Event under the auspices of the Ministry of Economy of Slovak Republic

Examination of insulation in fire retardant cables by time domain methods

Michal Váry
FEI STU UEAE

Jaroslav Lelák
FEI STU UEAE

Vladimír Ďurman
FEI STU UEAE

Juraj Packa
FEI STU UEAE


     Last modified: 2017-05-02

Abstract
Dielectric properties of the halogen free fire retardant cables were investigated with aim of choosing the most suitable measuring and analyzing method which can characterize the insulation state of these cables. Unlike of the previous research where we used the frequency domain measurements, the present work was directed to the time domain methods. Namely, it was the absorption current method and recovery voltage method. In this paper, the known procedures concerned with the time domain measurements are listed in details. Empirical and analytical approaches are compared and evaluated from the viewpoint of their effectiveness and accuracy in practical applications. Except of the previously known techniques, a new access to the recovery voltage data was proposed. This is based on the recovery voltage time integral. During experiments, the time domain measurements at various temperatures were performed on three types of fire retardant cables. The results of experiments were compared with the previous measurements of complex capacitance in the range of very low frequencies.
Conclusions from our work can be divided into the two sections: at first it is the interpretation of dielectric phenomena occurring in the cables, secondly it is the assessment of the used measuring methods with reference to their possible application in the ageing tests. As for the dielectric phenomena in the fire retardant cables, we have found a very slow polarization process with relaxation time of the order of 1000 s, which is probably caused by the interfacial polarization on the transition between the individual cores insulation. The insulation resistance is relative high because of an intermittent contact at insulation interfaces inside the cable. The contact irregularity generates some abnormal phenomena like the increase of resistance with temperature. This was not observed in three-core cables. As for comparison between the time domain and frequency domain method of measurements, we can conclude, that the time domain methods provide more reliability namely in calculation of insulation resistance, which is questionable at frequency domain measurements. The time domain measurements have also less demand on the measuring equipments.

 

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