The most critical function is to control the dump valve, if the device that is used to power the dump valve fails in a condition where power can’t be removed from the dump valve, the controller is not able to apply then SEMB.

This problem is overcome by using the following method; 3 relays are used to drive
the dump valve, 1 of these relays named the fail save relay, is never switched under load ensuring maximum life expectancy. The main relay that does the power switching is condition monitored; when a failure of this relay is detected the fail safe relay is used to switch off the dump valve. This will invoke a condition that will disable the unit in a safe mode and the unit will now be inoperable until it is serviced by BE. An output is available to disable the controller in the event of failure.
This concept ensures that the locomotive is operable only when the hydraulic en electrical subsystems of the SEMB are functional.