The most common categories of grounding for electrical equipment

At present, the common grounding classifications are as follows: repeated grounding, protective grounding, working grounding, lightning protection grounding, shielding grounding, and anti-static grounding.

1 Repeat grounding

Repeated grounding: In a system with the neutral point directly grounded, a metal wire is used to connect the grounding device at one or more places of the neutral line. In a low-voltage three-phase four-wire neutral point directly grounded line, the construction unit shall, during installation, The terminals of the neutral trunk and branch lines of the distribution lines should be grounded, and grounding should be done every 1 km on the neutral trunk.

For distribution lines that are more than 50 meters away from the grounding point, the neutral line connected to the user should still be grounded repeatedly, and the repeated grounding resistance should not be greater than 10 ohms.

2 Protective grounding

Under normal circumstances, the uncharged metal shell and metal bracket of electrical equipment are electrically connected to the earth, which is called protective grounding. Protective grounding is mainly used in power supply systems where the neutral point is not grounded.

If the protective grounding measures are not adopted, when the human body touches the charged shell, a loop is formed due to the distributed capacitance between the transmission line and the earth, so that the human body has current passing through and an electric shock accident occurs.

If the electrical equipment adopts protective grounding measures, then when the human body touches the live shell, the human body is connected in parallel with the resistance of the protective grounding device. Since the ground resistance is smaller than the resistance of the human body, it can be considered that the current passing through the human body is very small at this time, and the current almost does not pass through the human body, thus avoiding electric shock accidents.

3 working ground

The grounding network diagram is designed to enable the system and the instruments connected to it to operate reliably and ensure measurement and control accuracy. It is divided into machine logic ground, signal loop ground, shielding ground, and intrinsically safe ground in petrochemical and other explosion-proof systems.

4 lightning protection grounding

Lightning protection grounding is part of the lightning protection measures, and its function is to introduce lightning current into the earth.

Lightning protection for buildings and electrical equipment mainly uses lightning arresters (including lightning rods, lightning protection belts, lightning protection nets and lightning elimination devices, etc.).

One end of the arrester is connected to the protected equipment, and the other end is connected to the ground device. When a direct lightning strike occurs, the lightning arrester leads the lightning to itself, and the lightning current enters the earth through its down conductor and grounding device.

In addition, due to the side effect of electrostatic induction caused by lightning, in order to prevent indirect damage, such as house fire or electric shock, etc., it is usually necessary to ground the metal equipment, metal pipes and steel structures in the building; The TV antenna invades the house, causing insulation breakdown of the electrical equipment in the house, resulting in fire or personal electric shock casualties, so it is necessary to ground the line and the iron feet of the insulating porcelain bottle before entering the house.

5 Shield grounding

It is an effective measure to eliminate the harm of electromagnetic fields to the human body and an effective measure to prevent electromagnetic interference. High-frequency technology has been widely used in electric heating, medical treatment, radio broadcasting, communications, television stations, navigation, radar, etc. Under the action of the electromagnetic field, the absorbed radiation energy will have biological effects and cause harm to the human body, such as slight trembling of fingers, skin scratches, vision loss, etc. Install shielding devices on the equipment casings that generate magnetic fields, and ground the shielding body, which can not only reduce the electromagnetic field strength outside the shielding body, reduce or eliminate the harm of electromagnetic fields to the human body, but also protect the equipment in the shielding grounding body from external electromagnetic fields. Interfering effects.

6 Anti-static grounding

In order to prevent the harmful effects of static electricity and discharge it, it is the most important part of electrostatic protection.

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