Introduction. Intumescent coatings are used as a means of protection from heat flows, and their mission is to preserve the operability of wires and cables under fire conditions coupled with simultaneous current loading. However, the effect of insulation destruction on the operability of cables has not been studied for the case of a real fire regime.Goals and objectives. The purpose of the article is to evaluate the experimental operability of electrical wires and cables subjected to simultaneous effects of fire and current loadings.To achieve this purpose, an experimental testing unit was applied to conduct the experimental testing of wires and cables manufactured by various producers. At the same time, the temperature effect of the heated environment on electrical parameters of wires and cables, such as resistivity, inductance and capacitance, was evaluated.Theoretical background. In real fire conditions, dependence of indoor temperature, affecting the heating of cable insulation, differs essentially from the same dependencies in cases of various standard fire conditions. Therefore, the insulation destruction process may occur before the coating intumescence starts.Results and discussion. An experimental testing unit has been developed. This unit allows for the gradual cable heating with a pre-set temperature measurement interval and cable electrical characteristics. Dependencies of resistivity, inductance and capacitance of standard electrical cables on the temperature of the air surrounding the cable are obtained. It’s been discovered that the gradual heating of an electrical conductor or cable eventually leads to a short circuit between its conductive cores and further electric current transmission in electrical wires and cables. It is shown that phases and amplitudes of an input electrical signal can drastically change before the short circuit.Сonclusions. The simultaneous effect of fire and current loadings on standard electrical wires and cables causes a short circuit in the temperature range, in which no intumescence of flame retardant coatings is initiated on the insulation surface. Therefore, these coatings can ineffectively maintain the operability of electrical wires and cables.
Introduction. Signal cables of safety systems, installed at nuclear power plants (NPPs), retain the ability to conduct modulated signals during the time period needed to switch the reactor facility to a safe mode. However, the ability of signal cables to transmit signals correctly in the high temperature gas medium, which is typical for the early stage of a room fi re, has not been exposed to research.Aims and objectives. The co-authors offer a theoretical assessment of the ability of NPP safety system cables to correctly transmit modulated electric signals if exposed to fi re and current loads. The theoretical research into the temperature of the conductor of a signal cable at the initial stage of fi re has been performed towards this end.Theoretical background. The steady state heat conduction equation, describing heat transmission from the cable core to the environment through the cylinder-shaped insulation layer, is used to measure the temperature of the cable strand.Results and discussion. Temperature dependences describing the relation between the temperature of the conductor of a single - strand and single-wire cable KNEPng(А)-HF on the gas medium temperature are obtained. Relations between the temperature of the gas medium in the room on fi re and the current intensity in the electric cable (if the cable is laid vertically) are presented with account taken of the dependence between the specifi c resistance of the wire and the temperature if the maximal permissible operating temperature of cable strands is 70 °С, the maximal permissible operating temperature of cable strands in the overload operation mode is 80 °С, and the maximal cable strand heating temperature is equal to 160 °С when the short-circuit failure occurs. Maximal current intensity values are obtained for various operating modes in the condition of temperatures typical for the initial stage of an indoor fire, they allow to correctly conduct modulated signals within the time period needed to switch the reactor facility to a safe mode.Conclusions. The developed mathematical model and results of numerical experiments allow to assess the infl uence of the temperature in the room of a nuclear power plant in case of fi re on the ability of a signal cable of the safety system to transfer undistorted modulated signals depending on current loads and signal cable laying patterns (whether it is laid vertically or horizontally), and also to expand the range of the room temperature dependence on the current load provided in Electrical Installations Code (EIC).
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