DC EV charger testing: loop impedance
In the standard-compliant testing of DC charging stations, measuring the loop impedance plays a particularly important role. Fast charging stations handle significantly higher power levels than wallboxes and are part of the public charging infrastructure. Safe disconnection in the event of a fault is therefore of critical importance.
Unlike with AC charging stations, the structural differences between AC and DC charging columns must be taken into account during testing. In this guide, you will learn how to carry out a standard-compliant test of DC charging stations with regard to loop impedance measurement and which measuring devices are suitable for this purpose.
Loop Impedance Measurement at a DC Fast Charging Station
If you have ever measured a wallbox using an AC test adapter, you know that loop impedance is measured directly at the charging plug or the wallbox charging socket. At this measuring point, the impedance of all upstream parts of the installation up to the transformer is taken into account.
If the loop impedance of a DC charging station is measured at the charging plug, the measurement result provided by the test instrument for DC charging stations does not offer a sufficient basis for evaluating the fault loop impedance. The reason for this is the power converters used in the fast-charging column, which enable the transformation from AC to DC. If the loop impedance or network impedance is measured on the charging plug side, only the short section within the DC charging station itself is tested. The upstream AC-side installation is not measured.
Standard-Compliant Testing of DC Charging Stations: Loop Impedance
A standard-compliant test of DC charging stations requires, with respect to loop impedance and short-circuit current, primarily AC-side measurement at the supply point. The measurement must be taken inside the fast-charging station. Electrically speaking, the measuring point should be chosen as far as possible downstream from the protective device. In the event of a fault, it must be ensured that the upstream protective device trips within the specified time.
The result of a DC-side loop impedance measurement (at the charging plug) does not provide a sufficient basis for evaluating the entire installation.
Loop Impedance Tester for DC Charging Stations
Fast-charging stations handle high currents and have large conductor cross-sections in the supply line. A simple installation tester is no longer sufficient here; therefore, it is advisable to use loop impedance testers specifically designed for DC charging stations. The issue lies in the test currents being too low, which leads to insufficient resolution. For this reason, we recommend the high-current impedance testers (300 A test current) from the MZC series.
