When choosing cabling are important some of criteria that influence the choice of type and cross-section of the cable line.
The following main selection criteria when working with power cables lines of low and medium voltage.
Selecting the operating conditions of the cable.
The cable must be chosen according to the climate and temperature operating conditions. Also take into account the particular method of laying lines and requirements for safe operation.
Selecting the temperature of the cable.
Wires and cables is selected based on the permissible heating subject to normal and emergency modes. As well as non-uniform current distribution between the individual lines, as heat changes the physical properties of the conductor, increases its resistance, increases the useless consumption of electrical energy for heating the conductive parts and shortens the service life isolation. Excessive heat is dangerous to the insulation and contact connections, and can cause fire and explosion.
Allowable current calculation 2-phase short-circuit.
Cable must withstand current in 2-phase short-circuit. For this must be checking the cable shield on the thermal resistance, as well as to calculate the maximum allowable time for the current at which the cable does not lose its performance properties.
Calculation of dynamic and thermal short-circuit strength.
For all types of chains (individual wires or tires) is important to consider electrodynamic factor. To withstand the electrodynamic overload, wire must be firmly attached and firmly connected. For cable lines it is also important to make sure that the characteristics of electrodynamic stability, while passing short-circuit current, satisfactory. The maximum value of the current, limited to the protection line must be smaller than the current of electrodynamic resistance cables.
Calculation economic current densities.
This item calculates the power reserve for the selected cable and defined lifespan. Conside the feasibility of increasing the number of lines or circuits than necessary under the terms of the reliability of electricity supply. As well as replacement of existing wiring heavy gauge wire with an increase in the load in order to meet the economic conditions of the current density.
Calculation of relay protection.
Calculation for the design of relay protection and automation must be made, and determined the current setting, and the time intervals to disable lines in the accident.