For the determination of parameters for your single circuit line, you inject a test current into several different test loops. Each of the loops represents a possible fault scenario. Thereby, the measured loop impedances equal the loop impedances, which the connected protection device would determine during a real fault on the line. Based on these loop impedances, your required single circuit line's parameters are finally calculated.
Conventional test methods use high-power injection at mains frequency for suppression of interferences. You can freely select the frequency with modern systems: By injecting a signal with a frequency higher or lower than the mains frequency and applying a steep filter, you effectively suppress the interference.
Our test system uses this technique and thereby allows you to inject of much smaller currents for gaining accurate measurement results.