When voltage is applied to the cable lines, the set current loads are set for them. The requirement of technical operation rules is associated with heating the insulation under continuous loads. If the long-term permissible current of the cable exceeds the limit value, it will overheat and destroy the insulation layer with subsequent damage. Therefore, loads are selected so as to eliminate the risk of thermal destruction of the insulating layer.
Cable heating reason
The amount of heat generated during operation of the cable is found by the formula:
- Q = I 2 Rn W / cm, where I is the load current, A; n is the number of cores; R is the resistance, Ohm.
From the above expression it follows that the higher the current consumed in the electrical installation to which the cable is connected, the more the latter is heated. Moreover, the power released in the veins in the form of heat is in a quadratic dependence on the load.
Heat dissipation from a working cable
The heating of the cable will not constantly grow due to the fact that the heat must go somewhere. Moreover, its amount depends on the difference between the temperature of the cable and the environment. In the end, equilibrium sets in and the temperature of the conductors becomes constant.
How to calculate the permissible current strength according to the heating temperature of the cores
When the heat from the load becomes equal to the amount of heat dissipated by the cable, the operation mode becomes stable:
- P = ΞΈ / βS = (t W - t sr ) / (βS), where ΞΈ is the difference between the core and medium temperature, 0 ; t W - t cf - temperature difference, 0 C; βS - thermal resistance of the cable.
The more heat will leave the cable, the better the conductivity of the medium. The long-term allowable cable current is calculated as follows: I add = β ((t add - t cf ) / (RnβS)), where t add is the allowable core heating temperature (depends on the type of cable).
Heat transfer conditions
Heat transfer is best when the cable is in water. If it is laid in the soil, heat removal depends on the composition of the latter and its moisture content. In the calculations, soil resistivity is usually taken r = 120 Ohm deg / W, which corresponds to sandy-clay soil with a moisture content of 12-14%. To obtain accurate readings, it is important to know the composition of the soil, since the resistance varies widely and is in the tables. It can be reduced by changing the composition of the backfill of the trench with the cable and by careful tamping. Porous sand and gravel have a lower thermal conductivity than clay. Therefore, the cable is filled up with clay or loam that does not contain slag, construction debris and stones.
A cable through the air has poor heat dissipation. It becomes even worse when laying in cable channels, where additional air gaps appear, mutual heating of adjacent cables and wall resistance. For such cases, select the current load as low as possible.
To ensure favorable temperature conditions for the cable line, it is necessary to find the permissible current loads for two modes: emergency and long. The characteristics of the cables also give the value of the permissible temperature during short circuit, which for paper insulation is 200 0 , and for PVC - 120 0 .
The long-term permissible cable current is inversely proportional to its temperature resistance and heat capacity of the external environment.
It must be taken into account that over time, the conductivity of the cable insulation increases due to drying. Soil resistance is 70% of the total value and is decisive in the calculation of the total load.
Tables for determining the permissible current
If calculated manually, it is quite difficult to determine the long-term allowable current of the cable. PUE contain special tables, where its values ββfor different operating conditions are given. Below are the calculated data on the maximum permissible loads for different sections of a copper conductor at its temperature of 90 0 and ambient air 45 0 .
Using cables, the characteristics of which are given in the table, transmit and distribute electricity in DC and AC networks and in stationary installations. They do not withstand large tensile forces and are laid in the ground, in the open air, in cable channels. The long-term permissible core temperature is 70 0 C, and with a short circuit - not more than 160 0 C for 4 seconds. In emergency mode, the permissible heating of the cores does not exceed 80 0 C.
Mark | Design |
VVG | Cores - copper conductor with a cross section up to 50 mm 2 . Insulation - PVC. The outer shell is PVC. |
AVVG | Veins - a conductor from aluminum with a section up to 50 mm 2 . Insulation - PVC. The outer shell is PVC. |
AVBBSHV | Veins - a conductor from aluminum with a section up to 120 mm 2 . Insulation - PVC. Armor - steel tarred tapes. |
The characteristics of the conductors vary widely, depending on the marking, number of cores and other parameters. The long-term permissible current of the VVG cable depends on the cross section, which is determined by the number and type of cores. For example, the maximum cross-sectional area of ββa single - core cable is 240 mm 2 , and in a five- core cable it is 50 mm 2 .
The long-term permissible current of the AVVG cable is also determined by the cross section, which will be slightly larger than that of the VVG wire, since it is made of aluminum. The permissible operating and emergency operating temperatures are the same for both types.
The AVBBSHV cable has a peculiarity - it can be used in explosive and fire hazardous rooms due to the presence of double armor made of steel tape. It is widely distributed in construction. The long-term permissible current of the AVBBSHV cable, like the previous products, depends on the temperature, which should not exceed 75 0 , which is slightly higher. It is determined by the tables and depends on the cross-section of the cores and the method of laying.
Conclusion
In order for the conductors under constant load not to overheat, it is necessary to select the long-term permissible cable current according to the tables and calculate the heat removal to the environment. Wrong cable selection will lead to overheating and destruction of the insulating layer, which will entail premature failure of the product.