Is there down time in dataguard

common you missed main things to post here

What your Database Version..?

Then yes - there is minimal downtime is required.

yes it is considered as a down time.

If your Primary goes down, you would do a FAILOVER to the Standby  (instead of a graceful SWITCHOVER).  A FAILOVER certainly involves noticeable downtime.

Also, you must verify if your application servers / clients are configured to automatically attempt a connection to the Standby (i.e. their TNS connect-strings also have the Standby target).

Hemant K Chitale