Hot-Standby allows implementing a fail-over concept in a supervisory system. This concept consists in the possibility of having two servers (main and backup) working in contingency mode for each other, that is, if the main server fails, a backup server immediately takes over, without stopping the process. This backup server is said to be in Standby mode.

When starting to execute an E3 configured for a Hot-Standby system, the Windows Notification Area on these servers displays an icon represented by a yellow spinning bar , indicating that Domains are loading. After a few seconds, E3 recognizes the main server and indicates that action by an icon represented by a green arrow . This icon means the server is executing. The Standby server remains with an icon represented by two yellow bars , indicating that it is in Standby mode.