Grounding Terminal Testing

Grounding terminal inspection mainly conducts scientific testing and analysis on the grounding terminal connection resistance testing, grounding terminal continuity testing, grounding terminal mechanical strength testing, grounding terminal temperature rise testing, grounding terminal corrosion resistance testing, and grounding terminal marking and structure inspection, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Grounding terminal connection resistance testing: Measuring the contact resistances between the grounding terminals and the grounding conductors or grounding bodies is the core indicator of measuring the reliability of the electrical connections. The excessively large connection resistances will cause the impedance increase of the grounding paths, affecting the fault current dissipation, and possibly causing the electrical fires or equipment damages. The testing is usually carried out under the full loads or specific currents, to simulate the actual working conditions.

Grounding terminal continuity testing: Verifying whether there are the continuous and low-impedance electrical paths between the grounding terminals and the metal housings and structural components of the grounded equipment. This testing aims to ensure the integrity of the grounding loops, preventing the equipment housings from being energized due to the open circuits or false connections. It is the basic item of ensuring the personal safety.

Grounding terminal mechanical strength testing: Evaluating the mechanical properties of the terminals and their fixing devices, including the tensile strengths, torque resistance and loosening resistance. Through the application of the specified tensile forces or torques, whether the terminals can withstand the mechanical stresses during the installation, use and maintenance is tested, ensuring the long-term firmness and stability of the connections.

Grounding terminal temperature rise testing: Under the conditions that the terminals pass the rated working currents or fault currents, the temperature rise values of their contact parts are measured. The excessively high temperature rises are the typical manifestations of the poor contact, oxidation or overload, directly affecting the safety and lives of the connections. The testing needs to be carried out at the specific ambient temperatures, and the determination is made with reference to the relevant standard limits.

Grounding terminal corrosion resistance testing: Evaluating the property retention capabilities of the terminals in the corrosive environments such as the salt spray, heat and humidity and industrial atmospheres. Usually adopting the salt spray testing, sulfur dioxide testing and other methods, checking whether the terminal metal materials and coatings have the rust and powdering, ensuring their long-term effectiveness in the harsh environments.

Grounding terminal marking and structure inspection: The visual and dimension measurement inspection of the permanent markings (such as the grounding symbols), dimension specifications, material types and structural designs of the terminals. Ensuring that they meet the standard provisions, convenient for the correct identification, installation and maintenance, and preventing the misuse.

Testing Scope

Grounding terminals for the low-voltage power distribution systems: Covering the grounding terminals in the civil, commercial and industrial low-voltage power distribution cabinets, distribution boxes, sockets, switches and household appliances. The testing needs to comply with the standards such as GB/T 7251 and GB 16917, emphatically verifying their protective grounding continuity in the TN, TT and IT systems.

Metal housing grounding terminals of the electrical equipment: Including the dedicated grounding screws or terminals on the housings of the motors, transformers, frequency converters, control cabinets and the various industrial equipment. The testing ensures that when the insulation fails, the fault currents can be safely led into the earth through the terminals, preventing the electric shocks.

Lightning protection and overvoltage protection grounding terminals: Applied to the lightning protection devices such as the surge protective devices (SPD) and lightning rod down conductors. The testing requirements are extremely high, needing to verify their thermal and mechanical stability when dissipating the lightning currents or large surge currents, preventing the connection point melting or bursting.

Grounding terminals for the information technology and communication equipment: Involving the grounding terminals of the server cabinets, communication base stations, network equipment and shielded cables. In addition to the safety grounding, their effects on the signal reference grounds, shield grounding and electrostatic dissipation are also concerned, with the stricter requirements for the connection resistances and electromagnetic compatibility.

Grounding terminals for the rail transit and vehicles: For the grounding devices of the vehicle bodies and electrical systems of the high-speed railways, subways and electric vehicles. The testing needs to consider the high vibration, impact and complex electromagnetic environments, ensuring the high reliability of the grounding systems in the dynamic operation.

Building steel structures and equipotential bonding terminals: The outgoing terminals of the building foundation grounding bodies, grounding busbars and connection terminals in the equipotential bonding boxes. The testing verifies their connection qualities as the building grounding backbone networks, guaranteeing the effectiveness of the total equipotential and local equipotential bonding.

Testing Methods

DC low resistance testing method: The micro-ohmmeters or DC resistance testers are used to apply the constant DC currents (usually 10A-100A) between the grounding terminals and the connected conductors, and the four-wire method is used to measure the voltage drops at their two ends, calculating the precise connection resistance values. This method can effectively eliminate the effects of the lead resistances, and is the mainstream method of measuring the milli-ohm level resistances.

Loop impedance testing method: The grounding loop impedance testers are used. At the equipment installation points, the power frequency or specific frequency testing currents are injected into the grounding grids through the terminals, measuring the impedances of the whole grounding loops including the grounding wires, terminals and grounding bodies. This method can comprehensively evaluate the on-site effectiveness of the grounding paths.

Torque-resistance relationship testing method: When tightening the grounding terminal connection bolts, the calibrated torque wrenches are used to apply a series of increasing torques, and the connection resistances are measured at each torque point. Through the plotting of the torque-resistance curves, the optimal tightening torque ranges are determined, avoiding the poor connections caused by the insufficient or excessive torques.

Large current temperature rise testing method: According to the standards such as GB/T 14048.1, under the laboratory conditions, the tested terminals pass 1.1-1.2 times the rated currents until the temperatures reach the stability. The thermocouples or infrared thermal imagers are used to precisely measure the temperature rises of the terminal key contact points, judging whether their current-carrying capabilities meet the requirements.

Salt spray testing method: The grounding terminal samples are placed in the salt spray test chambers. According to the GB/T 10125 standard, the sodium chloride solutions of the specific concentrations are continuously sprayed, simulating the marine or industrial atmospheric environments. After the specified periods (such as 48 hours and 96 hours), they are taken out to check their appearance corrosion grades and electrical property changes.

Vibration and mechanical shock testing method: For the grounding terminals used in the mobile equipment such as the vehicles and aerospace, they need to be installed on the vibration tables, simulating the vibration spectra in the actual operation for the durability testing. After the testing, their electrical connection resistances and mechanical fastening states are re-measured, evaluating their vibration resistance.

Testing Standards

Domestic Standards:

1. GB/T 7251, Low-voltage switchgear and controlgear assemblies

2. GB 16917, Residual current operated circuit-breakers with integral overcurrent protection for household and similar uses (RCBO)

3. GB/T 14048.1, Low-voltage switchgear and controlgear — Part 1: General rules

4. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

1. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Micro-ohmmeter/DC low resistance tester: The core instrument, used to precisely measure the connection resistances of the milli-ohm to micro-ohm levels. With the four-terminal measurement functions to eliminate the errors, the testing currents are adjustable (usually above 1A-100A), and some high-end models integrate the data storage and temperature compensation functions, suitable for the laboratory and on-site testing.

Grounding loop resistance tester: Used to measure the complete grounding loop impedances including the grounding terminals on-site. Usually adopting the "clamp method" or "fall-of-potential method", the testing can be carried out without disconnecting the grounding wires. The operation is convenient, and it is the common equipment of the electrical safety inspection.

Digital torque wrench and screwdriver: Used to apply the precise and traceable tightening torques to the bolted grounding terminals. Their reading display functions ensure that the installation processes comply with the torque values specified by the standards. They are the key tools of realizing the reliable mechanical connections and used for the torque-resistance relationship research.

Infrared thermal imager and thermocouple temperature measurement system: Used for the non-contact or contact measurement of the temperature distributions and temperature rises of the terminals in the energized states. The infrared thermal imagers can quickly scan and find the local overheating points, while the thermocouples can provide the more precise single-point temperature data, often used for the temperature rise testing and fault diagnosis.

Salt spray corrosion test chamber: The dedicated equipment of artificially simulating the corrosive environments. It can precisely control the chamber temperatures, salt spray deposition rates, solution concentrations and testing periods, providing the standardized accelerated testing conditions for evaluating the corrosion resistance of the grounding terminal materials and coatings.

Vibration test table and impact testing machine: Used to assess the performance of the grounding terminals under the dynamic mechanical stresses. The vibration tables can simulate the continuous vibrations of the different frequencies and amplitudes, while the impact testing machines simulate the instantaneous shocks. The combination of the two can comprehensively evaluate the mechanical reliability of the terminals under the harsh working conditions.