Contact resistance is the core parameter for evaluating the quality of the conductive circuit connections in electrical equipment such as high-voltage switches, circuit breakers, and disconnectors. An abnormal increase in contact resistance indicates insufficient contact pressure, surface oxidation, or loose defects. Over a long period of continuous power supply, the contact points will heat up more, potentially leading to contact welding or even failure. Therefore, the measurement of circuit resistance is not merely a “measurement of a number”, but a crucial diagnostic method for exposing hidden contact defects before a fault occurs.
However, the accurate measurement of contact resistance is not an easy task. If the measurement is conducted using a milliampere-level small current with a common double-arm bridge, the presence of oxide films on the contact surfaces will cause the measured value to be several times larger, masking the true value of the circuit resistance. This requires the measuring instrument to be capable of outputting a sufficiently large direct current to break through the oxide films and obtain accurate, repeatable contact resistance data. A large current contact resistance tester, also known as a circuit resistance tester or a large current micro-ohmmeter, is a specialized device designed based on this engineering requirement.
Core requirements of the test procedure
The design, manufacture, and selection of large current contact resistance testers must strictly follow relevant standards and regulations of the power industry. The most core provisions come from the following aspects:
(1) Mandatory requirements for test current
National standards and the “Electrical Equipment Preventive Test Procedures” of the power industry clearly stipulate that the measurement of the conductive circuit resistance of circuit breakers should adopt the direct current drop method, and the test current must not be less than 100A.
The physical basis of this regulation is that contact resistance consists of contraction resistance and film resistance. The presence of oxide films causes the resistance value measured at a small current to deviate significantly from the true value. Only by applying a sufficiently large current can the oxide film be broken through, obtaining the contact resistance value that reflects the actual operating conditions.
(2) Technical specifications for measurement principle
The procedure requires the use of the direct current drop method (four-wire measurement) for circuit resistance measurement. The four-wire method, also known as the Kelvin connection method, outputs a constant direct current test current through two current wires and measures the voltage drop through two voltage wires, effectively eliminating the influence of test wire resistance and contact resistance on the measurement results. Modern circuit resistance testers generally adopt this principle, calculating the resistance value based on Ohm’s Law R = U/I.
(3) General technical conditions for equipment performance
Current technical standards set quantitative requirements for the technical performance of testing equipment. The measurement error is typically required to be better than ±(0.5% reading + several digits), and maintain extremely high repeatability under power frequency interference. In addition, the instrument must have the ability to continuously output large currents for a long time, overcoming the limitations of “pulse-type” power supplies that cannot work continuously and cannot effectively break through oxide films.
Selection basis: Key parameters and technical considerations
Based on the above procedure requirements, the selection of large current contact resistance testers should focus on the following key parameters:
(1) Selection of test current level
The configuration of the test current must match the rated parameters of the equipment being tested. For medium-voltage switch cabinets ranging from 10kV to 35kV, 100A of test current is usually sufficient; for ultra-high voltage GIS combinations or large-capacity generator outlet circuit breakers of 220kV and above, 200A, 300A, or even 600A of test current is required. The mainstream models on the market usually offer multiple selectable levels such as 100A, 200A, 400A, and 600A.
When selecting, attention should be paid to whether the instrument has the ability to continuously output for a long time. Equipment using high-frequency switching power supply technology can continuously output large currents, meeting the requirements for long-term operation, while pulse-type power supplies have limitations.
(2) Measurement range and resolution
The procedure requires the measured circuit resistance value to be in the range of several micro-ohms to several hundred micro-ohms. Mainstream instruments have a measurement range of 0 to 1999.9μΩ, with a resolution of 0.1μΩ. Some high-end models can achieve a resolution of 0.01 μΩ, which depends on the performance of the high-precision analog-to-digital converter inside the instrument.
(III) Measurement Accuracy
The procedure requires a measurement accuracy of typically 0.5 class, that is, ±0.5% of the reading. Some products can achieve an accuracy of 0.2 class. When selecting the instrument, attention should be paid to the accuracy indicators within the full range of the instrument, rather than just focusing on the performance at a specific point.
(IV) Four-wire Measurement Function
The four-terminal wiring method is a basic requirement to eliminate the influence of test line resistance and ensure measurement accuracy. The software and hardware design of the instrument should be able to effectively eliminate the influence of test wire resistance and the contact resistance of the test clamp on the test results, and there should be no impact on the test results due to changes in the test wire length.
(V) Environmental Adaptability and Anti-interference Capability
There is strong electromagnetic interference in the substation site. Therefore, an instrument with a good anti-interference design should be selected to ensure the stability and reliability of the measurement data. Under severe interference conditions, the last digit of the LCD screen data should be able to remain within ±1 character range, with stable reading and good repeatability. In addition, attention should be paid to the weight, volume, and protection level of the instrument to meet the requirements of field operations.
Application Scenarios and Engineering Practice
The application of the large current contact resistance tester runs through the full life cycle management of power equipment. The main scenarios include:
(1) Commissioning and Acceptance Test
According to relevant procedures, new installed or overhauled switchgear must undergo conductive circuit resistance measurement as an important basis for quality acceptance. This scenario requires the instrument to have data storage and printing functions to facilitate the formation of a standardized test report.
(2) Preventive Test
Regularly testing the circuit resistance of operating circuit breakers and disconnectors is an effective means to detect contact deterioration hazards. The procedures clearly specify the test cycle and qualification standards for various equipment. This scenario has higher requirements for the portability and operational convenience of the instrument.
(3) Fault Diagnosis and Maintenance
When equipment shows signs of overheating or abnormal temperature rise, it is necessary to diagnose the contact condition through circuit resistance measurement. Some high-end instruments support dynamic resistance measurement, which can continuously record the change curve of contact resistance over time during the operation of the circuit breaker, intuitively reflecting the wear condition of the contacts, and providing data support for predicting the remaining electrical life of the equipment.
(4) Other Large Current Micro-resistance Measurement Scenarios
Apart from power switchgear, this instrument is also applicable to the measurement of busbar joints, GIS basin insulators, cable wires, weld seam contact resistance, and the resistance of railway and subway car bodies and rails, etc.
Post time: Sep-10-2026