1, the distribution box shell is actually zero not grounded, because the distribution box is equipped with electrical equipment such as insulation faults, the shell will have a high potential, the human body contact with the distribution box there is a risk of electric shock, so zero, when the failure to ensure that the shell of the distribution box for the zero potential, while generating a short-circuit current so that the upper switch tripped the blackout.
2, the electrical equipment in the distribution box is not likely to fail and make the shell with a voltage that endangers personal safety, such as the internal installation is completely isolated from the system and safe voltage equipment, then the distribution box shell can not be connected to zero.
3, the shell of the electrical equipment and transformer center point of the zero line connected together called protection zero.
4, the role of grounding zero is when a phase of insulation damage to the phase line touching the shell, the fault phase conductor, equipment shell, zero line and transformer windings to form a closed loop, the short-circuit current is sufficient to make the line on the action of the protective device.
In the event of a short-circuit fault shell, even if the human body touches the shell, the current flowing through the human body due to its own resistance and shunt small, but also to ensure personal safety.
In order to improve the degree of safety, but also must be used to repeat the grounding, the so-called repeat grounding refers to the system in addition to the neutral point in the work of grounding, in the neutral line of one or more places to repeat the contact with the earth, when the occurrence of touching the shell or grounding short circuit, you can reduce the neutral line voltage to ground, to play the role of potential equalization.
Expanded Information:
Protection Zeroing p>Protection zero considerations
1, the conditions of using protection zero. In the actual operation process, if the power neutral point is well grounded, and the zero line can operate reliably, then you can use the protection of zero way to deal with. In the work of grounding, the system must be reliable, and grounding resistance is less than 4 ohms.
2, the work of the zero line repeated grounding. In the work, for the work of the zero line circuit, in order to avoid the phenomenon of disconnection, on the one hand, the neutral grounding, on the other hand, the work of the zero line to repeat the grounding process.
3, the cross sectional area of the zero line shall not be less than one half of the phase line. In the grid system, the zero line is usually not charged, or the current is very small, except for single-phase loads, compared with the phase line, so the cross-section of the zero line is relatively small.
4, the equipment of the protective zero line and the work of the zero line to be firmly connected, wire in the actual use of the process, only to connect firmly, the contact between the wire to ensure good sex.
5, single-phase load line shall not be borrowed working zero line instead of protective zero line. For sockets connected to the power supply on the zero line of the hole, in the process of connecting the three-eye socket, not allowed to protect the rest of the zero line of the hole in series processing, that is, not to borrow the work of zero line instead of the protective zero line.
6, in the same low-voltage power grid, protective grounding and protective zero can not be mixed. Otherwise, if the grounding equipment fails, the zero line potential will be raised. For the voltage, the contact voltage is comparable to the phase voltage, making the risk of electrocution further increase.
7, in the choice, adjust the rated current of the protection equipment, must strictly comply with the safety requirements. Zero protection from the essence, that is, when the electric equipment leakage accidents, through the zero line can form a circuit, so that the leakage current further increase, by increasing the current to a certain extent to stimulate the protection device to cut off the power supply.
8, the use of protective zero for the management of electrical equipment, and can not completely prevent electric shock. Appliance shell and power supply firewire connection triggered by serious faults, through the protection of zero can be avoided, if the shell of the electrical appliances triggered by leakage faults, through the protection of zero can not be ruled out, in order to eliminate the shell of the electrical appliances leakage faults, the need to cooperate with other protective measures.
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