Protective grounding and protection zero

Update date:2018-07-04 Source:MAXGE

Protective grounding and protection zero

 For a different purpose, a certain part of the electrical device is electrically connected to the earth through a grounding wire or a grounding body. Working grounding refers to grounding a certain point of the power system or equipment for operational needs, such as direct neutral grounding of the transformer; protective grounding refers to grounding a certain point of electrical equipment to prevent personal electric shock, such as metal of electrical equipment. The case is grounded, etc.

Under the premise that the electrical equipment is not grounded, consider the grounding of the neutral point of the power supply and the grounding of the grounding.(3 phase rcd)

1. The danger that the electrical equipment is not grounded in the power grid where the neutral point of the power supply is directly grounded.

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As shown in the figure, when the insulation of the electrical equipment is abnormal, the outer casing is electrically charged, and when the human body touches the outer casing of the device, the path of current flowing through the human body is: equipment casing->human body->earth->power neutral point. Assuming that the body resistance RB is 1700 Ω and the grounding resistance R0 is 4 Ω, the current flowing through the human body is:

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Beyond the safe range, causing personal injury.

2. The danger that the electrical equipment is not grounded in the power grid where the neutral point of the power supply is not directly grounded.

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If the electrical equipment fails, the outer casing is electrically connected, the human body touches the outer casing of the equipment, and the path of current flowing through the human body is: equipment casing->human body-> other two-phase line-to-ground distributed capacitance-> other two-phase power supply. The current flowing through the human body at this time is:

The value depends on the cable's resistance to ground. In general, this situation may result in personal electric shock.

Under the premise of grounding electrical equipment, consider the grounding of the neutral point of the power supply and the grounding of the grounding.(magnetic circuit breaker)

1. TT system: The neutral point of the power supply is directly grounded, and the metal casing of the electrical equipment is directly grounded by the three-phase four-wire low-voltage power distribution system through the respective protection line PE.

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When the electrical equipment casing is energized, it is assumed that the grounding resistance RE=R0=4Ω and the human body resistance RB=1700Ω. At this time, the fault current in the power line is:

 

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Even if a circuit breaker is installed in the line, it is usually not enough to play the overload protection function of the circuit breaker, so the power line is not cut off, which will cause the electrical equipment casing to have a voltage of 110V to ground for a long time. When the human body touches the charged casing, it flows through the human body. The current is:

 6.jpg

Can cause personal injury.

2. IT system: The three-phase three-wire low-voltage power distribution system in which the neutral point of the power supply is not grounded, and the metal casing of the electrical equipment is directly grounded via the respective protection line PE.

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In an IT system, when insulation damage of an electrical device causes the outer casing to be charged and the human body touches the outer casing, current flows from the grounding body and the human body at the same time. Since the resistance of the human body is much larger than the grounding resistance Re, the current flowing through the human body is much smaller than the current flowing through the grounding body. At this time, the grounding protection of the electrical equipment acts to protect personal safety.

In summary, the protective grounding is mainly applied to the grid where the neutral point is not grounded (IT system), and the working principle is the shunting action of the resistor in the parallel circuit. Therefore, the value of the grounding resistance is critical to the protection effect.


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