An output module comprises two management units each of which calculates the current demand for the module independently from one another. One management unit controls an output current, controller whilst the other unit monitors the current produced by the output current controller against an independently calculated demand. The output module has multiple output modules. In normal operation, each module provides a portion of the required output current, the total output current being equal to the sum of the currents output by each module. in the event of failure on one of the, modules, the other module or modules switch to providing the total current required and the failed module is switched to a fail safe mode by using an isolation circuit to switch off the output current from that module.