In the commissioning tests, preventive tests, and daily maintenance of power systems, resistance measurement is a fundamental and crucial task. DC resistance testers and loop resistance testers are two common resistance measurement devices. Although both are based on the principle of DC voltage drop, they have essential differences in design goals, test objects, and application scenarios, and should not be used interchangeably. The following is a review from multiple dimensions to provide reference for relevant technicians.
I. Core Differences
DC Resistance Tester
Main Measured Object: The winding resistances of equipment such as transformers, motors, and transformers.
Load Nature: Inductive load, including inductors and capacitors.
Typical Test Current: The current is relatively small, typically ranging from a few amperes to several tens of amperes, such as 1A, 5A, 10A, 20A.
Measurement Range: The range is wide, capable of measuring resistances from milliohms to ohms and even higher values.
Core Purpose: To evaluate the conductive performance of winding materials and detect equipment faults such as inter-turn short circuits and broken strands.
Key Requirements: Equipped with rapid demagnetization and discharge functions to ensure the safety of the equipment and personnel operation.
Loop Resistance Tester
Main Measured Object: The contact resistances of components such as circuit breakers, disconnectors, and busbar joints.
Load Nature: Pure resistive load, almost without inductance.
Typical Test Current: The output current is large, typically 100A, 200A, or higher grades.
Measurement Range: The range is biased towards the low-value range, specifically for micro-ohm (μΩ) level resistances.
Core Purpose: To test the contact condition of the components to avoid overheating due to poor contact when operating under high current conditions.
Key Requirements: Capable of outputting large currents to break the oxide film and contaminants on the surface of the contact points, obtaining the true contact resistance value.
II. Working Principle and Core Differences
Both instruments use the four-wire method, also known as the Kelvin connection method, to conduct measurements, eliminating the interference of the test leads’ own resistance on the measurement results. The core difference between the two lies in the size of the test current and the purpose of setting the current.
The measured objects of the DC resistance tester are mostly inductive loads such as transformer windings. Due to the influence of inductance, larger currents should not be used in the measurement process, otherwise, a very high reverse electromotive force will be generated, which not only interferes with the measurement accuracy but also poses safety hazards. Therefore, this device selects a relatively smaller test current and is equipped with a demagnetization circuit to ensure a safe and stable measurement process.
The measured object of the loop resistance tester is pure resistive loads such as the contacts of circuit breakers. A test current of 100A or even higher can be applied. The large current can break the insulating film formed by oxidation and contaminants on the surface of the contact points, obtaining the true resistance data of the metal contact surface.
III. Application Scenarios
Applicable Scenarios of DC Resistance Tester
Power Industry: Complete the commissioning tests and preventive tests of transformer, transformer, and reactor equipment;
Industrial Scenarios: Detect the winding resistances of motors and generators;
Cable Testing: Measure the DC resistance of cable conductors.
Applicable Scenarios of Loop Resistance Tester
Switching Equipment: Detect the contact resistances of main contacts of high-voltage circuit breakers, disconnectors, and load switches;
Connection Point Testing: Measure the contact resistances at the connection positions of substations busbars and the terminals of equipment leads;
Other Micro-ohm Measurements: Weld seams, grounding lead-down line connection points, etc., for low-resistance areas.
Post time: Sep-15-2026