Level Controls WC & WCS - Screwplug Liquid Level Control The WC Liquid Level Control consists of a low wattage heater and SPST latching cutout switch, physically designed as a screwplug heater. When the WC is in liquid, the heat of the small wattage heater is dissipated into the liquid. When the liquid level drops below the heater, the thermostat changes states (opens) and remains open until it is physically reset. This action can protect the main process heater and the process.