occurs.

• If the initial value exceeds the alarm limit, and no other value is received before the end of the time deadband, an alarm is triggered.

Example

For example:

• The HiHi alarm limit is set to 90.

• The alarm deadband is set to 5.

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

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

• The Time deadband is set to 5 seconds.

  • If the PV value exceeds 95 (limit plus deadband value), and the next value (whether or not it exceeds 95) is not received within 5 seconds, the HiHi alarm is triggered.

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

Sample sequence of events

Using the parameters described above (HiHi alarm is set to 90, alarm deadband is 5, and time deadband is 5), the following sequence of events describes how a HiHi alarm could be triggered. This sequence of events assumes that no PV values, other than the ones described here, have been received.

1. A HiHi alarm is not initially active.

2. The PV value changes to exactly 95 and remains that way for 5 seconds. The alarm is still not active.

3. The value increases to 95.1 and remains that way for 5 seconds. The alarm activates.

4. The value decreases to exactly 85 and remains that way for 5 seconds. The alarm remains active.

5. The value decreases to 84.9 and remains that way for 5 seconds. The alarm deactivates.