Alarm Hysteresis

EXCEL 50/100/500/600/800

ATTRIBUTES

maintained” will be 120 hours. By comparing the attributes "Maintenance Alarm" and "Last Maintained", the user can see that the next maintenance period will be after an additional 380 hours have elapsed. If the maintenance alarm is reached, and the maintenance work has been performed, the counter can be reset manually. The counter can also be reset manually before reaching the maintenance alarm if, for instance, the maintenance has been performed earlier. If the counter is not reset, on expiration of the maintenance alarm, e.g. after 500 hours, the elapsed hours run continues to be incremented, and an additional alarm will be generated when 1000 hours running has been reached.

Hysteresis

The following hysteresis attributes allow the user to control the triggering conditions based on a changing parameter under which actions are taken, such as alarm signaling, writing values to buffers, etc. A hysteresis can be used, e.g., to prevent an alarm from being generated unless the value being monitored changes by more than a given value. This eliminates unnecessary alarm generation and reduces bus communication traffic.

Alarm hysteresis (V1.5.x) In the case of analog inputs and pseudo analog points, the attribute "Alarm Hysteresis" is available. It provides variable hysteresis that can be implemented, e.g., in order to reduce the cost of communicating with a remote front-end. The hysteresis value is set from an MMI and can have a value anywhere in the range defined by [10-ato (Max1 - Min1)] where “a” is the number of decimal places set in the attribute "Engineering Unit". The minimum value for the hysteresis is 10-a. Alarms are generated under the following conditions:

•Max 1 Alarm (generated if MAX 1 is exceeded)

•Max 2 Alarm (generated if MAX 2 is exceeded)

•Max 2 Normal (generated if the value falls below MAX 2-Hysteresis)

•Max 1 Normal (generated if the value falls below MAX 1-Hysteresis)

•Min 1 Alarm (generated if the value falls below MIN 1)

•Min 2 Alarm (generated if the value falls below MIN 2)

•Min 2 Normal (generated if the value MIN 2+Hysteresis is exceeded)

•Min 1 Normal (generated if the value MIN 1+Hysteresis is exceeded)

The CARE default value (i.e. 1% of actual value and no less than 0.2) for this attribute is 0. Access to "Alarm Hysteresis" is also possible via RIA/WIA.

NOTE: The number of decimal places can be changed only using CARE.

NOTE: Attempting to enter a hysteresis value that is less than the allowed minimum will result in the attribute being assigned the minimum value as defined above.

Example:

In this example, the number of decimal places in the attribute "Engineering Unit" has been chosen to have a value of 2. Fig. 11 shows an example datapoint value as a function of time that increases and decreases over the range from Max 1 to Min 1.

21	EN2B-0092GE51 R0512