A, low-voltage grid grounding: 1, TT mode, the first letter T that the neutral point of the low-voltage power system grounding, the second letter T that the shell of the power equipment grounding, the system in addition to the neutral point of grounding outside the work of the zero line is not allowed to be grounded again, both our common "protective grounding. In accordance with the requirements of the regulations, the neutral point and equipment shell grounding resistance ≤ 4Ω. 2, TN mode, the first letter T indicates that the neutral point of the low-voltage power system grounding, the second letter N indicates that the shell of the electrical equipment connected to the zero line, which is common to us, "protective grounding. 3, IT mode, the first letter I indicates that the low-voltage power system neutral point of insulation to ground, the second letter T indicates that the shell of the electrical equipment grounded. This method is suitable for continuous uninterrupted power supply requirements are very high places of electricity, such as medical units in the process of surgery and mine drainage ventilation system and other places, these places of electricity do not allow an electrical equipment insulation failure and automatically cut off the power supply of the entire system. In the TT method, if the human body touches the phase line or the insulation of the electrical equipment is poor, resulting in the shell charged, the current will flow through the human body or the shell of the electrical equipment into the earth, and then back to the neutral point of the distribution transformer (when there is no second grounding point in the system), forming a closed loop. The current will cause injury when passing through the human body, and the grounding system is prone to leakage and fire. In the event of a breakdown short-circuit between the low-voltage windings of a low-voltage distribution transformer, the voltage to ground on the low-voltage side is all phase voltage due to the neutral point grounding. Comparatively speaking, the neutral point direct grounding operation mode is safer for electrical equipment and operation, and is suitable for large-capacity low-voltage power grids. This way to facilitate the installation of current-type leakage protector, and can be used to protect the total protection, split protection and terminal direct protection, improve the safety management of low-voltage power grid.
Two, leakage protection detection principle: any low-voltage lines, there is a leakage current to ground. The main reason for the generation of leakage current lies in the insulation resistance and distributed capacitance between the charged body and the earth. In the low-voltage grid TT grounding method, the phase line to the earth leakage, with the zero sequence current transformer detection is now a commonly used method. Zero sequence current transformer has high detection sensitivity, good transmission characteristics and other characteristics. At present, its iron core is generally used in the most advanced, coercive force is very small soft magnetic material - Po Mo alloy, such as; 1J85 and other models.
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