Buyer's Guide: Choosing the Right Micro-Ohmmeter

What is a Micro-Ohmmeter?

A micro-ohmmeter (also called a low-resistance ohmmeter or DC resistance tester) is a precision instrument designed to measure extremely low electrical resistance values—typically from microhms (μΩ) to milliohms (mΩ). Unlike standard multimeters that struggle with measurements below 1Ω, micro-ohmmeters use specialized circuitry and four-wire (Kelvin) measurement techniques to deliver accurate readings where contact resistance and test lead impedance would otherwise overwhelm the measurement.

Who Needs a Micro-Ohmmeter?

Power Utility Professionals use micro-ohmmeters to test transformer winding resistance, verify busbar connections, and measure contact resistance in circuit breakers and disconnect switches—critical preventive maintenance that identifies loose connections, degraded contacts, and potential failure points before they cause outages.

Electrical Contractors and Technicians rely on these instruments for grounding system testing, lightning protection verification, and quality control of high-current connections in substations, data centers, and industrial facilities.

NETA-Certified Testing Firms require micro-ohmmeters that meet InterNational Electrical Testing Association (NETA) specifications for acceptance and maintenance testing of electrical power equipment throughout North America.

Manufacturing Quality Control engineers use micro-ohmmeters to verify bond resistance in aerospace assemblies, test relay contacts, measure PCB trace resistance, and ensure proper connections in battery packs and electric vehicle components.

Telecommunications and Data Center operators test grounding networks, cable resistance, and bonding connections to ensure signal integrity and protect sensitive equipment from ground potential differences.

Key Selection Criteria

1. Test Current Range

The test current you need depends on your application:

• 5-10A models (like the UT620T) work well for general electrical testing, small transformer windings, and moderate current connections
• 100-200A models (like the UT625A/B) are essential for high-voltage circuit breaker contact resistance, large transformer windings, and heavy-duty busbar connections

Higher test currents break through surface oxidation and film resistance on contacts, providing more reliable measurements that reflect actual operating conditions. UNI-T offers adjustable current settings with 5A or 10A steps, letting you match the test current to your specific application.

NETA Compliance Note: NETA MTS and ATS standards specify minimum 100A DC test current for circuit breaker contact resistance testing. The UT625A (100A) meets this requirement, while the UT625B (200A) exceeds it for high-voltage applications.

2. Measurement Resolution and Range

Look for instruments offering:

• 0.1μΩ resolution for precision work on low-resistance connections
• Range up to 1MΩ or beyond to handle both low-resistance and moderate-resistance measurements without changing instruments

UNI-T micro-ohmmeters provide 0.1μΩ resolution across ranges from 0.1μΩ to 1MΩ (UT620T) or 2000mΩ (UT625A/B), with typical accuracy of ±0.4% reading ± 0.04% full scale.

3. Four-Wire (Kelvin) Measurement

This is non-negotiable for accurate micro-ohm measurements and is required by NETA standards. Four-wire testing uses separate current and voltage connections:

• Current connections (I+, I-) force a known current through the device under test
• Voltage connections (U+, U-) measure only the voltage drop across the test specimen

This separation eliminates test lead resistance and contact resistance from the measurement, which is critical when measuring resistances that may be smaller than the lead resistance itself. All UNI-T micro-ohmmeters feature proper four-wire Kelvin measurement.

4. Portability and Power Options

Field testing demands:

• Rugged, portable cases that protect the instrument and organize cables
• Dual power options: AC mains for workshop use and internal rechargeable batteries for field work

UNI-T models feature both AC power (up to 150W output) and high-capacity lithium-ion batteries (14.8V/5000mAh) that enable extended field operation. The UT620T weighs 7.1kg with a compact case design, while the UT625A/B offers up to 60 seconds of continuous testing per run.

5. Data Management Features

Modern testing workflows and NETA documentation requirements demand:

• Built-in thermal printers for immediate hard-copy records
• Internal memory (up to 9,999 records on UT620T, 200 records on UT625A/B)
• PC connectivity via USB for downloading data, generating Excel reports, and real-time analysis
• Automatic, manual, and circular save modes
• Timestamp, temperature, and test parameter recording for complete documentation

6. Safety Features

When working with high-energy circuits, safety is paramount:

• Automatic discharge circuits that safely dissipate stored energy after testing inductors and transformers
• Audible discharge alarms that prevent disconnection while energy remains
• Overheat protection with ventilation systems
• Back EMF protection when testing inductive loads
• High-voltage reverse polarity warnings

All UNI-T micro-ohmmeters include comprehensive safety systems.

7. Display and User Interface

A 7-inch color LCD touchscreen (standard on both UT620T and UT625A/B) provides:

• Clear visualization of measurement data
• Easy menu navigation
• Real-time environmental temperature display
• Test status indicators
• Comparison functions (Pass/Fail against preset values)

North American Testing Standards Compliance

UNI-T micro-ohmmeters are designed to meet the requirements of major electrical testing standards used throughout North America:

NETA Standards (InterNational Electrical Testing Association)

NETA provides the primary testing standards for electrical equipment acceptance and maintenance testing:

• NETA ATS (Acceptance Testing Specifications) - for commissioning new electrical equipment
• NETA MTS (Maintenance Testing Specifications) - for ongoing preventive maintenance programs

IEEE Standards

• IEEE C57.12.90 - Transformer testing standards
• IEEE 115 - Generator testing standards

National Codes

• NFPA 70 (National Electrical Code) - Grounding and bonding requirements
• NFPA 780 - Lightning protection system testing

Application-Specific Standards and Instrument Recommendations

1. Circuit Breaker Contact Resistance Testing

Standards: NETA MTS Section 7.5 / NETA ATS Section 7.5

Requirements:

• Minimum test current: 100A DC for low and medium voltage breakers
• Recommended: 100-200A DC for high-voltage breakers (>15kV)
• Four-wire Kelvin measurement required
• Measurements on each pole independently
• Contact resistance should not vary more than 25% between poles

Best Fit Instrument:

UT625A (100A) - Meets NETA minimum requirements for all low and medium voltage breakers up to 15kV class

• Range: 0.1μΩ to 1000mΩ
• Resolution: 0.1μΩ
• Accuracy: ±0.4% reading ± 0.04% full scale

UT625B (200A) - Exceeds NETA requirements for high-voltage circuit breakers (>15kV)

• Range: 0.1μΩ to 2000mΩ
• Resolution: 0.1μΩ
• Higher current breaks through surface films on heavily-used contacts
• Recommended for utility substations and industrial power systems

Key Features for Compliance:

• Adjustable test duration (10-60s) meets manufacturer specifications
• Pass/Fail comparison function against typical values (10-500μΩ)
• Built-in printer documents date, time, current, resistance, and pass/fail status
• Phase-to-phase comparison capability

2. Transformer DC Winding Resistance Testing

Standards: NETA MTS Section 7.13 / NETA ATS Section 7.13 / IEEE C57.12.90

Requirements:

• Four-wire measurement required
• Test current sufficient to overcome transformer inductance
• Temperature correction to 75°C or 85°C for comparison to manufacturer data
• All tap positions tested on tap-changing transformers
• Phase-to-phase comparison within 5% for three-phase units
• Readings must be within ±5% of manufacturer data (new) or ±3% of baseline (maintenance)

Best Fit Instrument:

UT620T (10A) - Ideal for distribution transformers up to 1000kVA

• Range: 0.1μΩ to 1MΩ
• Resolution: 0.1μΩ
• Handles both low-resistance primary windings and higher-resistance secondary/tertiary windings

Key Features for Compliance:

• Built-in temperature conversion to 75°C and 115°C - eliminates manual calculations and reduces errors
• Adjustable test time (10-60s) allows inductive windings to stabilize
• Automatic discharge protection - safely dissipates stored inductive energy (critical safety feature)
• Real-time ambient temperature display when using temperature probe
• Storage for 9,999 test records enables long-term trending

Why Temperature Compensation Matters: Copper resistance changes approximately 0.4% per degree Celsius. A transformer winding measured at 25°C vs. 35°C can show a 4% difference—enough to fail NETA's ±3% maintenance criterion. Built-in temperature correction ensures accurate comparisons regardless of ambient conditions.

3. Busbar and Bolted Connection Testing

Standards: NETA MTS Section 7.1.1 (Busway) / Section 7.15.3 (Switchgear/Switchboards)

Requirements:

• Minimum 10A test current (higher currents recommended for large bus connections)
• Four-wire measurement across each joint
• Detect loose/improperly torqued connections, corrosion, or oxidation
• Variation between similar joints typically <25%

Best Fit Instrument:

UT625A/B (100-200A) - High test current provides superior accuracy on busbar joints

• 100-200A current forces through surface oxidation to measure true connection resistance
• 0.1μΩ resolution detects subtle differences indicating developing problems
• Pass/Fail comparison against typical thresholds (10-75μΩ depending on material and size)

Typical Acceptance Values:

• Bolted copper bus joints: <50μΩ
• Bolted aluminum bus joints: <75μΩ
• Compression connections: <25μΩ

4. Grounding System Testing

Standards: NETA ATS Section 7.10 / NETA MTS Section 7.10 / NFPA 70 (NEC) / NFPA 780

Requirements:

• Grounding electrode conductor resistance verification
• Equipment grounding conductor continuity
• Bond resistance between metallic systems (<1Ω per NEC)
• Lightning protection down conductor continuity (<0.5Ω per 30m per NFPA 780)

Best Fit Instrument:

UT620T - Wide range and unique wire length calculation capability

• Range: 0.1μΩ to 1MΩ covers both low-resistance bonds and moderate-resistance grounding paths
• Wire length measurement mode - calculates conductor length from resistance and known resistivity
• Constant-voltage 12V mode for very long or high-resistance conductors
• Verifies as-built conditions against design specifications

Unique Advantage: The wire length calculation feature is particularly valuable for verifying lightning protection conductor installations where you need to confirm the installed conductor length matches design specifications without physically measuring.

5. Motor and Generator Winding Testing

Standards: NETA MTS Section 7.9 (Motors) / Section 7.8 (Generators) / IEEE 115

Requirements:

• Four-wire measurement of each phase winding
• Temperature correction to standard reference
• Phase-to-phase comparison within 1-3%
• Trending over time to detect insulation breakdown precursors

Best Fit Instrument:

UT620T - Temperature compensation and data storage ideal for rotating machinery

• 10A test current suitable for medium-sized motors (up to several hundred HP)
• Temperature correction to 75°C reference standard
• Data storage enables baseline comparisons and trending
• Phase imbalance >2% indicates potential problems

Documentation and Record-Keeping Compliance

NETA Requirements:

• Complete test records with date, time, environmental conditions, instrument calibration status
• Comparison to previous readings and manufacturer data
• Temperature correction documentation
• Professional test reports in industry-standard formats

How UNI-T Instruments Comply:

Built-in Thermal Printer:

• Immediate hard-copy records with all required information
• Timestamp, ambient temperature, resistance values, test current, pass/fail status
• No need to return to office before documentation

Data Management:

• Internal memory: 9,999 records (UT620T) or 200 records (UT625A/B)
• Automatic, manual, or circular save modes
• USB connectivity for data download

PC Software:

• Generates professional Excel reports meeting documentation standards
• Real-time data analysis
• Historical trending capabilities
• Customizable report templates

Calibration Verification:

• Product self-test mode using standard reference resistors (750μΩ for UT625A, 375μΩ for UT625B)
• Verifies instrument accuracy between formal calibration cycles
• Maintains compliance with quality programs
• Recommended: Annual NIST-traceable calibration (standard for NETA-certified firms)

Special Considerations by Application

Transformer Testing

When testing transformer DC winding resistance:

• Choose models with adjustable test times (10s to 60s) to allow inductive windings to stabilize
• Higher currents (10A minimum, 100A+ preferred for large transformers) provide better accuracy
• Temperature conversion functions (like the 75°C and 115°C presets on UT620T) let you compensate for ambient temperature differences

The UT620T can test transformers up to 1000kVA, while the UT625A/B is ideal for high-voltage circuit breakers and large transformers.

Circuit Breaker Contact Resistance

Power system maintenance standards often specify 100A or 200A test current for circuit breaker contact testing:

• UT625A: 100A maximum, 0.1μΩ to 1000mΩ range - meets NETA minimum requirements
• UT625B: 200A maximum, 0.1μΩ to 2000mΩ range - exceeds requirements for demanding applications

Both models provide the stepping current control (5A/10A increments) needed to comply with testing protocols and manufacturer specifications.

Grounding System Testing

For lightning protection systems and grounding networks:

• Look for wire length measurement functions that calculate conductor length from resistance measurements
• Constant-voltage modes help test high-resistance grounding paths
• Temperature compensation ensures consistent readings across seasons

High-Current Connections

When testing busbars, switchgear connections, and large bolted joints:

• Higher test currents (100-200A) are essential for breaking through oxidation and contact films
• These films can create misleadingly high readings with low test currents
• The difference between a 10A test and a 100A test on the same connection can be 50% or more
• Only high-current testing reveals the true operational resistance

Quick Selection Guide

Your Primary Application	Testing Standard	Recommended Model	Key Reason
Circuit Breakers (≤15kV)	NETA MTS 7.5	UT625A (100A)	Meets NETA 100A minimum requirement
Circuit Breakers (>15kV)	NETA MTS 7.5	UT625B (200A)	Exceeds standard, handles heavily-used contacts
Transformer Windings	NETA MTS 7.13 / IEEE C57.12.90	UT620T (10A)	Built-in temperature conversion to 75°C/115°C, auto discharge
Busbar Connections	NETA MTS 7.1.1, 7.15.3	UT625A/B (100-200A)	High current breaks through oxidation
Grounding Systems	NETA 7.10 / NFPA 70, 780	UT620T	Wire length calculation, wide range
Motors & Generators	NETA MTS 7.9, 7.8 / IEEE 115	UT620T	Temperature compensation, trending capability
General Electrical Maintenance	Multiple	UT620T	Most versatile for varied applications
Utility Substation Work	Multiple NETA Standards	UT625B (200A)	Meets highest current requirements

Why Choose UNI-T Micro-Ohmmeters?

NETA Standards Compliance: Four-wire Kelvin measurement, appropriate test currents, temperature compensation, and comprehensive documentation capabilities meet NETA ATS and MTS requirements across multiple equipment categories.

Accuracy and Reliability: True four-wire Kelvin measurement with 0.1μΩ resolution and ±0.4% accuracy provides confidence in critical safety measurements. This precision enables detection of subtle changes that indicate developing problems before they cause failures.

Field-Ready Design: Rugged cases, dual power systems, and built-in printers mean you can work anywhere without returning to the shop for documentation. Lithium-ion batteries provide sufficient power for full-day field testing campaigns.

Professional Data Management: USB connectivity and PC software turn raw measurements into professional reports that meet documentation requirements for safety inspections, NETA testing programs, and preventive maintenance. Excel export simplifies integration with existing maintenance management systems.

Safety-First Engineering: Comprehensive protection systems and clear visual/audible warnings protect both operator and equipment. Automatic discharge protection and audible alarms are particularly critical when testing transformers and other inductive equipment that stores energy.

Temperature Compensation (UT620T): Built-in conversion to 75°C and 115°C reference temperatures eliminates manual calculations that are error-prone and time-consuming. This feature alone can save hours on large transformer testing projects while ensuring accurate comparisons.

Affordable Professional Performance: UNI-T delivers the precision and features of instruments costing significantly more, making proper low-resistance testing accessible to smaller contractors and facilities that previously couldn't justify the investment. NETA-level performance without premium pricing.

Model Comparison Summary

Feature	UT620T	UT625A	UT625B
Maximum Test Current	10A	100A	200A
Current Adjustment	40mA to 10A (8 steps)	5A to 100A (5A/10A steps)	5A to 200A (5A/10A steps)
Resistance Range	0.1μΩ to 1MΩ	0.1μΩ to 1000mΩ	0.1μΩ to 2000mΩ
Resolution	0.1μΩ	0.1μΩ	0.1μΩ
Accuracy	±0.1% rdg ± 0.1% fs (constant current)	±0.4% rdg ± 0.04% fs	±0.4% rdg ± 0.04% fs
Temperature Conversion	75°C, 115°C (built-in)	Manual calculation	Manual calculation
Wire Length Measurement	Yes	No	No
Data Storage	9,999 records	200 records	200 records
Weight	7.1 kg	~7 kg	~7 kg
Best For	Transformers, motors, generators, grounding systems, general maintenance	LV/MV circuit breakers, medium busbars	HV circuit breakers, large busbars, utility substations
NETA Applications	MTS 7.13 (Transformers) MTS 7.9 (Motors) MTS 7.10 (Grounding)	MTS 7.5 (Circuit Breakers ≤15kV) MTS 7.15.3 (Switchgear)	MTS 7.5 (Circuit Breakers >15kV) MTS 7.1.1 (Busway) Heavy-duty applications

Getting Started

When ordering, consider:

1. What's your highest test current requirement?
   • Transformer and motor testing: UT620T (10A)
   • Circuit breaker and busbar testing: UT625A (100A) or UT625B (200A)
2. Do you need temperature conversion?
   • Essential for transformer testing per NETA and IEEE standards
   • UT620T includes built-in 75°C and 115°C conversion
3. Do you need wire length measurement?
   • Valuable for grounding system and lightning protection verification
   • Only available on UT620T
4. Standard accessories included:
   • Four-wire Kelvin test leads with heavy-duty clamps
   • Self-calibration reference resistors
   • USB cable for PC connectivity
   • Thermal printer paper
   • Rugged carrying case
   • AC power cord
5. Calibration considerations:
   • All instruments include self-test capability for field verification
   • NIST-traceable calibration recommended annually for NETA compliance
   • Consider purchasing calibration services at time of order

Real-World Applications

Utility Substation Maintenance: A utility company testing 138kV circuit breakers uses the UT625B (200A) to verify contact resistance on all three phases. The 200A test current breaks through any oxidation on the silver-plated contacts, and the 0.1μΩ resolution easily detects the 50-150μΩ typical values. Built-in printer provides immediate documentation for maintenance records, and phase-to-phase comparison quickly identifies any problems (readings should be within 25% per NETA).

Industrial Transformer Testing: An electrical contractor performing commissioning tests on a new 1000kVA distribution transformer uses the UT620T. The built-in temperature conversion automatically corrects measurements taken at 28°C ambient to the 75°C reference temperature specified by the manufacturer. With data storage of up to 9,999 records, baseline readings are saved for comparison during future maintenance testing per NETA MTS requirements.

Data Center Grounding Verification: A data center operator uses the UT620T's wire length measurement feature to verify that the lightning protection down conductors meet NFPA 780 requirements. By measuring the DC resistance and inputting the conductor size, the instrument calculates the actual installed length—confirming it matches the design specifications without having to physically measure conductors that run from roof to ground level.

Manufacturing Quality Control: An electrical equipment manufacturer uses the UT625A to verify contact resistance in every low-voltage circuit breaker before it leaves the factory. The Pass/Fail comparison function is set to the specification limit (typically 100μΩ), and the instrument immediately indicates whether each breaker passes. The data is uploaded via USB to the quality management system for traceability.

Support and Training

UNI-T provides:

• Comprehensive user manuals with step-by-step procedures
• PC software for data analysis and report generation (free download)
• Technical support for application questions
• Firmware updates to enhance functionality

For NETA-certified testing firms and utilities, proper training on micro-ohmmeter use is essential. While the instruments are designed for intuitive operation, understanding the theory behind four-wire measurement, test current selection, and temperature compensation ensures accurate results and proper interpretation.

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UNI-T's micro-ohmmeters provide professional-grade DC resistance measurement capability at prices that make proper testing accessible across industries. Whether you're maintaining utility infrastructure per NETA standards, performing commissioning tests on new installations, verifying manufacturing quality, or ensuring safety in telecommunications and data center grounding systems, these instruments deliver the accuracy, documentation, and reliability your work demands—backed by compliance with North American electrical testing standards.