Transformer Turn Ratio Test and Recommended Lead Length

1. Overview
The transformer turn ratio test is one of the core inspection and acceptance tests for power equipment. Its principle is to apply an AC voltage to one winding of the transformer (usually the high-voltage side) and measure the induced voltage on the other winding (low-voltage side). The ratio of these two voltages reflects the actual turns ratio of the transformer windings.

This parameter directly indicates the accuracy of the winding turns and the integrity of the magnetic coupling structure. Any deviation from the design ratio may imply shorted turns, open circuits, incorrect connections, or winding displacement. Therefore, the turn ratio test is essential for ensuring reliable transformer performance and detecting potential winding faults.

2. Why Lead Length Matters in Turn Ratio Testing

Voltage drop and accuracy – Long test leads may introduce extra impedance, causing slight voltage drops that affect the accuracy of low-voltage side readings.
Signal stability – Excessively long or thin leads are more susceptible to electromagnetic interference, leading to unstable measurement values.

Magnetic coupling – The precision of the voltage ratio measurement depends on maintaining clean and symmetrical connections; uneven or twisted long leads may cause phase shift errors.

3. Recommended Lead Lengths：

Recommended Test Lead Length for Turn Ratio Testing

Transformer Type	Recommended Lead Length (per side)	Notes
Small distribution transformers (≤ 630 kVA)	≤ 3–5 m	Keep the tester close to the transformer to minimize lead resistance.
Medium transformers (800–3150 kVA)	≤ 8–10 m	Use leads with a cross-section of ≥ 4 mm².
Large power transformers (≥ 4000 kVA)	≤ 10–15 m	Use 6~10 mm² thick copper leads; if necessary, extend current leads but keep potential leads short.

Practical Recommendations

Short leads first: Always keep test leads as short as possible to ensure voltage ratio accuracy.

Good contact: Ensure clamps or test clips are tightly connected to avoid contact resistance affecting readings.

Balanced wiring: Keep both sides’ leads of equal length to maintain phase consistency.

Environmental considerations: Avoid strong electromagnetic fields and vibration sources near the test area.

Other Related Articles:

The 5 Most Popular transformer turn ratio testers in the World
What is the Difference between "Ratio" and "Turn Ratio" in a Transformer Testing?
How to Choose the Right Turns Ratio of CT/PT?
Why is it so Important to Test Transformer Turn Ratio?
What are the Reasons for Abnormal Voltage Output of Power Transformers?

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