配電用ケーブルのトラッキング劣化による電気着火現象と電気火災防止装置の開発

Polyvinyl chloride (PVC) resin is widely used as an insulation or a jacket for distribution cable (F-Cable). The F-Cable would sometimes be ignited by an arc which occurs between deposited carbon caused by a scintillation discharge and an electric wire under wet polluted conditions. <br>Recently, the importance of fire breaking out in electric appliances and devices has been established, and tracking and ignition of polymers as an origin of fire has been scrutinized. Electric fire phenomena and these processes caused by tracking failure has been studied by many investigators, but no satisfactory solutions seem available as yet. <br>In this paper, first of all, in order to investigate the electric causes and processes of tracking failure and ignition, tracking tests were carried out by a test method which complied with the IEC (International Electrotechnical Commission) tracking test using the specimens with an artificial cutting groove of a commercial two-conductors F-Cable insulated by PVC resin. The electrolyte of 20 +3/-0 mm³ quantities is dropped in the center of cutting groove which has a width of 4 mm and a depth to conductor exposed. The electrolyte used is the same A-solution (0.1% NH₄Cl in distilled water) as the IEC test method provides. A constant voltage of 100 V (Effective value) was applied between one conductor and the other. <br>From these tracking test results of F-Cable, it is found that the free carbon is formed by the heat of scintillation, and that the waveform of current indicates the pulse shape. The pulse width of current measured with digital memory indicated from about 10 μs to 0.2 ms. After this stage, as the number of electrolyte drops increase, the arc occurs between the wet deposited carbon caused by a scintillation discharge and an conductor. The current waveform of the arc indicates half wave rectification. After this stage, the current waveform of the arc indicates distored alternating wave. The sheath or jacket is ignited by these arcs under test conditions in which the wet deteriorated parts are always in contact with the open air. These arcs will be the ignition source in this case. <br>Nextly, on the basis of the tracking model test results, the authors made a plan for electric fire protection equipment which responds to the various current signals in the tracking failure, and the electric fire protection equipment was manufactured for trial. <br>Thirdly, in order to reconfirm the response of present equipment against various current signals in the tracking failure under a load or no-load, the tracking model test was carried out. As the results obtained by the response reconfirmation test, the authors reconfirmed that this equipment had excellent frequency response, and responded to various current signals in the tracking failure under a load or no-load. On the other hand, it was clear that a fault current alarm of first class had no response to current signals in the tracking failure. <br>Finally, the trial manufacture equipment was proposed as the convenient method to protect the tracking failure and ignition of F-Cable for practical use.