DC Resistance Tester (Including Detailed Operating Steps)

The DC resistance tester is an instrument used in power plants, substations, and other field or laboratory settings to measure the DC resistance of windings in electrical equipment such as transformers, motors, current transformers, and switchgear. Featuring an integrated design, the instrument incorporates a programmable constant-current source, a high-precision signal acquisition module, and an automatic discharge protection circuit, and is controlled by a microprocessor with a Chinese-language menu interface. The test current source is generated by the instrument's internal power conversion module, delivering a stable and adjustable DC test current. Operating on a four-wire (Kelvin connection method) measurement principle, the instrument automatically eliminates interference from lead resistance and contact resistance, while also offering automatic charging/discharging, data storage, and printing functions to ensure measurement accuracy. It is suitable for various on-site applications, including equipment handover and preventive testing scenarios.

An illustrative analogy of the testing principle (Kelvin four-wire method): split-type hydrometer and water transmission pipeline

Roles involved in the testing process:

• Electrical windings (such as the copper windings in transformers or the contacts in switches) function like a pipeline for water transmission.
• The test current is analogous to water flowing through a pipeline at a precisely fixed velocity.
• Intact winding/contact = A pipeline with a smooth and unobstructed inner wall: When water flows through, the pressure difference across its ends is minimal, indicating low pipeline resistance.
• Defective winding/contact = A pipeline with internal blockage or a narrow joint: When water flows through it, the pressure difference across the ends increases significantly, indicating high pipeline resistance.

The magnitude of this "water pressure difference" corresponds to the DC resistance of the winding. The more blocked the pipeline and the narrower the joints, the greater the pressure difference and the higher the resistance, thereby increasing the operational risk. Poor electrical contact may lead to localized heating, potentially causing winding burnout and subsequent power outage incidents.

How to eliminate external interference?

The Kelvin four-wire method functions as a "modular" precision measurement system. It employs two independent "water pipes" (current leads) to deliver a constant flow of water through the pipeline, while two separate "pressure measurement tubes" (voltage leads)—through which minimal water flow passes—are placed directly across the pipeline to measure pressure. Since no water flows through these pressure measurement tubes, they completely ignore the resistance inherent in the upstream pipeline, thereby accurately detecting the true pressure difference across the pipeline. The DC resistance tester operates on this principle of the Kelvin four-wire method, utilizing a high-precision constant current source and independent voltage sampling to achieve precise resistance measurements.

Application scenarios for DC resistance testers:

• Power Grid Company: Preventive testing of substation transformers and switchgear, completion acceptance of new construction projects
• Power Engineering Company: Construction and Testing of Power Transmission and Transformation Projects, Equipment Handover Tests
• Industrial enterprises: Daily maintenance of transformers and motors in self-owned power plants of power plants, steel mills, and chemical plants
• Equipment manufacturer: Quality inspection of transformers, current transformers, and motors upon factory delivery
• Third-party testing institutions: Power equipment quality testing, forensic appraisal

Several different wiring methods for DC resistance testers:

Four-wire standard wiring: The most commonly used standard wiring configuration. The instrument's current terminals C1 and C2 deliver the test current to the specimen, while the voltage terminals P1 and P2 measure the voltage signals across the specimen. This configuration completely eliminates interference from lead resistance and contact resistance, making it suitable for the vast majority of low-resistance test objects, such as transformer windings, switch contacts, and cable joints.

Simple two-wire wiring: Suitable for measuring high-resistance objects (typically with resistance values> 1Ω), where the influence of lead wire resistance is negligible. Simply short-circuit the instrument's current terminal to its voltage terminal and connect it to both ends of the test specimen for simplified wiring.

Three-phase synchronous test wiring: An efficient wiring method for transformer winding testing that allows simultaneous connection of all three-phase windings, enabling one-time measurement of three-phase DC resistance without phase-by-phase rewiring. This significantly reduces testing time for large transformers and enhances on-site testing efficiency.

Here are several common wiring diagrams:

A:For the single-phase measurement method, see Figure 6 below.:

Figure 6

B,See fig. 7 ~ 9 for the connection by magnetic assist method (applicable to Y(N)-d-11 connection group). When measuring the low-voltage side of a large-capacity transformer, if the maximum current of the DC resistance tester is relatively small under the existing circumstances, or in order to speed up the measurement, the magnetic-assisted method can be selected for measurement. In the following figure, Figure 7, Figure 8 and Figure 9 respectively show the wiring methods for measuring low-voltage Rac, Rba and Rbc.

Figure 7

Figure 8

Figure 9

Fig. 7, fig. 8 and fig. 9 are wiring methods for measuring low-voltage Rac, Rba and Rbc respectively.

1. When measuring the inductive load, the test line cannot be directly removed, so as to avoid endangering the safety of testers and equipment due to inductive discharge. The output end of this machine is provided with a discharge circuit. After the instrument is reset, the inductor will release energy through the instrument. Do not remove the test line until the discharge instruction is completed.
2. For no-load voltage regulating transformer, it is not allowed to switch tap-changer during measurement.
3. If the power supply is suddenly cut off during the measurement, this machine will automatically start discharging. Please do not disassemble the wiring immediately, and wait at least 30 seconds before disassembling the wiring.
4. During measurement, other untested windings should not be short-circuited to ground, otherwise, the magnetizing process of transformer will be slowed down, the data stabilization time will be prolonged or the value will be incorrect.
5. Please check the power supply voltage before starting: AC 220 V 10%, 50Hz.
6. Please confirm that the equipment under test has been cut off and disconnected from other live equipment during the test.
7. The enclosure must be reliably grounded during the test.
8. Irrelevant items are not allowed to be piled up on and around the equipment panel during the test.
9. When replacing the safety tube and fittings, please use the same model as this instrument (see technical indicators for details).
10. this instrument pay attention to moistureproof, oil pollution prevention.
11. When selecting the current, refer to the range in the technical index column. If the current exceeds the preset value, the instrument is always in the "charging" state. At this time, press the reset button to reset the instrument and re-select a smaller current gea

Detailed steps for using the product

Next, using transformer testing as an example, we will illustrate the experimental steps by employing the most common four-wire standard wiring configuration.

Pre-test Preparation

1. The device under test must be completely powered off and disconnected from other energized equipment, ensuring full discharge of its windings.
2. The test casing must be reliably grounded, with a grounding resistance ≤4 Ω.
3. Remove all external connection wires of the winding under test to ensure the measurement is limited to the winding's own resistance.

Testing Process

I. Wiring

1. All non-test windings of the transformer under test must remain open-circuit; they must not be short-circuited or grounded. Ensure sufficient safety insulation distance between the leads to prevent induced interference during DC testing caused by changes in core magnetic flux.
2. Connect terminals C1 and P1 to one end of the winding under test, and terminals C2 and P2 to the other end, ensuring proper contact of the clamps.
3. Apply firm friction to the wiring contact points to remove the oxide layer and ensure reliable contact.

Reference Table for Choosing Wiring Methods

II. Begin operating the equipment

1. First, connect the instrument's power cord, then press the power switch to start the instrument.
2. After the display light turns on, select the appropriate test current setting based on the estimated resistance value of the winding under test and the instrument's range. The general principle is to use the smallest possible test current while ensuring adequate measurement sensitivity and range, thereby minimizing winding heating and shortening the test stabilization time. (For example, high-voltage windings with higher resistance values typically require smaller currents (e.g., 0.1A–1A), whereas low-voltage windings with very low resistance values necessitate larger currents (e.g., 10A–50A); refer to the instrument manual for specific specifications.)
3. After confirming the wiring is correct, click the "Measure" button; the instrument will automatically begin charging the winding.
4. Wait for the charging progress bar on the screen to complete; then the instrument enters the measurement mode. Once the resistance value stabilizes, the screen will display the final test resistance reading.
5. Click the "Save" or "Print" button as needed to save or print the test data.
6. After completing the test, click the "Exit" button and wait for the instrument to complete its automatic discharge. Only after the discharge notification on the screen disappears should you remove the test cable, and finally turn off the power switch.

III. Precautions During the Testing Process

1. It is strictly prohibited to remove the test lines during testing or before discharge completion. The release of energy stored in the inductive winding generates high voltage, posing a hazard to personal safety.
2. It is strictly prohibited to connect voltage wires and current wires in reverse; otherwise, significant measurement errors may occur, or even damage the instrument.
3. It is strictly prohibited to arbitrarily disconnect the instrument's power supply during testing to avoid safety hazards caused by incomplete discharge.
4. It is strictly prohibited to conduct tests on the equipment under test before it has been completely powered off and fully discharged.
5. When measuring the windings of each phase of the same transformer at the same voltage level, the same test current must be used to avoid introducing unnecessary systematic errors.

Common Issues and Solutions in the Four-Line Method

Characteristics of the Four-Line Method Results

1. The four-wire method measures the actual resistance of the test winding itself, completely eliminating interference from lead wire resistance.
2. The results are unaffected by the length of the test line or the contact resistance of the terminal clamp; even with a longer test line, accuracy is ensured.
3. If the three-phase resistance imbalance exceeds the allowable limit, further inspection should be conducted to check the contact condition of the tap changer and whether the winding connections are loose, in order to locate the defect.
4. For large transformers, after completing the DC resistance test, a certain amount of remanence is introduced into the iron core, which may affect subsequent tests or operational safety; demagnetization treatment can be performed as needed.

Actual product photo