Three-level distribution and two-level protection

Three-level power distribution: equipped with three standard electrical boxes: main distribution box, sub-distribution box and switch box. Switch box shall conform to one machine, one box, one brake and one leakage. All kinds of electrical appliances in Class III electrical boxes shall be qualified products;

Two-level protection: choose leakage protectors that meet the capacity requirements and qualified quality in the main distribution box and switch box.

This is not only conducive to the maintenance of on-site electrical system, but also can fully ensure the construction safety, and it is also the embodiment of on-site standardized management and civilized construction. Therefore, it is very important to set up "three-level distribution and two-level protection" reasonably.

1, "Three-level distribution and two-level protection"

(1) The Technical Specification for Temporary Electricity Safety on Construction Site (hereinafter referred to as the Specification) requires that distribution boxes should be set up in different levels, that is, there should be a distribution box below the general distribution box, a switch box below the distribution box, and electrical equipment below the switch box to form three-level distribution. This way, the distribution level is clear, which is convenient for management and fault finding. At the same time, lighting distribution and power distribution are required to be set separately to form an independent system, so that lighting will not be affected by power failure.

(2) "Two-level protection" mainly refers to the use of leakage protection measures. In addition to installing the leakage protector, the last level switch box should also install the first level leakage protector in the superior distribution box or the general distribution box to form two levels of protection as a whole.

2. About installing leakage protector

"Code" stipulates: "In addition to protecting the connection, all electrical equipment on the construction site must be equipped with a leakage protection device at the head end of the equipment load line".

Although TN-C was changed to TN-S in the construction site, the safety of power supply was improved. However, due to the limited protection sensitivity, it is impossible to quickly cut off the insurance for the shell collision fault of large-capacity equipment, and it is also impossible to cut off the insurance for the small-current leakage fault, which still poses the danger of electric shock to operators, so it is necessary to install a leakage protector for protection. When installing a leakage protector, the original protective grounding (grounding) measures shall not be removed.

3. Main parameters of leakage protector

(1) rated leakage action current. When the leakage current reaches this value, the protector acts.

(2) Rated leakage action time. Refers to the time from reaching the leakage action current to the circuit being cut off.

(3) Rated leakage current. When the leakage current is lower than this value, the protector should not act, and its value is 65438+ 0/2 of the leakage current.

(4) Rated voltage and rated current. Adapt to the protected lines and loads.

4. Selection and matching of parameters

(1) Two-stage leakage protectors shall match:

The specification stipulates: "The rated leakage action current and rated leakage action time of the two-stage leakage protectors in the main, distribution box and switch box should be reasonably matched, so that they have the function of grading and subsection protection".

"Two-level protection" refers to taking the trunk and branch lines of the power grid as the first level and the end of the line as the second level. The primary leakage protection area is large, and the impact is also great after power failure. The sensitivity of leakage protector should not be too high, and its leakage action current and action time should be longer than that of the secondary protection behind. This kind of protection mainly provides indirect protection to prevent leakage fire. If the parameters are too small, it will lead to misoperation and affect normal production.

The leakage non-action current of the leakage protector should be more than 2 times of the total leakage current of the power supply line and electrical equipment, and a high-speed leakage protector with leakage action current less than 30mA should be installed at the end of the line to form hierarchical and segmented protection, so that each electrical equipment has two levels of protection measures.

When grading protection, the scope of protection at all levels should cooperate with each other. When an accident finally occurs, the protector will not go beyond the level. When the lower leakage protector fails, the upper leakage protector will act to remedy the unexpected situation of lower failure.

(2) General distribution box (primary protection):

Under normal circumstances, the main distribution box should not adopt the leakage trip type. Once the main box power supply is cut off, it will affect the electricity consumption of the whole low-voltage power grid and affect production and life. The sensitivity of leakage protector is not too high, so medium sensitivity leakage alarm and delay protector can be selected.

The leakage action current should be selected according to 2 times of the measured leakage current of the trunk line, and the leakage action current value is generally 300 ~ 1000 Ma.

(3) Distribution box (secondary protection):

The installation of leakage protector in distribution box not only plays a monitoring role for lines and electrical equipment, but also plays an auxiliary protection role for switch box. The distribution box leakage protector mainly provides indirect protection, and the parameter selection should not be too close to the box closing, so as to form a graded and segmented protection function. Too much parameter selection will affect the protection effect, but too little parameter selection will lead to leapfrog tripping, and the distribution box will trip before the switch box.

The ability of human body to withstand electric shock is not only related to the current passing through human body, but also related to the duration of current in human body. According to this theory, the international safety limit for designing leakage protectors is 30 mA s. Even if the current reaches1000 ma, as long as the leakage protector cuts off the power supply within 0.3s, the human body will not cause fatal danger. This value is also the basis for providing indirect contact protection.

The distribution box leakage protector mainly provides indirect protection, and its parameters are selected according to 2.5 times of the measured leakage current value on the branch line. Generally, the optional leakage action current value is 100 ~ 200ma (which should not exceed the limit of 30mA.s).

(4) Switch box (tertiary protection):

The specification stipulates: "The rated leakage action current of the leakage protector in the switch box should be no more than 30mA, and the rated leakage action time should be less than O.1s..

The leakage protector used in wet and corrosive places should adopt splash-proof products, and its rated leakage action current should not be greater than 15mA, and its rated leakage action time should be less than O. 1s ".

Switch box is the last stage of hierarchical power distribution, and frequent use is dangerous. Indirect contact protection and direct contact protection should be provided, which are mainly used to protect people with fatal danger from electric shock.

Although the safety limit of the designed leakage protector is 30mA·s, when the human body is in direct contact with the phase line, the electric shock current passing through the human body has nothing to do with the action current of the selected leakage protector, but is completely determined by the electric shock voltage and human body resistance when the human body is electrocuted (the human body impedance changes with the contact voltage). Because the danger of electric shock is often more serious than indirect electric shock, it is stipulated in the temporary electrical code and national standard: "When used for direct contact electric shock protection, high voltage should be selected. Fast action type means that the action time is less than O.1s. Therefore, the parameter selection of direct contact leakage protector is 30mA×0. 1s=3mA s = 3ma s. This means that in case of direct contact electric shock accident, the current value should not be greater than the escape current; Judging from the duration of current passing through human body, it is less than one cardiac cycle and will not cause ventricular fibrillation. Under wet conditions, the leakage current is not more than 15mA due to the decrease of human resistance.

5. Testing of leakage protector

The examination content is divided into two items. The first item tests the sensitivity of the interlocking mechanism by pressing the test button of the leakage protector for three times. Open and close the switch with load for three times, and there should be no wrong action; The second test characteristic parameters, the test contents are: leakage action current, leakage non-action current and breaking time, and the test method is carried out by special leakage protector tester. The above tests shall be carried out after installation and before use, and shall be carried out regularly (monthly) after the leakage protector is put into operation, and the frequency shall be increased in thunderstorm season.

6. Circuit breaker

(1) disconnector is generally used for high-voltage distribution equipment. Considering the actual situation of the construction site, the code emphasizes that the main purpose of setting the power disconnector in the electrical box is to ensure that the electrical equipment is isolated from other operating electrical equipment during maintenance, and to provide visible disconnection points to the staff at regular intervals in the air to ensure the safety of maintenance work. The disconnector has no ability to extinguish the orphan, and it is absolutely forbidden to switch on with load. Otherwise, the electric arc formed between the contacts will not only burn the disconnector and other adjacent electrical equipment, but also cause accidents due to phase-to-phase or solitary light to the ground. Therefore, the isolating switch can only be opened after the load switch is turned off, and the load switch can only be closed first.

(2) According to the specification, isolating switches should be installed in the main distribution box, distribution box and switch box to meet the requirement of "electrical equipment can be used for power isolation in any case".

(3) The air switch cannot be used as an isolating switch:

Automatic air switch, referred to as air switch or automatic switch, is a kind of protective electrical appliance that automatically cuts off line faults. It can be used as short circuit, overload and undervoltage protection in the main circuit of the motor, but it cannot be used as a power isolating switch. The main reason is that the air switch has no obvious disconnection point, the disconnection point is short, it is easy to breakdown, it is difficult to ensure reliable insulation, and the contacts sometimes get stuck. In view of the above inertia, knife switches, knife switches and fuses can generally be used as power disconnectors. Knife switches and knife-shaped changeover switches can be used as power disconnectors for no-load on-off circuits, as well as for directly controlling lighting and power circuits not exceeding 5.5kW. Fuses are mainly used for short-circuit protection of circuits and can also be used as power disconnectors.