What are the requirements of thermal stability for the cable section of household distribution box?

What are the requirements of thermal stability for the cable section of household distribution box?

In the private 10 / 0.4kV substation, the selection of the cable section of the self use distribution box used in the substation is much larger than that of the distribution box in other places, because the distribution box set in the substation is close to the transformer. When the cable section of the distribution box is short circuited, the short circuit current is large. If the cable section is small, it can not meet the requirements of thermal stability.

(1) Calculation of short circuit current when short circuit occurs at the incoming line of distribution box.

The electrical design case of a residential project designed by our company is selected for calculation. In this project, a substation is set in the basement, two 800kVA transformers are set inside the substation, and a distribution box is set inside the substation (supplying power for the blower, lighting and repair socket of the substation). The capacity of the distribution box is 10kW, the calculated current is 16a, and the power supply is connected with the low-voltage bus of the transformer, The cable specification is nh-yjv-1kv5x16 and the length is 5m. The power distribution diagram from the substation to its own distribution box is as follows

Simple diagram of power distribution from substation to self use distribution box, wiring of self use distribution box.

The short circuit current in case of three-phase short circuit at K1 is calculated as follows

1) Resistance of transformer: S = 800kVA, RT = 1.90, XT = 13.27; 2) resistance of copper bar [tmy-3 (100x8) + (80x8)] from low-voltage main circuit breaker to feeder circuit of self-service distribution box: L1 = 6m; R1 = 0.04x6 = 0.24M; X1 = 0.182x5 = 1.092m ω; 3) resistance of cable [nh-yjv-1kv5x16] from low-voltage distribution box to self-service distribution box: L25M; R2 = 1.097x55 = 1.4.45m; x2 = 1.0820.08x5m ω; 3) Resistance of cable [nh-yjv-1k5x16] from low voltage distribution box to self use distribution box: l225m; r22000000000m; r = 1. R2=1000000000x9m;R

(2) Minimum cross section calculation and thermal stability check of wiring cable in distribution box 1) the minimum cross section calculation of wiring cable is based on the requirements of article 3.2.14 of code for design of low voltage distribution (GB 50054-2011): the selection of conductor cross section area shall meet the requirements of s ≥ I * √ T / K, where s is conductor cross section area (mm2) I is the root mean square value of short circuit current passing through the cable (a) t is the action time of automatic cut-off current protection of electrical appliances (s) k is Coefficient. The short-circuit current I is calculated as 13.84ka through the second section, that is, t of 13840a is the instantaneous tripping time of the case circuit breaker in the low-voltage box, and generally takes 20ms, that is, 0.02sk as the coefficient. The K value of cables of different materials is different. Through the investigation of Appendix A of code for design of low voltage distribution (GB 50054-2011), the minimum cross-section of k = 143 cable is calculated as follows: s ≥ I * t / k = 13840 * √ 13840 * * 143 = 13.7 (mm2). According to the above calculation, the cable of self use distribution box must meet the requirements of thermal stability, and its cross section shall not be less than 13.7mm2. 2) The incoming cable specification adopted in the original design is nh-yjv-1kv5x16, and its cross section is 13.7mm2 larger than the minimum cross section calculated above, meeting the requirements of thermal stability. The power consumption capacity of the self use distribution box is small, and the calculated current is only 16A. Considering the normal distribution, the wiring only needs 6mm2, but because it is set inside the substation, the cross section of the wiring cable must be expanded to 16mm2 to meet the requirements of thermal stability.

(3) This paper summarizes that in the power distribution design of civil buildings, the closer the electrical equipment is to the substation, that is to say, the closer the power supply is to the transformer, the smaller the impedance of the distribution line and the larger the short-circuit current. The selection of the cross section of the incoming cable should not only consider the calculation current of the electrical equipment, but also pay attention to whether the cable can meet the requirements of thermal stability. Therefore, the closer the distribution line is to the substation, the more difficult it is The selection of cable needs thermal stability verification.