Volume 3 nr 1 / 2011 - Academia Oamenilor de Stiinta din Romania

128 Alexandru Ionuț Chiuță, Liviu Mihai Sima, Nicoleta Doriana Secăreanuperturbation (distortion) introduced in the signal line, measured between IMGBDepot and Unirii 2 stations, and is correlated with the "hopping" potential levels.They are variable along the tunnel and approximately equal to the average of thelatter. From measurements it was found that the disturbances level that penetratesthe coaxial line is favoured by low insulation resistance of the coaxial cablebetween stations. This parameter shows large fluctuations over time. For example,the insulation resistance between the signal cable exterior armature to the groundbelt between Piata Sudului and Constantin Brancoveanu stations varies between400kΩ/1kV and 100MΩ/1kV. The minimum acceptable insulation resistance isconsidered to be at least 10MΩ/1kV (15MΩ/1kV).Under these conditions the following situations are possible:- CCTV signal cable has the screen interrupted or portions without screen (cablequality);- CCTV cable grounding is not done properly.Measurements made show that the "hopping" potentials on the tunnel andelectromagnetic influences make impossible the idea of changing the arrangementof the tunnel coaxial lines to ensure a minimum acceptable of the insulationresistance.Penetration of the disturbance in the CCTV installation takes place from electricpower feeding and earth contact, as well. Assessment of the perturbation level inthe feeding power network was done by measuring on all 20 kV busbars andgeneral distribution panels (GDPs) in all stations of the entire Subway Line II.Also, measurements were made at TGDs in the Unirii 1 station: dispatch area, onthe camera and monitors.Grounding belt of CCTV installations in stations is heavily perturbed.Comparison of the segments of earth belt between stations shows, in terms ofgrounding of the disturbances, a critical sizing (for CCTV installations) and / oran untidy construction and / or inappropriate execution of grounding of theelectrical equipment in Subway Line II and the CCTV system respectively.Interaction between the CCTV cable and grounding belt is so strong that, evenwith decoupling, disturbances are still very high.3.4.3. Susceptibility to disturbances of CCTV chainRegarding the susceptibility to disturbances of the CCTV chain and possibility ofincreasing immunity to disturbances, it was found that CCTV equipment is notqualified for operation under conditions of disturbances.From this point of view, the standards of industry products mention, in the chapterof "environmental conditions", that the equipment is designed for operation inenvironments free from strong electromagnetic fields and disturbances.Copyright © Editura Academiei Oamenilor de Știință din România, 2011Watermark Protected