Long-term admissible cable current: loads, technology

When voltage is applied to the cable lines, the set current loads are set for them. The requirement of the rules of technical operation is related to the heating of insulation under prolonged loads. If the long-term permissible cable current exceeds the limit value, it will overheat and destroy the insulation layer with subsequent damage. Therefore, the loads are selected so as to eliminate the risk of thermal destruction of the insulating layer.

Cause of cable heating

The amount of heat released during the operation of the cable is given by the formula:

- Q = I ² Rn W / cm, where I - load current, A; N is the number of cores; R is the resistance, Ohm.

It follows from the above expression, the higher the current consumption in the electrical installation to which the cable is connected, the more the latter is heated. And the power released in the veins in the form of heat, is in a quadratic dependence on the load.

Dissipation of heat from the working cable

The heating of the cable will not grow constantly due to the fact that the heat must go somewhere. And its quantity depends on the difference between the temperature of the cable and the environment. Eventually there will be an equilibrium, and the temperature of the conductors will become constant.

How to calculate the allowable current from the heating temperature of cores

When the heat release from the load becomes equal to the amount of heat dissipated by the cable, the operating mode becomes stable:

- P = θ / ΣS = (t _ж - t _ср ) / (ΣS), where θ is the difference between the core temperature and the medium, ⁰ С; T _ж - t _ср - temperature difference, ⁰ С; ΣS is the thermal resistance of the cable.

The heat will escape from the cable the more the better the conductivity of the medium. The long-term permissible current of the cable is calculated as follows: I _add = √ ((t _additional - t _cp ) / (RnΣS)), where t _additional is the permissible temperature for heating the cores (depends on the cable type).

Heat transfer conditions

The best heat transfer occurs when the cable is in the water. If it is laid in the ground, the heat removal depends on the composition of the latter and the moisture content in it. In calculations, the specific resistivity of the soil is r = 120 ohm-deg / W, which corresponds to sandy-argillaceous 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 found in tables. It can be reduced by changing the composition of the backfilling of the trench with the cable and by careful ramming. Porous sand and gravel have a lower thermal conductivity than clays. Therefore, the backfill of the cable is produced by clay or loam, which do not contain slag, construction debris and stones.

The cable carried through the air has a poor heat transfer. Even worse, it becomes when laying in the cable channels, where there are additional air layers, mutual heating of the adjacent cables and the resistance of the walls. For such cases, the current loads are selected as low as possible.

To ensure favorable temperature conditions of 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 permissible temperature for a short circuit, which for paper insulation is 200 ⁰ C, and for PVC - 120 ⁰ С.

The long-term permissible current of the cable is inversely proportional to its thermal resistance and the heat capacity of the external medium.

It must be taken into account that over time the conductivity of the cable insulation increases due to drying. The soil resistance is 70% of the total value and is the determining factor in calculating the total load.

Tables for determining the permissible current

If it is calculated manually, it is rather difficult to determine the long-term admissible current of the cable. PUE contain special tables, where its values are given for different operating conditions. Below are the calculated data of the maximum permissible loads for different sections of a copper conductor at a temperature of 90 ^° C and an ambient air of 45 ^° C.

With the help of cables, the characteristics of which are given in the table, transmit and distribute electricity in the networks of DC and AC voltage 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 ^° C, and in the case of a short circuit - no more than 160 ^° C for 4 seconds. In emergency mode, the permissible heating of the veins does not exceed 80 ^° C.

The characteristics of the conductors vary widely, depending on the marking, the number of cores and other parameters. The long-term admissible current of the VVG cable depends on the cross section, which is determined by the number and type of conductors. For example, the maximum cross-sectional area of a single-core cable is 240 mm ² , and in a five-core cable it is 50 mm ² .

The long-term admissible current of the AVBG cable is also determined by the cross section, which will be somewhat larger than that of the VVG wire, since it is made of aluminum. The permissible operation and emergency operation temperature for both types is the same.

AVBbShV cable has a feature - it can be used in explosive and flammable premises 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, just like that of the previous products, depends on the temperature, which should not exceed 75 ⁰ C, which is somewhat higher. It is determined by the tables and depends on the cross-section of the veins and the method of laying.

Conclusion

To ensure that the conductors are not overheated during a continuous load, it is necessary to select a long-term permissible cable current through the tables and calculate the heat transfer to the environment. Incorrect cable selection will lead to its overheating and destruction of the insulating layer, which will lead to premature failure of the product.