Long-term allowable current of the cable: load technology

When the cable line is energized, shall be subject to the specified load current. The requirement of the rules of technical operation due to the heat insulation during long loads. If the permissible current of the cable exceeds the limit, it will be overheating and destruction of the insulating layer, with consequent damage. Therefore, the load is selected so that there is a risk of thermal destruction of the insulating layer.

The cause of the heating cable

The Number released by the operation of the cable heat is given by:

- Q = I²Rn W/cm, where I is the load current; n - number of cores; R is the resistance in Ohms.

From the above expression it follows that the higher the current consumption on electrical installation which are connected to the cable, the more the latter is heated. Moreover, the capacity allocated in the veins in the form of heat, is a quadratic function of load.

Heat Dissipation from the running cable

Heating cable will not be expanding due to the fact that heat has to go somewhere. And the amount depends on the difference between the temperature of the cable and the environment. In the end, come to an equilibrium, and the temperature of the conductors will become permanent.

How to calculate ampere temperature reheat

When the heat load becomes equal to the amount of heat dissipated by the cable, the mode becomes stable.

- P = θ/∑S = (t_W - t_CP)/(∑S), where θ is the difference between the temperature of the conductor and the environment ⁰; t_W - t_CP - temperature difference ⁰; ∑S is the thermal resistance of the cable.

The Heat will leave the cable the more, the better the conductivity of the medium. Long-term allowable current of the cable is calculated as follows: I_DOP = √((t_Extra - t_CP)/( Rn∑S)), where t_Extra is the allowable temperature of heating of tendons (depending on cable type).

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Heat transfer

The best heat transfer occurs when the cable is in the water. If it is laid in the ground, the heat depends on the composition of the latter and the content of moisture. The calculations usually take the resistivity of soil r = 120 Ohm deg/W, which corresponds to the sandy-clay soil with a moisture content of 12-14 %. To obtain accurate readings it is important to know the composition of the soil, because resistance varies widely and is on the table. It is possible to reduce a change in the composition of the backfill of the trench with the cable and by thorough ramming. Porous sand and gravel have a lower conductivity than clay. Therefore, the filling of the cable produce a clay or clay loam that does not contain slag, debris and stones.

The Cable held in the air has poor heat transfer. Even worse it becomes when laying in cable ducts, where additional air gap, the mutual heating of adjacent cables and the resistance of the walls. For such cases, choose the load current as small as possible.

To ensure favorable temperature conditions for operation of cable line should be found allowable load current for the two modes: emergency and long-term. The characteristics of the cables also shows the value of the permissible temperature at short circuit, for which the paper insulation is 200⁰ C, and for PVC - 120⁰ C.

Long-term allowable current of the cable is indirectly proportional to its thermal resistance and heat capacity of the environment.

It is Necessary to take into consideration that over time the conductivity of the cable insulation increases because of drying. The resistance of the soil is 70% of the total and is decisive in the calculation of the total load.

Table to define the permissible current

ESL to be calculated manually, it is difficult to determine the long-term allowable current of the cable. The EMP contain a special table which gives its value for different operating conditions. Below are the calculated data maximum permissible loads for different cross sections of copper conductor when the temperature of 90⁰ C and ambient air 45⁰ C.

Using cables, the characteristics of which are given in the table, transmit and distribute electricity to DC and AC voltage and in stationary installations. They can not withstand large tensile forces and are laid in the ground, outdoors, in cable ducts. Durably allowable temperature of conductor is 70⁰ C, and if a short circuit is not more than 160⁰ For 4 seconds. Emergency operation is permissible heating lived does not exceed 80⁰ C.

Characteristics of conductors vary widely, depending on the marking, number of strands, and other parameters. Long-term allowable current of the cable VVG depends onsection, which is determined by the number and type of live. For example, the maximum cross-sectional area single core cable is 240 mm², and five-core - 50 mm².

Long-term allowable current of the cable AVVG is also determined by the cross section which is slightly larger than the wire VVG because it is made of aluminum. Permissible temperature of operation and emergency mode of operation of both types is identical.

Cable Avbbshv has a feature - it can be used in explosive and pozharoopasny areas due to the presence of double armour of steel tape. It is widely distributed in the construction. Long-term allowable current of the cable Avbbshv, as well as the previous products depends on the temperature, which should not exceed 75⁰ C, which is slightly higher. It is defined in the table and depends on the cross section lived and the method of installation.

Conclusion

To conductors prepostional load does not overheat, it is necessary to choose a long permissible current of the cable on the tables and calculate the heat dissipation into the environment. The wrong choice of cable will lead to its overheating and destruction of the insulating layer, which will cause premature product failure.