Megger MTO250 Transformer Ohmmeter

Transformer Ohmmeter (Megger MTO Series Winding Resistance Tester)

A transformer ohmmeter is a high-current, low-resistance test instrument used to measure winding resistance on power and distribution transformers. The Megger MTO250 transformer ohmmeter and the wider MTO range are the test equipment Irish utilities, electrical engineers, and electrical contractors use to confirm that transformer windings, tap-changer contacts, and bushing connections are within specification, on commissioning, after a heat run, and through the working life of the asset.

Winding resistance is one of the most diagnostic measurements in a transformer test plan. A change in measured resistance against the factory data, or against the previous service reading, points directly at loose connections, bad joints, tap-changer wear, shipping damage, or overheating that would otherwise stay invisible until something fails.

What a transformer ohmmeter measures

• Winding resistance on each phase, at every tap position, against the factory test sheet.
• Tap-changer contact resistance through the tap range, picking up worn or carbonised contacts before they trip a transformer protection.
• Bushing and lead connections, where contact resistance can give early warning of overheating risk.
• Comparison between phases, where a sudden imbalance points at a problem in one winding.
• Trended winding resistance measurements across return visits, supporting condition-based maintenance decisions.

Why measurement accuracy and test current matter

Winding resistance values on a power transformer are small, often in the milli-ohm or micro-ohm range. Measuring them accurately needs a stable, high test current that fully magnetises the core and a precise measurement chain that is not pulled around by noise on site. A general-purpose multimeter cannot do this; a dedicated transformer ohmmeter can.

The Megger MTO range is built for that test. Stable DC test current up to the levels needed for large power transformers, dual-channel measurement so two windings can be tested simultaneously where the design supports it, and built-in discharge circuitry to safely discharge the unit under test at the end of each measurement.

Specifications: what to confirm before you order

For full datasheet detail, send us your transformer types, top winding rating, and target resolution and we can confirm the right MTO model before you order.

Target users and applications

• Utilities and DNOs running commissioning, routine maintenance, and fault investigation on power and distribution transformers
• Electrical engineers and test specialists carrying out acceptance testing and trending over time
• Electrical contractors and service providers performing site testing, refurbishment, and outage work
• Maintenance teams in industrial, pharmaceutical, and data-centre estates running reliability programmes
• Renewable site engineers testing wind, solar, and BESS step-up transformers

Typical applications include heat-run testing in the works, on-site acceptance testing during commissioning, periodic winding resistance measurements as part of preventive maintenance, post-fault diagnostics, and tap-changer condition assessment.

As authorised Megger distributor in Ireland, Powerpoint Engineering supports the MTO transformer ohmmeter range with in-house calibration and repair, accessory supply, and operator training for transformer testing teams.

FAQs

Why can a multimeter not measure transformer winding resistance?

Transformer windings are inductive and the resistance values are very small. A general-purpose multimeter cannot deliver enough stable test current to magnetise the core, and its resolution is not fine enough at the milli-ohm range. A transformer ohmmeter delivers the high DC test current and precise measurement chain the test actually needs.

What test current does the MTO use?

It depends on the model and the transformer being tested. Larger power transformers need higher test current to magnetise the core and stabilise the reading. The MTO range covers from low-current models suitable for distribution transformers up to high-current units for large power transformers.

Why does the unit need to discharge after a test?

A transformer winding stores energy in its inductance during the test. The transformer ohmmeter discharges that stored energy safely through built-in circuitry at the end of each measurement so that operators can disconnect the test leads without an arc or shock hazard.

To match the right transformer ohmmeter to your transformer fleet, test current needs, and resolution requirement, request a quote or speak to our team for compatibility advice.