The basic purposes of grounding and neutral connection are twofold: first, grounding is required according to the working requirements of the circuit; second, grounding or neutral connection is necessary to ensure the safety of personnel and equipment. Based on their function, they can be divided into four types: A. Working grounding; b. Protective grounding; c. Protective neutral connection; d. Repeated grounding.
01
Working ground
In a low-voltage power system using 380/220V, four wires are typically drawn from the power transformer: three phase wires and one neutral wire. These four wires serve both power and lighting purposes. Power is provided by the three phase wires, while lighting uses one phase wire and the neutral wire. In such a low-voltage system, to ensure reliable operation of electrical equipment under both normal and fault conditions, and to protect personnel and equipment safety, the system's neutral point is generally directly grounded, which is called the working ground. The wire drawn from the three coils of the transformer is also called the neutral wire, or zero wire, and this point is called the neutral point.
1.1 Function of working ground
The function of working grounding is twofold: firstly, to reduce the danger of a single-phase grounding; and secondly, to stabilize the system potential, limit the voltage to a certain range, and reduce the risk of high voltage entering low voltage.
02
Protective grounding
Protective grounding is a grounding system implemented to prevent the metal casings of electrical installations, the frames of power distribution equipment, and power line towers from becoming energized and endangering personal and equipment safety. Protective grounding involves reliably connecting the metal parts of electrical appliances (i.e., metal structural parts insulated from energized parts) that are normally not energized but may become energized due to insulation damage or other circumstances, to a grounding electrode using a conductor.
2.1 The application scope of protective grounding:
Protective grounding applies to ungrounded power grids. In such grids, regardless of the environment, any metallic parts that may present dangerous voltages due to insulation failure or other reasons should be protected by grounding measures, unless otherwise specified. These measures mainly include:
(1) Metal casings, bases and transmission devices connected thereto of motors, transformers, switchgear, lighting fixtures and other electrical equipment;
(2) Metal or reinforced concrete frame of indoor and outdoor power distribution equipment, and metal barriers or fences near live parts;
(3) The metal frame or shell of the power distribution panel, control panel, protection panel and power distribution cabinet (box);
(4) The metal casing of the cable junction box, the metal sheath of the cable, and the steel conduit for wiring;
In addition, metal and reinforced concrete poles of certain overhead power lines, as well as the secondary coils of instrument transformers, should also be grounded.
03
Protective grounding
Protective grounding involves directly connecting the normally non-energized metal parts of equipment to the system using a wire. This method ensures personal safety and prevents electric shock accidents.
3.1 Working principle of protective grounding
Connecting the metal casing of electrical equipment to the neutral wire of the power grid is a safety measure to protect personal safety. In a neutral-grounded power grid with a voltage below 1000 volts, if the metal casing of electrical equipment becomes energized due to insulation damage or an accident, forming a single-phase short circuit between the phase line and the neutral line, the protective device on the line (automatic switch or fuse) will quickly trip, cutting off the power supply. This prevents the metal parts of the equipment from being exposed to dangerous voltage for an extended period, thus ensuring personal safety. In multi-phase AC power systems, the neutral point of a star-connected winding is directly grounded to make it equipotential with the earth, i.e., zero potential. The conductor drawn from the grounded neutral point is called the neutral wire. It is not permissible for some equipment powered by the same power source to use protective neutral grounding while other equipment uses protective grounding (see Grounding). When the protective grounding equipment casing is energized, if its grounding resistance r′D is large, the fault current ID is insufficient to make the protection device operate. Then, due to the existence of the working resistance rD, there will always be a voltage U0=IDrD on the neutral line. At this time, there will be a dangerous voltage U0 on the casing of the protective grounding equipment for a long time, which will endanger personal safety.
04
The difference between functional grounding and protective neutral grounding
Functional grounding is used to discharge residual electricity generated during operation into the earth to prevent electric shock. Any grounding required for equipment operation is called functional grounding. Without it, the equipment cannot operate. For example, the neutral point grounding of a transformer.
The protective neutral wire, also known as the ground wire, is the wire that allows the residual current circuit breaker (RCCB) to trip promptly when it comes into contact with an object, preventing injury.
Both wiring methods play an important role in protecting personal safety.
05
The difference between protective grounding and protective neutral grounding
5.1 Different Principles
Protective grounding limits the voltage to ground after equipment leakage, ensuring it does not exceed a safe range. In high-voltage systems, besides limiting the voltage to ground, protective grounding can also, in certain situations, trigger the operation of power grid protection devices. Protective neutral grounding uses a neutral line to create a single-phase short circuit due to equipment leakage, causing the protection devices on the line to operate and disconnecting the power supply to the faulty equipment. Furthermore, in a protective neutral grounding network, the protective neutral line and repeated grounding can also limit the voltage to ground when equipment leaks current.
5.2 Different Scope of Application
Protective grounding is applicable to both ungrounded high and low voltage power grids, as well as low voltage power grids with other safety measures (such as the installation of leakage current protection devices); protective neutral grounding is only applicable to low voltage power grids with a directly grounded neutral point.
5.3 Different line structures
If protective grounding measures are adopted, there is no working neutral wire in the power grid; only a protective grounding wire is required. If protective neutral grounding measures are adopted, a working neutral wire must be installed and used for neutral grounding protection. Switches and fuses should not be connected to the protective neutral grounding wire. When fuses or other switching devices are installed on the working neutral grounding wire, a separate protective grounding wire or neutral grounding wire must also be installed.
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