When LISN (Line Impedance Stabilization Network) are not applicable to EMI conducted test because the current flowing is too high, the conducted interferences are measured by means of a high impedance voltage probe, according to C.I.S.P.R 16-1 subclause 12.2.
The passive probes are designed to comply with CISPR 16-1 regulation and thus include a blocking capacitor to decouple the measured signal from any AC supply voltage which may be present on the line under test, they have a total internal resistance between line and earth of more than 1500 ohm.