Loop Resistance Tester
The loop resistance tester (also known as contact resistance tester or micro-ohmmeter) is a specialized electrical test equipment designed to measure the loop resistance of conductive circuits in power equipment such as high-voltage switches, circuit breakers, and disconnectors. Its core function is to evaluate the contact condition at conductive connections, preventing serious safety hazards such as local overheating, accelerated oxidation, or even burnout caused by excessive contact resistance.
Working Principle and Core Technology
The loop resistance tester commonly uses the DC voltage drop method (four-wire Kelvin measurement method) for measurement.
Four-wire principle: The instrument applies a stable large DC test current to the resistance under test through a pair of current terminals (I+, I-), while simultaneously measuring the voltage drop across the resistance with high precision through another pair of voltage terminals (V+, V-). The internal system automatically calculates and displays the resistance value based on Ohm's law (R=U/I).
Advantages of high current: The four-terminal connection method effectively eliminates interference from the test leads' own resistance and contact resistance. Additionally, applying a large current of no less than 100A can effectively break through the oil film and oxide layer on the contact surface, thereby measuring the true contact resistance. Key Standards and Selection Criteria
When selecting equipment and conducting tests, it is necessary to strictly follow relevant power industry standards and focus on the following core technical indicators:
Test current: According to the "Preventive Test Procedures for Power Equipment" (DL/T596) and the latest industry standard (DL/T 845.4-2019), the measurement current for circuit breaker conductive loop resistance must not be less than 100A DC. Currently, mainstream instruments offer adjustable test currents ranging from 50A to 300A in multiple levels to accommodate different equipment testing needs.
Resolution, accuracy, and repeatability: Resolution represents the smallest step displayed by the instrument, accuracy represents how close the measurement result is to the true value, and repeatability represents the consistency of multiple measurement results. When selecting, these three aspects should be considered separately. High-quality instruments can achieve resolutions of 0.1μΩ or even 0.01μΩ.
Data management and anti-interference capability: Modern intelligent instruments are typically equipped with LCD screens, thermal printers, data storage, and communication interfaces, and feature multi-level automatic range switching and adaptive noise reduction technology to handle continuous testing in complex field environments.
Reference for Mainstream Brands in the Market
The market for loop resistance testers is mainly divided into imported and domestic brands, each with its own focus:
Mainstream imported brands:
Megger: Offers a wide product range, covering from portable micro-ohmmeters to high-current field testing, emphasizing portability, data repeatability, and safe grounding conditions.
DV Power: Focuses on micro-ohm measurement and comprehensive circuit breaker diagnostics, integrating contact resistance, circuit breaker timing, coil current, and other tests into a comprehensive analysis platform, suitable for condition diagnostics and maintenance decision-making.
AEMC/Chauvin Arnoux: Emphasizes low-resistance measurement methods and data management, with in-depth technical support for four-wire Kelvin method and error analysis.
Mainstream domestic brands:
Beijing Kanggaote (KGT): Its self-developed "Baiju Pro" series is positioned as a handheld, battery-powered field micro-ohm tester, with output current adjustable up to 300A smoothly, suitable for scenarios requiring frequent site changes such as substation maintenance and switchgear maintenance.
Wuhan Guodian Xigao: Its GDHL series intelligent high-precision loop resistance tester uses high-frequency switching power supply technology, featuring compact size, strong anti-interference, and continuous high-current output, supporting custom test time and data storage.
Testing Precautions and Result Evaluation
Safety isolation: Before testing, it must be confirmed that the object under test is completely de-energized, and safety isolation and grounding measures should be implemented.
Wiring specifications: Ensure that test leads are securely connected with good contact, avoiding data fluctuations or measurement failures due to loose connections.
Scientific evaluation: When determining whether loop resistance is abnormal, it is not advisable to simply use a rule like "below a fixed micro-ohm value." A more reliable approach is to establish a historical baseline for the same equipment and, under consistent test conditions, observe whether there is a continuous upward trend in resistance or significant differences among the three phases.
