WPU regulates conductivity of the water filled in the CTT. Thermal losses of the cable heat up the water and WPU cools it by means of a built in heat exchanger WPU has an application specific software suite which makes it easy for the operators to set up AC or LI or SI tests and to switch between them. The software also monitors temperature, pressure water flow and conductivity. It also issues warning messages, status of interlocks, and pressure values. the functions are operable via a touch screen