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Power Distribution Box

1、 Classification

The technical code for safety of temporary electricity on construction site (JGJ46-2005) clearly stipulates that the electricity on construction site must be three-level distribution. Level I refers to the general distribution box on the construction site, level II refers to the distribution box, and level III refers to the distribution box of each equipment. In other words, each equipment has its own separate distribution box, which is called three-level distribution box. Then, several three-level distribution boxes are summarized to become a two-level distribution box. Then, the primary distribution box is summarized by multiple secondary boxes.

The requirements of level I, level II and level III distribution boxes are different, including control load, control current and control mode, which shall be determined according to the specific project, number of equipment and power.

Three level distribution box, also known as final protection device. It is the switch box, which can only be responsible for one household or one equipment. The so-called "one machine, one switch, one box and one leakage" is for the switch box. Since the third pole distribution box is generally indoors, it has no particularly strict mechanical strength and waterproof requirements.

Class Ⅰdistribution box

The primary distribution box is the centralized point for controlling all power supply on the construction site and should be set near the power supply area. The power supply is connected by the cable from the low-voltage side of the power transformer on the construction site, and the current transformer, active watt hour meter, reactive watt hour meter, ammeter, voltmeter, main switch and sub switch are installed. The switches in the main distribution box shall adopt knife fuse switch (or leakage protection switch). The incoming and outgoing lines shall pass through pipes and have waterproof bends.

Class II distribution box is the distribution box, also known as sub box, which is generally responsible for a power supply area.

Main circuit: main isolation + main circuit fuse or circuit breaker; shunt: shunt isolation + shunt fuse or circuit breaker.

Class Ⅲ distribution box

Tertiary distribution system

The neutral point directly grounded system with special protective zero line is called TN-S neutral protection system, commonly known as three-phase five wire system. The neutral conductor and protective conductor of the whole system are separated. Three-level power distribution, i.e. general distribution, distribution and switch box, three-level power distribution and step-by-step protection, so as to achieve "one machine, one switch, one leakage, one box and one lock".

Ⅱ.Overview

1) The power distribution system shall be equipped with indoor general distribution panel and outdoor distribution box or outdoor general distribution box, distribution box and switch box to implement three-level power distribution.

2) The power distribution box and lighting distribution box should be set separately. If they are combined in the same distribution box, the power and lighting lines should be set separately, and the lighting line wiring should be connected to the upper side of the power switch.

3) The switch box shall be distributed by the final distribution box. One machine and one switch shall be set in the switch box. Each electric equipment shall have its own switch box. It is strictly prohibited to use one switch to directly control two or more electric equipment.

4) The main distribution box shall be located near the power supply, and the distribution box shall be installed in the area with relatively concentrated electrical equipment or load. The distance between the distribution box and the switch box shall not exceed 30m, and the horizontal distance between the switch box and the fixed electrical equipment controlled by it shall not exceed 3M.

5) The distribution box and switch box shall be installed in dry, ventilated and normal temperature places. It shall not be installed in gas, flue gas, steam, liquid and other harmful media with serious damage. It shall not be installed in places vulnerable to impact of foreign solids, strong vibration, liquid splashing and heat source baking. There shall be enough space around the distribution box and switch box for two people to work at the same time, and no articles hindering operation and maintenance shall be stacked around them.

6) The distribution box and switch box shall be installed correctly and firmly, and the mobile box shall be installed on a solid support. The vertical distance between the lower skin of fixed distribution box and switch box and the ground shall be greater than 1.4m and less than 1.6m. The vertical distance between the lower skin of mobile distribution box and switch box and the ground is 0.8 ~ 1.6m. The distribution box and switch box shall be made of iron plate or high-quality insulating material, and the thickness of iron plate shall be greater than 1.5mm.

7) The inlet and outlet of the conductor in the distribution box and switch box shall be set at the lower bottom of the box, and it is strictly prohibited to set at the upper top, side, back or door of the box.

Ⅲ.Installation of electrical equipment

1) The electrical appliances in the distribution box shall be first installed on the metal or non wood insulated electrical appliance mounting plate, and then fastened in the distribution box as a whole. The metal plate shall be electrically connected with the distribution box. 

2) Various electrical appliances in the distribution box and switch box shall be fastened on the mounting plate according to the specified position without skew and looseness. Moreover, the distance between electrical equipment and around the equipment and the Board shall meet the requirements of relevant process standards.

3) The working zero line in the distribution box and switch box shall be connected through the terminal board, and shall be set separately from the protective zero line terminal board.

4) The connecting wire in the distribution box and switch box shall be insulated conductor, and the model and section of the conductor shall strictly comply with the section marked in the temporary electricity drawing. The connecting wire between various instruments shall be insulated copper core conductor with a section of not less than 2.5mm2, and the conductor joint shall not be loose and there shall be no exposed live parts.

5) The metal frame, metal box, metal electrical installation board of various boxes, as well as the normally uncharged metal base and shell of electrical appliances in the box must be connected to neutral, and the protective zero line shall be connected through the wiring terminal board.

6) The wires behind the distribution box shall be arranged neatly, bound into bundles, and fixed on the panel with clips. Appropriate redundancy shall be reserved for the outgoing and incoming wires behind the panel for maintenance.

7) The wire core shall not be damaged at the wire exploitation, and the wire indenter shall be firm and reliable. Multi strand wires shall not be crimped by coils, and crimping terminals shall be installed (except for crimping holes). If it is necessary to crimp with jacking wire for perforation, multi strand wires shall be brushed with tin before crimping, and the number of wire strands shall not be reduced.

8) Clearly mark the name of the control circuit and equipment.