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The Event Storage is not perpetual; the factory default storage capacity setting is 10.000 events per category.
A system may contain a large number of different event categories. E.g. AC800M adds the following:
- "Process Condition Event"
- "Process Simple Event"
- "System Condition Event"
- "System Simple Event"
Depending on filter setting, storage situation, etc. you may very well be looking at the "tail" of the storage period; there it is perfectly normal for events to drop out due to the FIFO theorem.
Keep in mind that when including multiple event categories into the same event list two categories may wrap at different points in time unless their event rate matches.
Find out what category you are seeing this problem for by calling up the Event Details... window; then create a temporary event list and filter object filtering ONLY that category. With default storage settings; you should be able to page down 19 times (each time another 500 events are listed; and after 19 clicks you are now at the "tail" where events are prone to disappear due to the FIFO theorem).
It is possible to increase the storage capacity up to 50.000 events per category. Search performance will drop slightly since we are not talking about a SQL database with fancy indexing possibilities.
Do you have more than one aspect server? Then check the same event list at both server nodes - if they differ the event storage database might be corrupt. Try first by restarting the event storage provider you believe to be incorrect; if that does not help, perform a cold start by stopping it; erasing its data folder (\OperateITData\Event Storage\*.*) then starting it again.
If nothing matches; I suggest that you contact your regional ABB support center and ask them to file a support case.