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Abstract

When writing this abstract the master's work is not completed yet. Final completion: June 2019. The full text of the work and materials on the topic can be obtained from the author after the specified date.

Contents

Introduction

In connection with the growth of automated processes in various industries, agriculture and housing and communal services are directly related to the increase in the number and increase in power of objects that are sources of ignition. Combustible materials (substances) heated to a high temperature, parts and assemblies of electrical products, as well as arcs, sparks, gas emissions, etc., which occur in emergency conditions, often act as sources of ignition. The way to prevent fires is to stop using ignition sources or reduce their energy to a safe level. In many industrialized countries, about 20–25 % of the total number of annual fires occur due to fires caused by malfunctioning or improper operation of electrical devices, while the trend continues to grow.

1. Theme urgency

In various industries and national economy fires occur, the causes of which are overloading of cable products. According to statistics, the most flammable electrical products are cable products. When the load exceeds the allowable value, the current increases dramatically, which leads to heating of the wire and melting of the insulation. In turn, this may lead to a short circuit. And the consequences of this situation are predictable - open fire and fire! [3]

The technical basis for ensuring the safety of electrical installations are fuses and circuit breakers that are used to protect external power supply systems. However, despite the improvement of protective equipment, the efficiency of the existing power system remains low. One of the reasons is the complexity of implementing sensitive protection. Also, the low protection efficiency is due to the spread of the characteristics of the operation of fuses and circuit breakers. The actual response time of the protection can vary significantly due to the presence of a scatter zone. Temporal selectivity consists in setting various time exposures (settings) in protection apparatuses located at different levels of the power supply system and reacting to the overcurrent arising in the circuit. The closer the unit is to the source, the longer its response time should be (the larger the setpoint). Suppose such a situation, for whatever reason, the switch that was closer to the electric receiver failed, and the switch behind it did not turn off the circuit in time. While the group switch was disconnected, the cable located directly at the receiver burned out. An obvious disadvantage of temporal selectivity in addition to the existence of the foregoing limits of application is also the need to spend some time waiting for the solution of the electrical installation protection system. This waiting time is necessary based on the principle of organization of this method. At the same time, the power supply system is exposed to emergency current, which leads to additional equipment wear and higher requirements for its thermal and dynamic resistance. Reducing the waiting time on the steps of the time delay is possible with a decrease in the spread of values in the time-current characteristics of the circuit breakers (improving the accuracy of operation), which is certainly associated with a significant increase in the cost of equipment. The use of microprocessor trip units in devices, also associated with an increase in cost, improves the speed of the protection system by increasing the accuracy of work. It should be noted that, despite the obvious shortcomings of this method, it is used to coordinate the protective characteristics of devices in the vast majority of electrical installations [1].

Conclusion

The developed model will allow to take into account the dependence of the thermophysical and electrical properties of the cable material on temperature, to follow the temperature change of the core and insulation over time, it is possible to describe the dynamics of processes with time varying current strength. The performed calculations of stationary temperatures will make it possible to make a base of constant heating of cables of different brands and sections, which will help in the analysis of the algorithms of the current protection device operation.

References

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  2. Аушев И.Ю, Станкевич Ю.А, Степанов К.Л. Динамика нагрева многожильного изолированного проводника электрическим током. // Вестник Командно-инженерного института МЧС Республики Беларусь, № 2 (16), 2012.
  3. Современные проблемы электроэнергетики. Алтай – 2014 : сборник статей II.
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