Loop Resistance TesterAlso Known AsContact Resistance TesterMicro-ohmmeteris used in power systems for measuring high-voltage switchgear
(such as circuit breakers, disconnecting switches, busbar connections, etc.) conductive loop resistance.
Its core purpose is to detectwhether contact connections are good. If the loop resistance is too high, excessive heat will be generated when large current passes through, leading to contact welding, insulation aging, or even equipment explosion. Therefore, it is a critical item in preventive testing.mandatory testingcritical item.
Based on the latest technical standards and market developments of 2025-2026, the following is a detailed analysis:
1. Core Principles and Technical Standards
- Testing Principle: AdoptingDC Voltage Drop Method(Four-Terminal Method). The instrument outputs a large DC current (typically 100A, 200A, 400A, or even 600A) while measuring the voltage drop across the test object, calculating the resistance value according to Ohm's Law R=U/I.
- Why High Current?
- Break Through Oxide Film: Switch contact surfaces often have oxide or oil films that small currents cannot break through, causing inflated measurement values. Only high current reaching or approaching the rated current can truly reflect contact resistance under operating conditions.
- Eliminate Thermoelectric Potential Interference: High current can effectively suppress the influence of contact potential and thermoelectric potential.
- Applicable Standards：
- GB 50150-2016"Standard for Hand-Over Test of Electric Equipment in Electric Equipment Installation Engineering"
- DL/T 846.4-2017"General Technical Conditions for High-Voltage Test Equipment - Part 4: Loop Resistance Tester"
- IEC 62271-100 (High-Voltage AC Circuit Breaker)
2. Mainstream Technical Parameters for 2025-2026
Modern high-end Loop Resistance Testers have fully replaced older manual-adjustment types, featuring the following characteristics:Table
| Parameter Item | Typical Specification (2026 Model) | Description |
|---|---|---|
| Output Current | 100A / 200A / 400A / 600A (Optional) | For 500kV and above equipment or large-capacity generator outlet switches, 400A-600A is recommended; for conventional 110kV-220kV, 200A is sufficient. |
| Measurement Range | 0 - 2000 μΩ (Micro-ohm) | Resolution typically reaches 0.1 μΩ。 |
| Measurement Accuracy | ±0.5% ±0.1μΩ | High precision ensures even tiny changes can be captured. |
| Open Circuit Voltage | ≥ 10V (Some high-end models reach 20V) | High open-circuit voltage helps break through thicker oxide layers. |
| Continuous Operating Time | Long-Duration Continuous Output | Features intelligent temperature control and overload protection, supporting long-duration testing without overheating. |
| Anti-Interference Capability | Strong Magnetic Field Filtering | Capable of stable readings in the strong electromagnetic environment of substations. |
| Intelligent Features | Touchscreen + Mini Printer
USB/Bluetooth/WiFi |
Automatically saves data, generates curves, one-click upload to phone or computer, supports historical data comparison. |
| Cable Design | Lightweight Test Cables | Uses new flexible cables, reducing weight by 30% at the same current level for easier on-site handling. |
3. Representative Models and Manufacturers (2026 Reference)
A. High Current / Extra-High Voltage Type (400A-600A)
- Wuhan Moeorw - MEHL-600A
- Features: Maximum output 600A, specifically designed for UHV and large generator outlet switches. Features constant current source technology with extremely strong anti-interference capability, equipped with heavy-duty flexible test cables.
- Wuhan Huatian Electric Power - HTHL-500A
- Features: 500A output, 10-inch color screen, built-in expert diagnostic system that can automatically determine whether resistance exceeds limits and trigger alarms.
B. Conventional Portable Type (100A-200A)
- Yangzhou Haiwo Electric - HWHL-200A
- Features: Compact size (approximately 10kg), 200A output, suitable for daily inspection of 110kV and below substations, extremely cost-effective.
- Guodian Xigao - GDHL-100A
- Features: Classic model, simple operation, rugged and durable, suitable for mass deployment in grassroots teams.
- Shanghai Sute - STHL-200
- Features: Features Bluetooth transmission, supports direct data reading and report generation via mobile APP, the top choice for paperless office operations.
4. Purchase Recommendations
- Current Level Selection：
- 10kV - 35kV Switchgear：100A is sufficient.
- 110kV - 220kV Circuit Breakers: Recommended 200A。
- 500kV and Above / Large Generator Outlet: Must select 400A or 600A, to ensure full breakdown of oxide film and obtain real data.
- Test Line Quality: High-current testing places extremely high demands on cables. Poor-quality cables generate severe heat and high voltage drops, directly affecting accuracy. When purchasing, pay close attention to the cable'smaterial (oxygen-free copper)andflexibility。
- Constant Current Source Technology: Confirm whether the instrument hasfully automatic constant currentfunction. When the grid fluctuates or load changes, it can automatically adjust output voltage to maintain constant current, which is the prerequisite for accurate data.
- Data Management: For O&M teams that need to frequently issue reports, it is recommended to choose models withBluetooth / wireless transmissionandbuilt-in storageto significantly improve work efficiency.
5. Market Price Reference
- Basic Type (100A, digital tube display): Approx. 3,000 - 6,000 RMB。
- Standard Type (200A, color screen, printing): Approx. 8,000 - 15,000 RMB。
- High-End High-Current Type (400A-600A, intelligent analysis, lightweight cables): Approx. 20,000 - 40,000 RMB。
6. Testing Precautions (Critical!)
- Wiring Sequence: First connect current leads (C1, C2), then voltage leads (P1, P2).Voltage leads must be clamped inside the current leads, to eliminate the influence of the test lead resistance itself (the core of the four-terminal method).
- Grinding Contacts: If the test point is severely rusted, simple grinding should be performed first, but the primary approach relies on high-current breakdown.
- Closed State: Testing must be performed with the circuit breakerfully closedstate.
- Discharge: After testing inductive loads (such as long busbars or large coils), the instrument automatically discharges, but before disconnecting, always confirm the discharge completion indicator has turned off to prevent arc injury.
- Temperature Effect: Resistance values are affected by temperature. For precise comparison, record the ambient temperature and perform conversion (typically normalized to 20℃).
How to select cables suitable for high-current testing?
Representative Models and Manufacturers (2026 Reference)
A. High Current / Extra-High Voltage Type (400A-600A)
Wuhan Moeorw - MEHL-600A
Features: Maximum output 600A, specifically designed for UHV and large generator outlet switches. Features constant current source technology with extremely strong anti-interference, equipped with heavy-duty flexible test cables.
Wuhan Huatian Electric Power - HTHL-500A
Features: 500A output, 10-inch color screen, built-in expert diagnostic system that automatically determines whether resistance exceeds limits and triggers alarms.
B. Conventional Portable Type (100A-200A)
Yangzhou Haiwo Electric - HWHL-200A
Features: Compact size (approximately 10kg), 200A output, suitable for daily inspection of 110kV and below substations, extremely cost-effective.
Guodian Xigao - GDHL-100A
Features: Classic model, simple operation, rugged and durable, suitable for mass deployment in grassroots teams.
Shanghai Sute - STHL-200
Features: Bluetooth transmission function, supports direct data reading and report generation via mobile APP, the top choice for paperless office operations.
4. Purchase Recommendations
Current Level Selection:
10kV - 35kV Switchgear: 100A is sufficient.
110kV - 220kV Circuit Breakers: 200A recommended.
500kV and Above / Large Generator Outlet: Must select 400A or 600A to ensure full breakdown of oxide film and obtain real data.
Test Line Quality: High-current testing places extremely high demands on cables. Poor-quality cables generate severe heat and high voltage drops, directly affecting accuracy. When purchasing, pay close attention to cable material (oxygen-free copper) and flexibility.
Constant Current Source Technology: Confirm whether the instrument has fully automatic constant current function. When grid fluctuates or load changes, it can automatically adjust output voltage to maintain constant current, which is the prerequisite for accurate data.
Data Management: For O&M teams that need to frequently issue reports, it is recommended to choose models with Bluetooth/wireless transmission and built-in storage, which can significantly improve work efficiency.
5. Market Price Reference
Basic Type (100A, digital tube display): Approx. 3,000 - 6,000 RMB.
Standard Type (200A, color screen, printing): Approx. 8,000 - 15,000 RMB.
High-End High-Current Type (400A-600A, intelligent analysis, lightweight cables): Approx. 20,000 - 40,000 RMB.
6. Testing Precautions (Critical!)
Wiring Sequence: First connect current leads (C1, C2), then voltage leads (P1, P2). Voltage leads must be clamped inside the current leads to eliminate the influence of test lead resistance itself (the core of the four-terminal method).
Grinding Contacts: If test points are severely rusted, simple grinding should be performed first, but the primary approach relies on high-current breakdown.
Closed State: Testing must be performed with the circuit breaker in the fully closed state.
Discharge: After testing inductive loads (such as long busbars or large coils), the instrument automatically discharges, but before disconnecting, always confirm the discharge completion indicator has turned off to prevent arc injury.
Temperature Effect: Resistance values are affected by temperature. For precise comparison, record the ambient temperature and perform conversion (typically normalized to 20℃).
If you need reference standard values for specific circuit breaker models (such as SF6 circuit breakers vs. vacuum circuit breakers), or need to look up operation videos for a particular instrument, please feel free to contact us!
