The principle of resistance probe discrimination between water and steam is well-established and accepted by the power plant industry as the most reliable, convenient, and maintenance-free method of detecting alarm levels and initiating protective actions. One, two, three or four independent probe channels in one electronic enclosure means the installed cost per detection point can be appreciably lower than traditional float level switches. The system incorporates a probe fault circuit that can distinguish whether probes are in a healthy state. The process fault circuit indicates a change in the normal operating state of each probe (for example, from steam to water). The Probe Insert Assembly is welded into pipework or vessels at the selected detection points. This provides internal shrouding of the probe for defined water conductivities and a robust housing for external protection of the probe. Multiple probe pipework assemblies can be supplied to suit particular applications. The system is micro-processor based, using a handheld programmer for set-up and adjustment.