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- Title: Cable and its standards in the distribution network
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For the economic use of cables, the optimal selection of the cross section is of particular importance. In this booklet, the effective factors in cable selection are examined, it should be noted that for the optimal selection of the cross-section, it is necessary to calculate losses and economic calculations, which are not discussed in this part.
Cable selection criteria can be divided as follows:
a) Nominal voltage.
b) Selection of the cross-section according to the current flow of the cable.
c) Considering the permissible voltage drop.
T) Tolerating the short circuit current by the cable.
The nominal voltage of the cable should be suitable for the system in which the cable is used. According to the first and second volumes of the standard of the cables used in the distribution network, this voltage should be according to Table 2-1.
U0 kV (R.M.S) 191235/66/0
U0 kV (r.m.s) 3320111
The current carrying capacity of the cables
In this part, the factors affecting the flow of cables are examined and the relevant tables are presented.
The most important reference used in this part is the IEC-287 standard under the title "Calculation of the continuous rated current of cables at a load factor of 100%", which is the criterion of the above standard in any part where more complete information was needed.
Determining the permissible current limit of the cables depends on the losses created in the cable and how the created heat is transferred to the surface of the cable and the surrounding environment. The IEC-287 standard gives the current limit by taking into account the losses created in the cable and the thermal resistance of the different layers of the cable and the ground in certain conditions. In this part of the brochure, it is assumed that the current value of the cables Specific conditions should be specified by the manufacturer. (This limit should be included in the technical documents of the tender), if the relevant information is not available, you can use the attached tables - A and B.
Factors affecting the rated current capacity of the cable
The important factors affecting the rated current capacity of the cable can be divided into the following groups:
Temperature is one of the important factors in determining the rated current capacity of the cable, which includes the temperature of the environment, the temperature of the installation site, and the temperature allowed for the insulation of the cable and its structure.
b) Cable design
In addition to the permissible temperature of the cable insulation, the type of cable design and the different layers used in it are important in determining the permissible current. These layers determine how heat is transferred from the conductor to the outer surface of the cable.
c) Installation conditions
Installation conditions such as installation in the air, buried in the ground, in the conduit, type of soil, etc. are factors affecting the flow of cables.
d) Effects of adjacent cables
If the cable is adjacent to other cables or pipes, appropriate coefficients should be considered to reduce the permissible current of the cable.
1- Ambient temperature
The average ambient temperature is different for each country and each region, which depends on the weather conditions of the region, cable installation conditions. In the standard IEC-287, the ambient temperature of the cable is given for several countries, in the standard for other countries, approximately the numbers in Table 3-1 are suggested.
Weather conditions, ambient temperature, temperature at a depth of one meter
Minimum Maximum Minimum Maximum
Table 3-1 Ambient and ground temperature in degrees Celsius
The values in the above table are approximate and should be used carefully. The nominal cable current limits should be calculated for the most extreme conditions throughout the year.
Cable working temperature
The maximum working temperature of the cable according to the IEC-287 standard for different cables should be according to Table 2-3:
Maximum insulation temperature of the conductor
Table 3-2 Maximum working temperature of the conductor for different cables
The effect of installation conditions on the rated current limit of the cable
Cable burial depth
Minimizing the damage to the cable is the reason for determining the depth of the cable burial. The higher the voltage of the cable, the greater the depth of the cable burial. As the amount of humidity increases, in this case, with the increase in temperature, the current capacity of the cable decreases, but with the increase in humidity, this value increases.
Specific thermal resistance of soil
The presence of moisture has a decisive effect on the specific resistance of each type of soil, this value must be measured for each region, if this number is not available, the following values are suggested according to the IEC-287 standard.
Climate and soil conditions, thermal resistance KM/W
Continuous wet very wet 7.0
wet rainy 1
Rarely rainy and dry 2
No rain or very dry rain 3
Table 3-2 specific thermal resistance of soil
Distribution cables are generally not used permanently at full load, so the issue of soil drying is not much of an issue, in conditions where the soil can be assumed to be wet, the thermal resistance value of the soil can be considered between 0.8-1Km/W. . In places where the soil is not always completely moist, but its type is a mixture of clay and garden soil, the value of 1.2 Km/W is a good value. If the soil consists of sand, after drying, some air will be created in the empty space of the sand. If this situation happens in several months of the year, the thermal resistance value of the soil can be considered between 2-3Km/W according to the following explanations:
Type A: Cables that carry a constant load throughout the year.
While the load is permanent or periodic, the value of the maximum thermal resistance of the soil should be considered, although this value may occur in some years and for a short time in summer or autumn, the suggested values are:
All soils except the soils below 1.5 km/W
Gypsum soil with small pieces of gypsum 1.2Km/W
Soil with a mixture of decayed plants 1.2Km/W
Stony soil 1.5Km/W
2.5 km/W of sand that has been drained
The applied soil is 1.8Km/W
If the soil is covered with an impermeable layer such as asphalt. The value of thermal resistance related to the first row in all types of soils may be reduced to 1.2Km/W.
Type B: cables with variable load and maximum load in summer
All the soils except the soils under 1.2Km/W
Stony soils 1.3Km/W
Sandy soil that has drawn water 2 km/W
Type P: cables with variable load and maximum load in winter
All soils except the soils below 1 km/W
Clay soil 0.9Km/W
Gypsum soil with small pieces of gypsum 1.2Km/W
1.5 Km/W for sandy soil that has drawn water
The applied soil is 1.2Km/W
When the clay soil is placed under an impermeable cover, its thermal resistance may decrease up to 0.8Km/W.
Standard conditions and nominal coefficients to correct the nominal value of full yield
The specified current values in the tables at the end of this section are based on the specified parameters below, and if the cable is used in the specified conditions, appropriate correction factors should be included.
Cables installed in the air
A) Ambient air temperature is considered to be 25°C for distribution cables and 30°C for indoor cables.
b) The air flow is not significantly restricted and for the cables installed on the wall, there should be at least 2 cm free space to the wall.
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