Primary test and power transformer diagnostic system STS 5000 & TD 5000 Knowledge of the behavior of the principal electrical apparatus installed in a substation is mandatory and testing and maintenance are necessary to understand any inconvenience it may occur during normal and fault condition of the system. STS 5000 test equipment is the most accurate multifunction substation and maintenance system for current, voltage and power transformer. It becomes also a capacitance and Tan Delta diagnostic system with the optional module TD 5000.