A safety control system includes: a first control unit arranged to control a controlled system, a second control unit arranged to detect a fault with the controlled system and arranged to transmit messages wirelessly to the first control unit, wherein the second control unit includes: a first controller and a second controller, each of the first and second controllers being arranged to detect a condition of the controlled system and output messages indicative of whether or not the condition has been detected; a transmitter arranged to transmit wirelessly to the first control unit; and a multiplexer arranged to connect each of the first and second controllers in turn to the transmitter so that messages from each of the first and second controllers can be transmitted to the first control unit.