I. Classification of DC Resistance Testers
DC resistance testers are mainly classified based on measurement principles, output current amplitude, and functional structure. Different types of devices have distinct technical characteristics and application scenarios.
Classification by measurement principle
Traditional bridge-type testers complete the measurement using single-arm or double-arm bridge principles. They require manual adjustment to balance, resulting in a cumbersome overall testing process and measurement accuracy susceptible to human operation. These testers have gradually faded out of use scenarios.
Digital DC resistance testers are the current mainstream type. They use constant current sources combined with four-wire Kelvin measurement method to achieve fully automatic testing and directly present the resistance values. The equipment is equipped with a high-precision analog-to-digital converter and microcontroller control unit, offering high measurement accuracy, excellent anti-interference performance, and is also equipped with complete data storage and printing functions. They are widely used in power test-related work.
Classification by output current
Micro-ohm and milli-ohm testers have output currents generally lower than 1 ampere, suitable for measuring small resistances such as contact resistances of switch contacts, bus joint resistances, and conductive circuit resistances of circuit breakers.
Medium current testers have output currents in the range of 3 amperes to 10 amperes, mostly used for the resistance measurement of small-capacity distribution transformers and motor windings.
Large current testers have output currents ranging from 20 amperes to 100 amperes or more, specifically designed for large power transformers with large inductance characteristics. The large test current can quickly establish a stable magnetic field, significantly reducing the testing time and improving operational efficiency.
Classification by functional structure
Single-phase or single-channel testers use a phase-by-phase measurement mode and can be used for single-phase equipment or for phase-by-phase detection of three-phase equipment. The equipment structure is simple and the usage cost is low.
Three-phase or multi-channel testers support three-channel synchronous measurement and can complete the detection of three-phase winding DC resistance in one go, automatically calculating the three-phase imbalance rate, significantly improving the on-site testing efficiency, especially suitable for preventive tests of large power transformers.
Portable DC resistance testers are small in size, lightweight, and equipped with built-in rechargeable batteries. They are suitable for outdoor high-altitude operations and mobile on-site testing scenarios and are commonly used equipment in on-site tests.
II. Application Areas of DC Resistance Testing
DC resistance testing covers the entire process of power equipment production and operation and is the core technical method for equipment quality control and fault detection.
Transformer full-process testing
During the factory and commissioning tests of transformers, the measurement of winding DC resistance can verify the compliance of conductor cross-sections, welding quality, and connection status of tap changers, and is a necessary test item before the transformer is put into operation.
For on-load tap-changing switches, DC resistance testing has a key role in fault diagnosis. The failure rate of this component is high. Measuring the DC resistance step by step can promptly detect issues such as contact erosion, spring fatigue, and abnormal operation of the switching mechanism. Under normal conditions, the resistance values of adjacent steps change uniformly, and significant deviations in the values indicate potential equipment abnormalities.
In daily preventive maintenance of transformers, regular DC resistance testing is conducted and the measured data is compared with historical data after temperature conversion to effectively monitor progressive faults in the windings. The normal requirement is that the inter-line DC resistance imbalance rate should not exceed 2%, and if it exceeds the standard, the fault cause should be promptly investigated.
Motor and transformer detection
In the manufacturing and maintenance of motors, measuring the DC resistance of stator and rotor windings can determine whether there are issues such as missing strands, broken wires, or virtual connections in the windings. If the three-phase winding imbalance rate exceeds 5%, it requires repair.
At the same time, this test can also detect the contact status of the commutator, brush, and winding of the motor, and avoid contact failure risks in advance. Current transformers and voltage transformers can also be tested using DC resistance measurement to assess the overall integrity of the winding.
Other industry extended applications In the electronics manufacturing industry, DC resistance testing is commonly used for detecting copper foil resistances in printed circuit boards, screening electronic components, and inspecting the quality of solder joints.
In the automotive industry, it is mainly used for measuring the internal resistance of battery packs in new energy vehicles, in order to assess the health status of battery cells and the welding quality of current leads.
In the communication and infrastructure fields, it can complete the detection of grounding resistance for communication equipment and the testing of DC resistance for various cables, etc.
III. Explanation of the Impact of Temperature on Winding Resistance
The resistance of copper windings is significantly affected by temperature changes, with a temperature coefficient of approximately 0.393 percent per degree Celsius. When comparing test data from different periods and different environmental temperatures, the measured values need to be uniformly converted to the same reference temperature.
Conversion formula: R2 = R1 × (T2 + k) ÷ (T1 + k)
Where the constant k for copper windings is fixed at 234.5
Post time: May-14-2026