How to choose a grounding wire What you need to pay attention to when choosing a grounding wire

Choosing the right grounding wire is a critical step in ensuring the safety of electrical equipment and systems. The correct selection and use of grounding wire can effectively prevent electrical equipment leakage or electrical faults caused by electric shock, fire and other hazards. The grounding wire is mainly used to connect the shell and conductor of electrical equipment or system with the earth to ensure that the current can flow smoothly into the ground, so as to protect the personnel and equipment safety.

1. Ground wire selection points

a. Ground wire conductive material

Copper wire: copper has a low resistance and excellent conductivity, so copper wire is a common grounding wire material.

Galvanised steel wire: If the cost of copper wire is high or the requirement of anti-mechanical strength is high, galvanised steel wire can be used. Galvanised steel wire is slightly less conductive than copper, but its tensile and corrosion resistance is stronger, commonly used in underground grounding systems.

Aluminium wire: Aluminium wire is also an economical choice, although the conductivity of aluminium is not as good as copper, but it is lighter and cheaper than copper, often used for low current grounding requirements.

b. Cross-sectional area of grounding wire

The cross-sectional area of a grounding wire is directly related to its conductivity. According to international standards (e.g., IEC 60364, IEEE 80, etc.) and relevant Chinese codes (e.g., Design Code for Electrical Installation of Buildings GB 50303-2019), the following factors should be considered when selecting a grounding wire:

Current strength: the earthing wire needs to be able to carry the maximum possible fault current and quickly lead it to ground. Generally speaking, the cross-sectional area of the grounding wire should be not less than the cross-sectional area of the protective conductor, and occasions with large fault currents require the use of grounding wires with larger cross-sections.

Current rating of the electrical system: For high voltage equipment or power systems, the cross-sectional area of the grounding wire usually needs to be larger. Common grounding conductors range from 4mm² to 150mm², depending on the rated current of the electrical equipment and the environmental conditions.

Safety margin: The cross section of the earthing conductor should have a sufficient safety margin to ensure that no excessive current overload occurs in the event of a short circuit or fault.

c. Length of grounding wire

The length of the grounding wire should be as short as possible to minimise resistance and current loss. Longer grounding wires may result in increased grounding resistance, reduced grounding effectiveness, and may not even be effective in directing fault currents.

The use of bent or twisted grounding wires should be avoided in the design as much as possible, as these may increase the grounding resistance and reduce the grounding effectiveness.

d. Insulation of the grounding wire

Uninsulated wire: In most cases, an earth wire does not need to be insulated because it is connected directly to the ground or to earth through an earth electrode. However, the grounding wire needs to be sufficiently resistant to corrosion.

External environment: If the grounding wire is installed in a place where it may be subject to mechanical damage or corrosion from the external environment (e.g. underground, wet environment or chemically corrosive environment), a grounding wire with a corrosion-resistant outer protection layer is required.

e. Corrosion resistance of the grounding wire

Grounding wire installation environment has high requirements for its material, especially in corrosive environments (such as groundwater or chemically corrosive environment), grounding wire may be damaged. Selection should take into account its corrosion resistance. Especially for grounding wires exposed to air for a long time, galvanised steel wires or copper wires coated with anticorrosive coating should be used.

Anti-corrosion measures: The service life of the grounding wire can be improved by the addition of protective sleeving, the use of corrosion-resistant materials, or regular maintenance.

f. Grounding resistance requirements

Grounding resistance value: the requirement of grounding resistance value is usually determined by national standards, industry standards or project-specific requirements. Under normal conditions, grounding resistance should be as low as possible. Common standard requirements are:

For low voltage equipment, the general grounding resistance value should be less than 4 ohms.

For high voltage equipment, the grounding resistance value may need to be lower, generally requiring no more than 1 ohm.

Grounding system design: In order to reduce the grounding resistance, the grounding system design should be a reasonable choice of the type of grounding electrode and installation. Common grounding electrodes include grounding rods, grounding nets, and grounding plates.

2. Precautions for grounding wire selection

a. System Electrical Characteristics

When selecting a grounding wire, it is necessary to determine the proper size based on the electrical characteristics of the system. For example, in substations, high-voltage equipment and industrial control systems, grounding wire sizes are usually larger than those for systems in general residential or commercial buildings.

The more stringent grounding requirements of high-voltage electrical equipment systems require the use of a larger cross-section of grounding wire to ensure that it is capable of carrying the fault currents that may occur.

b. Fault Current Estimation

When designing the earthing system, it is necessary to consider the maximum fault current that may occur and ensure that the earthing conductor is able to conduct the fault current into the ground in the shortest possible time, thus effectively protecting the electrical equipment and personnel safety.

c. Connection Method

Connection of grounding wire and grounding electrode: the grounding wire must be firmly connected to the grounding electrode, and the joint should have good conductivity and mechanical strength. The selection of grounding electrode should be determined according to the local soil resistivity and other factors.

Connection between grounding wire and equipment: when the grounding wire is connected to the equipment casing, make sure that the connection point is clean, free of oil and dirt, and connected by bolts, welding and other reliable ways.

d. Environmental factors

For grounding wires installed in humid, corrosion-prone environments or high-temperature environments, special attention should be paid to the material's corrosion resistance, oxidation resistance and high-temperature resistance.

When installing, make sure that the grounding wire is not subject to external mechanical damage to avoid loosening or breaking of the grounding wire due to external force.

e. Regular Inspection and Maintenance

Regularly check the integrity of the grounding wire and grounding system to ensure that the grounding resistance meets the standard requirements. If the grounding resistance increases or the grounding wire is damaged, it should be replaced or maintained in a timely manner.

3. Summary

When selecting an earth wire, it is important to consider the material, cross-sectional area, length, environment and other factors. The appropriate grounding wire should have good electrical conductivity, adequate mechanical strength and corrosion resistance, to ensure the safety of the electrical system. In addition, regular inspection and maintenance of the grounding wire is also the key to ensure the long-term stability of the grounding system.