The selection of the current for the DC resistance tester is essentially a trade-off between measurement speed, signal stability, and preventing overheating while ensuring measurement accuracy. In simple terms, for large transformers or low-resistance measurements, a high current is suitable; for small equipment, high resistance, or easily overheating windings, a low current is preferable.
Core trade-off between high current and low current
High current
Advantages: Fast measurement speed, stable readings, can quickly magnetize the iron core of large transformers to eliminate inductive interference, making the measurement values stable and quick; when measuring micro-resistances in the milliohm range, the signal-to-noise ratio and resolution are higher, and the measurement accuracy is better.
Disadvantages / Risks: If the measured resistance is too large or the measurement time is too long, the thermal effect of the current will change the resistance value, introducing measurement errors.
Low current
Advantages: Can effectively prevent overheating, avoid changes in small resistance values due to high current, and ensure the accuracy and authenticity of the measurement results; suitable for measuring high-resistance windings or contacts, more friendly for portable equipment.
Disadvantages / Risks: For large inductive equipment, low current is difficult to saturate the iron core quickly, and the measurement values are prone to fluctuate for a long time, with a longer waiting time for stability.
Choose the test current based on application scenarios
During actual use, the test current can be determined by combining the type, capacity, and resistance range of the measured equipment.
Select based on transformer capacity
1000kVA and below: recommended 2A – 5A
2000kVA – 6000kVA: recommended 5A – 20A
6000kVA – 10000kVA: recommended 20A – 30A
60000kVA and below: recommended 30A – 50A
60000kVA – 100000kVA: recommended 40A – 60A
100000kVA and above: recommended 50A – 100A
Special techniques: When measuring the low-voltage side of a transformer with a delta connection, the high-voltage winding can be connected in series to the current, increasing the excitation ampere-turns, accelerating iron core saturation, and significantly reducing the testing time.
Select based on estimated resistance value
Less than 50 mΩ: choose 10A, 20A or higher current, suitable for low-voltage windings, copper bars, contacts
1 mΩ – 10 Ω: choose 5A – 10A, common range
1 Ω – 50 Ω: choose 1A, 200mA, suitable for high-voltage windings
50 Ω – 20 kΩ: choose 1mA – 40mA, suitable for high-resistance windings
Greater than 20 kΩ: choose less than 5mA / 1mA, suitable for insulation or extremely high resistance checks
Theoretical reference: You can choose the transformer’s rated current’s 2% – 10% as the test current. This range can ensure core saturation and control the heating effect.
Practical tips and suggestions
Make good use of automatic modes: If unsure about the range of the measured resistance, you can first use the instrument’s automatic mode for a preliminary measurement, and the device will automatically match the appropriate current.
Pay attention to temperature correction: The resistance of windings will change with temperature. It is recommended to use an instrument with a temperature conversion function, converting the measured values to the standard temperature (such as 75°C) for convenient data comparison.
Remember safety operation: After testing transformers and other inductive equipment, wait for the instrument to complete discharge. Wait until the discharge indicator light goes out or the discharge prompt is given before removing the test leads to avoid electric shock and instrument damage.
The core of current selection is to match the test object. If there is a specific measured equipment, provide relevant information to obtain more accurate current recommendations.
Post time: Apr-10-2026