The normally closed contacts of the de-energised relay route the power to the Backup system. When the relay is energised the power is routed to the Master system. The relay can be energised by means of the closing health relay or contact output1 from the Master unit. When the health relay or contact output opens, the power switches from the Master to the Backup system.

An override switch enables the power to be ‘forced’ to the Master system for initial system commissioning so that the Master system is set to a system fault-free condition. The override switch can also be used to ‘force’ power to the Master system so that the cause of the changeover to the Backup can be investigated and rectified