Transformer winding resistance testing

Q: How do I choose a suitable test current?

To get stable readings, the test current should be: above 1% of rated current (to saturate the core and reduce inductance effects) below 15% of rated current (to avoid heating the winding) Most measurements fall comfortably between these limits. Too low a current leads to noisy, inconsistent values; too high a current risks temperature rise and inaccurate readings.

Megger transformer ohmmeters measure winding resistance with fast stabilization, high test currents and automated OLTC diagnostics, giving you clear insight into transformer health.

Winding resistance testing reveals problems you cannot see from the outside: loose connections, deteriorating contacts, shorted turns and issues inside on-load tap changers (OLTCs). These faults increase heating, create imbalance and shorten transformer life.

Megger transformer ohmmeters deliver stable test currents up to 100 A, helping the core reach saturation quickly so measurements settle fast. With automated OLTC measurements and dynamic resistance recording, you can see transition issues and contact wear early—before they turn into failures.

Whether you’re commissioning a new transformer, checking for transport damage or carrying out routine maintenance, winding resistance testing gives you a clear picture of what’s happening inside the active part.

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Industry-leading measuring

Megger’s winding resistance testers deliver stable high-current injection, rapid core saturation and repeatable results—even on large MVA units. Our instruments capture smooth current curves, low-noise voltage readings and clean transitions, making subtle changes visible. Automated demagnetization ensures safe energization and reduces the variability that often affects resistance tests.

Accuracy and speed

Fast stabilization matters. Megger technology injects enough current to minimize inductance effects, helping the current settle quickly so you get reliable results without waiting. Clear connection diagrams, guided workflows and automatic temperature compensation help streamline the process, reducing retests and shortening substation downtime.

Safety first

Winding resistance testers are designed for safe field use, with automatic discharge, interlocks, and alerts for incorrect connections. Shielded leads, clear prompts and built-in demagnetization protect both the operator and the transformer.

Predictive maintenance — from static to dynamic OLTC testing

Winding resistance is essential for OLTC condition assessment. Megger supports both: Static resistance measurement — checks for loose joints, poor connections, thermal damage and imbalance. Dynamic Resistance Measurement (DRM) — captures the OLTC transition in real time, revealing worn contacts, slow transitions, intermittent behaviour and resistor-bridge issues. DRM provides insight you cannot get from resistance alone. Trends over time help plan maintenance before tap changer problems lead to outages.

Frequently Asked Questions

When should winding resistance measurements be carried out?

Winding resistance tests should be performed:

after delivery, to detect transport or handling damage

during commissioning, to create baseline values

during routine maintenance (one of the most important periodic tests)

following faults, overloads or tap changer problems

whenever unusual heating or imbalance is suspected

Many transformer issues cause a change in DC winding resistance, making this an essential diagnostic tool.

Is a “ripple test” the same as dynamic resistance measurement (DRM)?

No. A ripple test measures how much current dips during a tap change and expresses it as a percentage. It gives a general indication of transition behaviour but not detailed timing or resistor values.

DRM, on the other hand, records current and/or voltage continuously during the tap change. This reveals:

contact timing

transition resistor condition

slow or sticking tap operations

intermittent or noisy transitions

DRM is a much richer diagnostic.

How do I choose a suitable test current?

To get stable readings, the test current should be:

above 1% of rated current (to saturate the core and reduce inductance effects)

below 15% of rated current (to avoid heating the winding)

Most measurements fall comfortably between these limits. Too low a current leads to noisy, inconsistent values; too high a current risks temperature rise and inaccurate readings.

How do you test the resistance of a transformer?

Winding resistance is measured by injecting DC into a transformer winding and recording the stabilized voltage and current. Key steps include:

connect the test leads according to the diagram

energize the selected test current

allow the current to stabilize

record the resistance value

compare to nameplate, baseline or sister-unit values

(if OLTC-equipped) perform dynamic measurements during tap transitions

demagnetize the core after testing

Correct setup, saturation and demagnetization are essential for repeatable results.

Find out more about our transformer resistance testing systems

Megger’s winding resistance and OLTC diagnostic tools help you assess transformer condition confidently—whether you’re commissioning, troubleshooting or performing routine maintenance. Our experts can help you choose the right instrument for your test strategy.