SQL queries for alarms and events

Note: The alarm and event history functionality described in this chapter captures detailed histories from Application Server. This functionality should not be confused with the Classic Event subsystem, which allows for some basic events tracking and is based on historical data. The examples provided in this chapter only work when the Historian is configured to use high-speed alarms and events storage.

AVEVA Historian captures process data about your plant. In addition to real-time and historical data, this includes information about events.

Events are like other process data – for example, temperature – because their values can change over time. Events differ from other process data in these ways:

• Events usually change more slowly.

• Events usually are more complex than simply a value, time, and quality.

Event data answers questions like "When did this setpoint change and who changed it?" The event record could include the name of the operator, the workstation from where the change was made, any comment about the change, the name of the person who verified the change, and other related details.

Alarms are a specific kind of event. They represent state changes and have an associated lifecycle. This lifecycle includes these states (usually in this order):

• Set – For example, when a temperature goes too high.

• Acknowledged – That is, when an operator recognizes it as an alarm and, ideally, addresses it.

• Clear – For example, when the temperature returns to normal.

Alarms may also have other states, but these are the key ones.

You can query the Events view, which references the History table, to track and analyze alarms and other events. Because Events is actually an extension table (like History), its data is stored in blocks, not in SQL Server tables.

Note: The Events view does not expose all application-specific columns that may be stored by Historian. (Such columns can be queried from the REST/OData interface.) Also, it is not unusual for Events columns to contain many NULL values.

For more information about the Events view, see Events in the AVEVA Historian Database Reference.