In the operation and maintenance testing of equipment such as high-voltage switches, circuit breakers, and busbar connections, the precise measurement of loop resistance is the core method for determining the contact state of the conductive circuit. The test current must reach 100A or more to effectively break through the oxide and oil films on the contact surface and obtain the true contact resistance value. The regulations in DL/T 596 and GB 50150 clearly stipulate that the test current should be no less than 100A. This requirement constitutes the fundamental constraint for the design of loop resistance testers.
For a long time, equipment that meets this current requirement has mainly been in the form of desktop models, characterized by large size, reliance on mains power supply, high measurement accuracy, but poor mobility. With the gradual shift of substation maintenance mode towards condition-based maintenance and non-stop power operations, the demand for on-site mobile detection has continued to increase. Handheld loop resistance testers have emerged accordingly and have rapidly evolved in recent years. Based on the framework of test standards, this paper sorts out the technical characteristics and applicable boundaries of handheld and desktop equipment to provide reference for equipment selection and on-site application.
I. Technical Requirements under the Standard System
The loop resistance tester has formed a relatively complete technical specification system. DL/T 845.4-2019 “General Technical Conditions for Resistance Measurement Devices – Part 4: Loop Resistance Testers” is the core industry standard, which stipulates relevant technical requirements, test methods, and inspection rules. The accompanying JJG 1052-2025 “Verification Regulations for Loop Resistance Testers” was released in 2025 and is applicable to the first verification and subsequent verifications of digital loop resistance testers with a rated working current of no less than 100A. In addition, T/ZDL 038-2025 “Digital and Intelligent Loop Resistance Testing Technical Specifications” is a group standard, which adds intelligent functions such as Bluetooth communication, voice recognition, and probe triggering, reflecting the technical development direction of this equipment.
From the standard provisions, the lower limit of the output current is clearly defined as 100A. Regardless of the structure and form of the equipment, compliant products must meet this indicator. The differences between the two types of equipment mainly lie in the current holding time and output quality. DL/T 845.4-2019 has relevant provisions on allowable errors in working current and holding time. Desktop equipment can generally continuously output current and is suitable for strict test conditions; handheld equipment is limited by battery capacity and heat dissipation conditions and mostly uses a rapid mode to complete the measurement.
II. Traditional Advantages of Desktop Machines: Precision and Continuity of the Baseline
The design focus of desktop loop resistance testers is on measurement accuracy and operational stability. The equipment is powered by AC 220V mains power supply and is equipped with a high-power linear power supply or high-frequency switching power supply, with a power supply capacity of 1 to 3kW, capable of continuously outputting 100A, 200A, or even 600A of high current for a long time. Typical models can weigh up to 30kg and need to be used on an experimental table or inspection vehicle. The current range covers 100A to 600A, and at the 600A range, it can maintain output for 10 seconds.
The core advantages of desktop equipment:
Excellent current ripple and stability performance. The large-power power supply scheme can control the output current ripple coefficient and stability within a small range. According to reference technical materials, at the 100A range, the ripple coefficient is no more than 1.5% rms, the stability is ±0.3%, and the measurement results are less affected by current fluctuations, and the data repeatability is better.
Continuous output capability is suitable for aging and temperature rise tests. Some test scenarios require long-term application of current to observe the temperature rise of the contact or conduct aging verification. This continuous power supply capability is difficult to be replaced by handheld equipment.
The upper limit of measurement accuracy is higher. Typical desktop equipment can achieve an accuracy of ±(reading × 0.5% + 0.2 μΩ), combined with high-precision sampling circuits, it has good resolution capabilities for contact resistances at the tens of micro-ohm level.
There are obvious shortcomings of desktop equipment: the overall weight is 5 to 15 kg, it relies on an external power source, and the wiring and debugging process takes a long time. When used in scenarios such as multi-interval inspections in substations, high-altitude operations, and narrow spaces in GIS, the equipment is cumbersome to operate and the operation efficiency is low.
III. Technological breakthroughs in handheld devices: From portability to usability
Early handheld loop resistance testers had the problem of strong portability but insufficient practicality. The current output current range was low, the battery life was short, and the anti-interference ability was weak. After product iterations in recent years, the relevant performance has been fundamentally improved.
Current indicators meet the standard. Current mainstream handheld devices can output 100A current, meeting the basic requirements of DL/T 596 and other regulations for test current.
Independent power supply and battery life. The device is equipped with a lithium-ion battery pack, with common specifications of 16.8V/6AH or 14.8V/5AH. No external power supply is required. According to technical specifications, a single charge can complete no less than 500 rapid tests, and the actual working condition can last for approximately 8 hours, supporting multi-interval inspections in a single day.
Environmental adaptability is improved. The product’s protection level can reach IP54 or above, capable of withstanding dust and splashing water; equipped with industrial-grade anti-interference circuits, the measurement deviation in a strong electromagnetic environment of the substation can be controlled within 1%. Some high-end models have a protection level of IP65 or IP67, suitable for harsh conditions such as outdoor and underground.
Test efficiency has significantly improved. The handheld device’s rapid measurement mode can obtain stable data in 2 to 5 seconds, and a complete test takes no more than 5 minutes, which is a significant improvement compared to the dozens of minutes of debugging and testing cycle of desktop equipment.
Handheld devices have performance trade-offs: the continuous output time of current is limited, the 100A output level generally lasts within 60 seconds, and the continuous time for 200A and above output levels is even shorter; the upper limit of accuracy is slightly lower than that of high-end desktop equipment, with typical accuracy ranging from ±(reading × 0.5% + 0.2 μΩ) to ±(reading × 0.5% + 2dgt). Therefore, handheld devices are more suitable for inspection-type measurements and are not suitable for precise calibration work.
IV. Selection logic: Scenarios determine form
There is no superiority or inferiority between the two types of equipment; the core difference lies in different applicable scenarios.
Desktop equipment is suitable for laboratory calibration, factory tests, product type tests, and aging tests and temperature rise tests that require long-term continuous large current output. These scenarios prioritize stability and continuous output capabilities, and portability is not a key consideration. The 200A to 600A current range is more effective for large cross-section busbars and heavy switch contact oxide film breakdown.
Handheld devices are suitable for substation site inspections, circuit breaker commissioning tests, GIS equipment interval tests, and test points that handheld equipment cannot reach easily. In these scenarios, reaching the test point is the primary difficulty, and portability becomes a necessary condition. The 100A to 200A current combination with the rapid measurement mode can meet the needs of most circuit breaker contact resistance detection.
In actual maintenance work, many units adopt a one main one auxiliary combination scheme: desktop equipment is used as the laboratory reference instrument for difficult data verification; handheld devices undertake daily inspections and on-site rapid determination, balancing operation efficiency and detection reliability.
V. Operational key points: Field norms under the standard framework
Regardless of using handheld or desktop equipment, the basic operation norms for loop resistance measurement follow a unified standard and are not affected by the equipment form:
Four-terminal connection is the key to ensuring measurement accuracy. The current lines I+ and I- are clamped on the outside, while the voltage lines U+ and U- are clamped on the inside. The voltage sampling points are set inside the current circuit to eliminate the measurement interference caused by the resistance of the test lines and the contact resistance. This is a general principle for loop resistance testing.
Do not perform live testing. Before testing, confirm that the tested circuit is de-energized and reliably grounded. Performing live work will endanger personal safety and may also cause damage to the instrument.
Wait for the energy release to complete before removing the wires. After the large current test is completed, the internal inductor of the instrument stores energy. It is necessary to wait for the instrument to prompt that the energy has been recovered before removing the test wires to prevent the generation of electric arcs.
Handheld devices need to manage the batteries properly. When the battery voltage is low, do not forcibly use the high current setting to avoid accelerating battery wear; when the equipment is idle for a long time, it should be fully charged and stored.
The mature application of handheld loop resistance testers is a typical manifestation of the migration of power detection equipment from laboratories to on-site operations. It does not replace the desktop equipment as the benchmark for high-precision measurement, but achieves the feasibility of on-site operations by making a moderate performance compromise within the allowable standards. For most on-site detection scenarios in substations, it can conveniently complete the detection and has more practical value than extreme precision. This is the core breakthrough of handheld devices. With the continuous implementation of digitalization technologies, functions such as Bluetooth communication, voice interaction, and automatic data interpretation will gradually improve, and the capability boundaries of such devices will continue to expand. The overlapping application scenarios with desktop devices will also increase.
Post time: Sep-28-2026