What are protective grounding, protective neutral grounding, repeated grounding, and working grounding?

Protective grounding: Grounding carried out to prevent the metal enclosures of electrical equipment, the frames of distribution installations, and utility poles from becoming charged, thereby protecting both people and equipment from danger. Protective zero connection: A electrical safety measure that ensures the reliable connection between the metal casing of electrical equipment and the neutral wire in the power grid, thereby protecting human safety. Repeated grounding: In a system where the neutral point is directly grounded, metal wires are used to connect grounding devices at one or more points along the neutral conductor. Working ground: In TN-C and TN-C-S systems, it is the grounding that enables circuits or equipment to meet the requirements for proper operation, such as the grounding of the transformer neutral point. This grounding is referred to as working ground or distribution system ground. Protective grounding is a form of grounding carried out to prevent the metal enclosures of electrical devices, the frames of power distribution installations, and utility poles from becoming charged, thereby protecting both people and equipment from potential hazards. Protective grounding refers to a protection wiring method in which the metal parts of electrical equipment that are normally not charged, but may become charged in the event of damage to the insulating materials or under other circumstances (i.e., the metal structural parts that are insulated from the live parts), are reliably connected to a grounding electrode using wires. Grounding protection is generally used in power supply systems where the neutral point of the distribution transformer is not directly grounded (three-phase three-wire system), to ensure that the voltage relative to ground generated when electrical equipment leaks current due to insulation damage does not exceed safe levels.

This post was last edited by Zhaixingnongyue on March 28, 2011, at 15:38. It is an electrical safety measure that involves reliably connecting the metal enclosures of electrical equipment to the neutral wire of the power grid, thereby protecting personal safety. In a neutral-connected power grid where the voltage is below 1000 volts for protection against ground faults, if the metal casing of an electrical device becomes charged due to insulation damage or some other accident, resulting in a single-phase short circuit between the phase wire and the neutral wire, the protective devices installed in the circuit (such as automatic switches or fuses) will activate immediately to cut off the power supply. This prevents the metal parts of the electrical device from being exposed to dangerous voltages for an extended period, thereby ensuring personal safety. On electrical equipment powered by the same power supply, it is not permitted for some of the equipment to use protective neutral connection while another part uses protective grounding (see Grounding). This is because when the enclosure of a device connected to protective ground becomes charged (as shown in Figure 1 at B), if the resistance of the protective grounding connection is high, the fault current ID is not sufficient to activate the protective device. As a result of the presence of the resistance rD of the working ground connection, a voltage U0 = IDrD remains present on the neutral wire. At this point, a voltage U0 will appear on the enclosure of the device connected to protective neutral grounding (as shown in Figure 1 at A), and contact with this enclosure can lead to electric shock. Protective neutral connection: In multi-phase AC power systems, the neutral point of the star-connected windings is connected directly to the ground, making its potential equal to that of the earth; this is known as zero potential. The wire leading from the grounded neutral point is called the neutral wire. In China, it is stipulated that 380-volt AC power supplies are generally connected in a three-phase star configuration, with the neutral point directly grounded. Therefore, the neutral wire is the zero wire. In power grids that use protective neutral grounding, the neutral wire must be repeatedly grounded as specified, to prevent the risk of electric shock resulting from the energized state of the grounded enclosures of electrical equipment in the event of a break in the neutral wire. 　　Repeated grounding is a measure that involves installing grounding devices on the neutral wire at or near the points where the power grid is grounded. In the absence of redundant grounding, when the neutral wire breaks accidentally (Figure 2), any device connected to the protective ground system will have its casing become charged due to insulation damage. This voltage is transmitted through the neutral wire to the casings of all devices connected to the ground system, and thus operators who come into contact with the casings of any of these devices are at risk. With a repeated grounding device (Figure 3), a grounding current ID is generated when the above situation occurs. If the wire resistance of the phase conductor and the neutral conductor is ignored, the grounding current is equal to the voltage on the neutral conductor after a break. By appropriately arranging the location and number of repeated grounding devices to keep r sufficiently small, the voltage U0 appearing on the neutral conductor can be made lower than the safe voltage level. Repeat grounding is generally installed at electrical equipment with large capacity, at the branching points of circuits, and at the ends of circuits. Protective earth connection: Modern large high-voltage testing laboratories are often designed to be fully shielded, forming a **Faraday cage**. The cage is connected to the ground electrode only at one point. At this point, the current in the test system, including that due to stray capacitance, flows through the shielding cage to form a circuit; the grounding electrode merely serves to maintain a constant potential, with no current flowing through it. Therefore, no overly high requirements need to be placed on the grounding impedance of fully shielded high-voltage test laboratories; the requirements for lightning protection grounding in ordinary buildings are sufficient. (See color illustration) When performing impulse voltage (or current) tests in a high-voltage testing laboratory, due to the rapid changes in the voltage (or current) waveform and the large discharge current, current flows through the outer sheath of the measurement cables connected to the grounding electrode. The noise voltage generated by this current is added to the signal being measured, resulting in common-mode interference. To reduce this interference, the grounding of the impact measurement system should follow these principles: ① If no grounding grid is installed underground during construction, metal plates can be laid on the ground as necessary to connect the grounding terminals of various testing devices together. ②The area enclosed between the measurement cable and the ground grid should be as small as possible. When a ground grid is present, the measurement cable should be routed close to the lower side of the ground grid. Otherwise, a metal plate or metal strip can be laid between the voltage divider and the oscilloscope chamber, with the measurement cable passing beneath it. ③The impedance of the grounding wire of the voltage divider should be as low as possible; generally, a copper strip about 200 millimeters wide is used, and its length should be as short as possible. The voltage divider should be placed near the point where the grounding electrode emerges.