Measurements of existence and level of the Partial Categories (PC) are carried out for the purpose of detection isolation defects in cable systems 6-500 kV with isolation from the sewed polyethylene, at the earliest stages of their emergence, tracking their development, assessment of current state of isolation, and possibility of identifying the location of PC. Over time the arising PC develop into the spark and arc categories which lead to accidents.

Registration pulses of the partial discharges can usually be made by sensors of three types which differ with operating range of frequencies:

• The low frequency range (LF) thrilling frequencies to hundreds of kHz. Normally it is acoustic and akustiko-emissive sensors.

• The high-frequency range (HF) occupying frequencies to tens of MHz. In this range transformer sensors, coupling capacitors sensors of surface spreading high-frequency currents are used.

• Ultra-high frequency range (UHF) stretching to several GHz. In this range various electromagnetic antennas work.

In case of existence of PD the place of their emergence can be defined by the analysis of a time delay during the passing and reflection of various signals which have resulted from PD. For this, we use the most widespread method of diagnostics measurement of coefficient reflection by method of form observation (TDR) at which measurement of timе passing signals of PD and their reflections is performed.

For operational observations of a CL condition we apply the measuring system MPD 600 of the OMICRON Company. This system gives the chance of carrying out operational, effective, exact measurements and localization of PD in full accordance with recommendations of IEC 60270 both in the conditions of factory laboratories and on installation.