Alarm hold-off

Alarm hold-off is used to temporarily suppress child state alarms that result from data krunching. Other types of alarms, such as command failure, execute as normal. Alarm hold-off is supported for analog, rate, multistate, and status records.

Communication Order and Alarm Suppression

Communication order can cause real-time events to be reported in an order that differs from the actual order in which they occurred. A value for a dependent device (child) can even be read before the recently changed value of its parent. This can result in false alarms that would normally be suppressed using alarm suppression. The alarm hold-off timer helps delay alarm processing to counter the effects of communication order.

For example, a breaker (parent) and a voltage sensor (child) exist on different Remote Terminal Units (RTUs). If the breaker trips, a low voltage alarm from the child is expected. Since the data for the voltage sensor could be processed before the data for the breaker, an alarm hold-off for the voltage sensor is configured. An alarm hold-off can suppress the child’s alarm until the breaker data has been updated and processed, preventing a nuisance alarm.

If the telemetry protocol does not provide a method to force the processing of the parent before its dependent child, the alarm hold-off can be used to suppress the child alarm until it is reevaluated at a later time. Both alarm and return-to-normal (RTN) alarm events are generated for the child regardless of alarm suppression. The alarm hold-off timer postpones the alarm long enough to receive the updated parent value.

The above figure shows a parent (real parent) going into alarm in real-time. However, the telemetry parent value is not processed until after the child’s value is read.

Note: In the above figure, the telemetered child value matches the real child value for example, there is no significant delay between when the child value changed and when the data was received in the AVEVA Enterprise SCADA system).

The alarm hold-off timer is triggered when the child’s alarm state changes and neither its parent alarm nor its RTN alarm suppresses it. Alarm hold-off saves the child’s previous value so it can compare it with the child’s value when the timer expires.

When the parent value is processed, the telemetered parent goes into alarm and triggers the alarm suppression timeout that cancels the alarm hold-off timer. Since the child returns to a normal state before the alarm suppression timeout expires, no alarm is generated.