In transformer maintenance, commissioning, and routine diagnostics, Winding Resistance Testing is instrumental in detecting potential hazards such as winding breakage, loose joints, and poor tap changer contact. Meanwhile, Transformer Turns Ratio (TTR) testing can effectively identify inter-turn short circuits, winding displacement, and off-nominal tap positions. Both tests serve as critical early warning mechanisms for faults. During Factory Acceptance Testing (FAT), they verify the compliance of winding configurations, conductor connections, and tap changer assemblies—timely eliminating manufacturing defects such as poor welding, incorrect turn connections, and turn deviations to guarantee product eligibility before shipment.
Consequently, these two tests are the most fundamental and vital among all transformer evaluation items, positioning them at the very top of the testing sequence. A failure in either Winding Resistance Testing or TTR indicates inherent physical defects at the manufacturing level (e.g., miscalculated turns or defective welding). This renders the transformer unqualified to proceed with subsequent high-voltage and performance tests.
Below is a curated list of the world's top 8 manufacturers of transformer Winding Resistance and TTR test equipment. Renowned for their exceptional measurement precision and long-term industrial reliability, these brands have become the benchmark choice for power utilities and transformer OEMs globally, delivering precise, code-compliant data to support your equipment commissioning, operation, and maintenance.
Founding Year: 1889
Current Development Status:
Founded in London, UK, in 1889 as Evershed & Vignoles, the company is the inventor of the insulation resistance tester (megohmmeter). During World War I and World War II, it served as a key supplier of military-grade electrical testing equipment. Since the 1990s, the company has acquired several small-to-medium European and American testing brands, including AVO, Biddle, and Multi-Amp. In 2000, it consolidated its global manufacturing facilities, establishing its global headquarters in Dover, UK, and its North American headquarters in Pennsylvania, USA.
Today, it stands as the world's longest-standing enterprise in electrical testing instrumentation. Ranking No. 1 globally in the electrical testing industry, the company is the global benchmark for insulation testing. It is a contributing member to IEC and ANSI testing standards and serves as the universal reference brand for high-voltage testing instruments, effectively securing its position as a Tier-1 supplier to national grids worldwide.
Core Business Scope: High-voltage test equipment, cable fault location, protection relay testing, battery diagnostics, insulation testing, and transformer analytics.
The company specializes in full-category predictive/preventive maintenance testing for power equipment, relay protection calibration, cable fault location, insulation testing, comprehensive transformer testing, earth/ground resistance testing, and power quality analysis.
Its operations span R&D, instrument manufacturing, and operations and maintenance (O&M) testing services, tailored for the power grid, power generation, rail transit, petrochemical industries, and renewable energy power plants.
Key Characteristics & Advantages: Unrivaled brand history; premium ruggedized field designs. Outstanding advanced software capabilities, multi-phase testing safety, and deep industry standard compliance.
Flagship Products:
Turns Ratio: TTR300 Series (Three-Phase Transformer turns ratio tester)
DC Winding Resistance: MTO300 Series (Automated Six-Winding Transformer Ohmmeter)
Website: https://www.megger.com
Official Email: sales@megger.com
Founding Year: 1984
Current Development Status: Top-tier global leader in power system testing and diagnostics. Headquartered in Klaus, Austria, it maintains 22 international offices and supports electrical grid utilities and manufacturers in over 170 countries.Founded in Vienna, Austria, in 1984 by a team of visionary power engineers, the company is the world’s premier brand for high-end protection relay test sets, starting its journey with a dedicated focus on relay testing. It expanded into North American and Asian markets in the 1990s and, since 2005, has broadened its portfolio to include transformer, partial discharge (PD), and power quality products. Operated under a family-and-capital holding structure, the company is deeply committed to high-end, precision electrical power testing. Today, it stands as the preferred choice for Extra-High Voltage (EHV, 220kV/500kV) and Ultra-High Voltage (UHV) testing applications.
Core Business Scope: Advanced testing and diagnostic solutions for primary and secondary substation assets, including power transformers, protection relays, circuit breakers, instrument transformers, and rotating machines.The company's core business covers power system protection relay calibration, primary equipment high-voltage testing, online and offline partial discharge (PD) detection for GIS (Gas-Insulated Switchgear) and transformers, power quality monitoring, and renewable energy grid-integration testing. Its operations primarily cater to high-end precision laboratories, electric power research institutes, and ultra-high voltage (UHV) maintenance sectors within the power grid.
Key Characteristics & Advantages: Industry gold standard for multi-functional, modular testing architecture. Exceptional software integration (Primary Test Manager), advanced automated tap-changer testing, built-in safety discharge features, and industry-leading partial discharge and insulation diagnosis.
Flagship Products:
Turns Ratio: TESTRANO 600 (Three-phase power transformer test system), CPC 100 + CP SB1 (Multi-functional test system with automated switch box)
DC Winding Resistance: TESTRANO 600 / CPC 100 (Features rapid three-phase simultaneous winding resistance measurement and automatic core demagnetization)
Website: https://www.omicronenergy.com
Official Email: info@omicronenergy.com
Founding Year: 2000
Current Development Status: Prominent European developer operated by IBEKO Power AB.Headquartered in Stockholm, Sweden, it supports power utilities and field service companies in over 100 countries.
Core Business Scope: Substation test solutions focusing on transformers, circuit breakers, and industrial battery testing.
Key Characteristics & Advantages: Renowned for inventing radically lightweight, portable, and mathematically advanced testing solutions using cutting-edge power electronics technology.3-year standard warranty.
Flagship Products:
Turns Ratio: TRT Series (TRT63A, TRT33A Three-Phase Ratiometers)
DC Winding Resistance: TWA Series (TWA40D, TWA25A Three-Phase Winding Ohmmeters)
Website: https://www.dv-power.com
Official Email: sales@dv-power.com
Founding Year: 1991 (Acquired by Doble Engineering Company in 2017)
Current Development Status: Operating as a specialized subsidiary under the Doble/ESCON Group brand. Based in Ontario, California, USA, serving international grid infrastructure operators.Established in Texas, USA, in 1991, the company was founded with the aim of developing microprocessor-driven, rugged, and durable industrial-grade power testing equipment. Around 2017, it was acquired by the Doble Group—a pioneer in the field of power diagnostics and testing services—thereby further integrating into its global ecosystem.
Core Business Scope: Substation diagnostic equipment for circuit breakers, power transformers, and vacuum bottles.
Key Characteristics & Advantages: Known for producing highly reliable, automated instruments with massive integrated memory storage, quick graphic printouts, and robust field construction.
Flagship Products:
Turns Ratio: ATRT-03-S2, TRF250A (Multi-Phase Transformer turns ratio testers)
DC Winding Resistance: TRM403, TRM-20 (Transformer Winding Resistance Meters)
Website: https://www.doble.com
Official Email: customerservice@doble.com
Founding Year: 1976 (AEMC) / 1893 (Parent company Chauvin Arnoux Group)
Current Development Status: Key brand of the Chauvin Arnoux Group. Headquartered in Dover, New Hampshire, USA, acting as a major distributor across North and South American industrial markets.
Core Business Scope: Professional electrical power quality, ground resistance, insulation testers, data loggers, and micro-ohmmeters.
Key Characteristics & Advantages: Backed by French precision engineering. Excellent data management integration via proprietary DataView software; durable field enclosures.
Flagship Products:
Turns Ratio: DTR Model 8510 (Digital Transformer Ratiometer)
DC Winding Resistance: Model 6240 / 6255 (Rugged 10A micro-ohmmeters for winding tests)
Website: https://www.aemc.com
Official Email: sales@aemc.com
Founding Year: 1995
Current Development Status: Leading independent high-tech manufacturer based in Bremgarten, Switzerland, with an established international subsidiary network (Raytech USA).
Core Business Scope: High-accuracy automated test systems and single/multi-phase test instruments for the international power industry.
Key Characteristics & Advantages: 100% Swiss-engineered quality.Extraordinary high precision, sleek interface designs, modular software (T-Base), and dynamic resistance measurement capabilities.
Flagship Products:
Turns Ratio: TR-Mark III (Three-Phase Turns Ratio Meter), Mini-ATOS
DC Winding Resistance: WR50-12 / WR14 Series (Winding Resistance Meters up to 50A)
Website: https://www.raytech.ch
Official Email: welcome@raytech.ch
Founding Year: 2013
Current Development Status: Rapidly growing manufacturer based in Epsom, Surrey, United Kingdom, with a rising commercial presence across European and Latin American markets.
Core Business Scope: Substation test instrumentation, transformer analyzers, and circuit breaker diagnostic instruments.
Key Characteristics & Advantages: Focuses on highly competitive cost-to-performance instruments built according to strict international standards. Features intuitive touchscreen color graphical interfaces.
Flagship Products:
Turns Ratio: TURA-01, TURA-03 (Three-Phase Turns Ratio Testers)
DC Winding Resistance: WINRES-20, WINRES-50 (winding resistance testers with demagnetization)
Website: https://www.hightest.co.uk
Official Email: info@hightest.co.uk
Founding Year: 2009
Current Development Status: Kingrun is one of China's earliest manufacturers of transformer testing equipment; indeed, China's very first fully automatic DC winding resistance tester was developed by Kingrun. Leveraging its reputation for stable quality and exceptional cost-effectiveness, Kingrun currently commands nearly one-third of the Chinese market for DC resistance and turns ratio testers, and has established itself as a trusted, long-term supplier to Hitachi, Siemens, and ABB.
Core Business Scope: Specialized diagnostic tools for power transformers, distribution networks, and insulating oil testers.
Key Characteristics & Advantages: Exceptional manufacturing agility. Pioneer of handheld palm-sized 3-phase TTR testers with ultra-long battery runtimes. Highly competitive pricing, fast customization, and robust technical support.
Flagship Products:
Turns Ratio: JYT-A, Palm Turns Ratio Tester (1.6 kg portable unit)
DC Winding Resistance: JYR-10S, JYR-20S / 40S / 50S (Three-channel auto-magnetizing resistance testers)
Website: https://www.kritester.com
Official Email: info@kritester.com
