The measurement of partial discharges (PD) in HV installations by means of high-frequency current transformers (HFCT) is being extended progressively in recent years, as they are affordable non-invasive sensors that can be easily installed. Several manufacturers provide in their data sheets the technical characteristics considered most relevant. However, there is no formal definition of these characteristics and/or minimum requirements to be met for such type of sensors, nor objective procedures to measure and quantify them. A new European EMPIR Project [1] aims to develop a characterization procedure for PD measuring systems. The main objective of this article is to analyze some of the most relevant characteristics of an HFCT sensor and to present testsetups to be used for obtaining them, especially its frequency response, as well as its EMC immunity, and their impact on the measured PD signals.