Abstract: This invention relates to fault detection in electrical circuits. The invention provides a switch for a fault tolerant digital output module comprising: an output transistor pair for enabling and disabling an alternating output current; and a current limiter for clamping the output current to a predetermined maximum value.

Abstract: This invention relates to overload protection for a circuit driving a direct current (DC) load.

Abstract: This invention relates to fault detection in electrical circuits. The invention provides a method and apparatus for testing an input circuit by generating a periodic test signal having a predetermined phase and a predetermined amplitude; summing the test signal and an input signal to provide a summed signal; processing the summed signal to provide an output signal; generating an extracted test signal from the output signal; comparing the extracted test signal with a reference signal representing said periodic test signal; generating an error signal in dependence upon the result of said comparing step. The invention also provides a method and apparatus for testing a plurality of adjacent input circuits.

Abstract: This invention relates to a speed monitor apparatus for monitoring speed and acceleration of rotating turbine equipment (turbomachinery) and for operating an overspeed trip to shut down the equipment in the event that the speed or acceleration exceed predetermined thresholds. The invention provides a speed monitor module wherein the speed monitor module may be may be plugged in to the same chassis as a module which is controlling the speed of the turbine, and wherein the turbine speed monitor is arranged to receive configuration parameters relating to the overspeed threshold and the overacceleration threshold only when the turbine is operating at a speed lower than a predetermined threshold.

Abstract: This invention relates to a method and apparatus for output current control. The invention provides an output module slice for controlling an output current comprising: an output field interface controller for controlling an input voltage to a first field effect transistor; an analogue to digital converter connected to read the current flowing through said first field effect transistor and to provide a digital signal dependent thereon to said output field interface controller.

Abstract: This invention relates to an emergency valve shutdown system having an integrated test facility. The invention provides an apparatus for testing emergency shutdown valves in a process industry comprising: a valve having a plurality of limit switches each limit switch arranged to indicate that the valve has reached a corresponding one of a plurality of predetermined valve positions; a valve monitor module arranged to send a signal to said valve to initiate movement of said valve and to receive feedback relating to said limit switches; wherein said feedback is provided via a single feedback signal between the valve and the valve monitor module.