Loop resistance tester, also known as contact resistance tester or micro-ohmmeter



is a specialized instrument used in power systems to measure the contact resistance of conductive circuits such as high-voltage switches, circuit breakers, disconnectors, and busbar connections.

Core Pain Points and Importance:

If the contact resistance of the conductive circuit is too high, severe heating will occur when large currents pass through (

 Rt ), leading to equipment burnout, contact welding, or even explosion accidents. Traditional bridge methods, due to their small test current (milliampere level), cannot break through the oxide film on contact surfaces, often resulting in distorted measurements. Modern loop resistance testers must adopt the DC voltage drop method, with test currents typically reaching 100A, 200A, or even above 600A, to truly reflect the contact condition of equipment under heavy loads.

Based on the latest market trends and technical standards from 2025-2026 (such as DL/T 845.4-2019, GB 763), here is a detailed guide:

1. Core Functions and Technical Evolution

High-current constant current source: Outputs stable DC high current (100A/200A/400A/600A), effectively breaking through oxide layers to measure true contact resistance (typically in the micro-ohm μΩ range).

Four-terminal measurement method: Strictly adopts Kelvin four-wire connection (C1, P1, P2, C2) to eliminate the influence of test lead resistance and contact resistance, ensuring high precision.

Anti-interference design: Built-in high-speed ADC sampling and digital filtering algorithms for strong electromagnetic environments in substations, ensuring stable readings under induced voltage interference.

Continuous test mode: Some high-end models support "long-duration high-current output" (e.g., lasting over 60 seconds) to monitor resistance changes of contacts under thermal steady-state conditions.

Intelligent diagnostics: Automatically determines if the circuit is open or short-circuited, and prompts wiring errors; built-in historical data comparison function to analyze resistance growth trends.

Data management: Supports USB export, Bluetooth printing, and some models can connect to mobile apps or upload to production management systems (PMS).

2. Mainstream Brands and Market Landscape in 2026

The domestic market is highly mature, with the Wuhan industrial cluster still being the absolute main force, offering extremely high cost-performance products that fully meet the stringent bidding requirements of State Grid and China Southern Power Grid.

Leading Domestic Brands (Highly Recommended)

Wuhan UHV Electric Power Technology Co., Ltd.:

Position: Industry leader, known for "high constant current accuracy" and "strong anti-interference capability."

Core Technology: Uses high-frequency switching power supply technology combined with closed-loop feedback control, maintaining constant output current (fluctuation <1%) even with significant grid voltage fluctuations.

Features: Equipped with "one-key test" function, ready to test upon startup; large LCD display with real-time curve plotting; built-in micro printer for on-site reports.

Applicability: Full scenario coverage from 10kV distribution networks to 1000kV UHV switchgear.

Wuhan Huadian Keyi / Wuhan Weian Electric Power:

Features: Established manufacturers with rich product lines, offering portable (suitcase) and desktop models in various specifications.

Advantages: Good durability, comprehensive after-sales service network, high cost-performance, and a mainstream choice for power supply companies in various regions.

Yangzhou Tuopu / Shanghai Shengxu:

Features: Focus on lightweight design, suitable for field, high-altitude, or space-constrained switchgear operations.

Internationally Renowned Brands

USA Megger (DLRO series), Germany Omicron:

Features: Exquisite craftsmanship, excellent long-term stability, some models offer extremely high resolution (0.1μΩ) and advanced diagnostic functions.

Disadvantages: Expensive (usually 5-10 times the price of domestic products), long repair cycles.

Applicability: Foreign-related projects, research institutes, and projects with strict brand requirements.

3. Key Selection Indicators

When purchasing, please pay special attention to the following parameters:

Test current level:

100A: Suitable for low-voltage switches, small circuit breakers, and busbars.

200A: Market mainstream, suitable for most 10kV-35kV vacuum circuit breakers and SF6 circuit breakers.

400A/600A and above: Suitable for 500kV and above UHV circuit breakers, GIS switchgear, or high-current generator outlet switches.

Recommendation: Choose based on the rated current of the device under test; typically, the test current should be no less than 10%-20% of the device's rated current, or follow the manufacturer's instructions (generally at least 100A).

Measurement range and accuracy:

Range: 0-1999.9μΩ (or wider).

Accuracy: ±(0.5% reading + 2 digits) or higher (0.2 class).

Resolution: At least 0.1μΩ to detect minor contact issues.

Continuous operation time: Confirm whether the device supports long-duration (e.g., >60 seconds) high-current output, which is important for thermal stability testing.

Protection functions: Must have over-temperature protection, over-voltage protection, and open-circuit protection to prevent damage to the instrument or test object.

Power supply adaptability: Prefer support for AC 220V wide voltage input; some high-end models support vehicle DC 12V/24V power supply.

4. Price Reference Range (2026 Market Conditions)

Basic type (100A): ¥3,000 - ¥6,000.

Suitable for general factory electricians and small distribution room maintenance.

Professional type (200A, mainstream configuration): ¥8,000 - ¥15,000.

Market mainstay, suitable for power supply company operation and maintenance teams and repair companies, with complete functions and high accuracy.

High-end type (400A-600A+, imported or domestic top configuration): ¥20,000 - ¥50,000+.

Suitable for UHV substations, large power plants, and electric power research institutes.

5. Usage Precautions and Safety Regulations

Power-off operation: Must be performed after the switch under test is completely de-energized and grounded for discharge; live testing is strictly prohibited.

Four-wire connection:

Current leads (C1, C2): Clamp on the outside, thick wires to carry large current.

Voltage leads (P1, P2): Clamp on the inside (near the contacts), thin wires to collect voltage signals.

Note: The voltage clamps must be clamped inside the current clamps; otherwise, the resistance of the current leads will be included in the measurement, resulting in higher readings.

Good contact: The test clamps must have tight and clean contact with the test points, removing oxide layers and oil stains; otherwise, test stability will be affected.

Test time: After pressing the test button, wait until the current stabilizes and the reading no longer fluctuates significantly before recording data (usually 3-5 seconds). For inductive loads (such as some switches with coils), wait longer to eliminate inductance effects.

Data judgment:

The measured value should comply with the manufacturer's technical specifications (usually ≤1.2 times the factory value).

The resistance values of each phase in a three-phase switch should be balanced, with mutual difference not exceeding 20% of the average value (refer to DL/T 596 regulations).

If the resistance value is abnormally high, possible causes include: contact erosion, insufficient spring pressure, loose connection bolts, surface oxidation, etc.

Heat dissipation: High-current testing generates heat; when testing multiple devices continuously, pay attention to instrument heat dissipation to avoid overheating protection shutdown.

6. Typical Application Scenarios

Circuit breaker maintenance: Check the contact condition of vacuum interrupter and SF6 arc-extinguishing chamber contacts.

Disconnector maintenance: Check if the knife-edge contact pressure is sufficient and if there is an oxide layer.

Busbar connection inspection: Verify the tightness of bolts at busbar lap joints.

New equipment acceptance: Verify whether the loop resistance after installation meets factory standards.

Fault troubleshooting: Locate high-resistance points when switches experience overheating.

Summary Recommendations:

For the vast majority of power operation and maintenance units, the 200A DC loop resistance tester from Wuhan UHV Electric Power or similar domestic professional brands is the best choice. They are fully comparable to international standards in accuracy, stability, and anti-interference capability, and are priced at only about 1/5 of imported equipment, offering extremely high cost-performance. Only in special ultra-high current requirements (e.g., >1000A) or foreign-related projects should international brands like Megger be considered.