Low resistance problems are easy to miss.

A connection may look tight. A bus joint may look clean. A breaker contact may look serviceable. A ground bond may look complete.

But if resistance is higher than it should be, that connection can heat up, waste energy, reduce system performance and eventually fail.

That is why low resistance testing matters.

The original JM Test article explains that abnormally high resistance can create unwanted heating, safety risk and energy loss. It also notes that low resistance testing is useful in manufacturing, power generation, distribution, motor testing, circuit breakers, bus bars, coils, ground bonds, switches, weld joints, lightning conductors, small transformers and resistive components. 

For Canadian electrical contractors, utilities, industrial maintenance teams, commissioning groups and plant reliability teams, a digital low resistance ohmmeter helps answer a simple question:

Is this connection still electrically sound?

What Is a Digital Low Resistance Ohmmeter?

A digital low resistance ohmmeter is an instrument used to measure very small resistance values.

These are often in the micro-ohm or milli-ohm range.

A normal digital multimeter is usually not the right tool for this type of work because lead resistance, contact resistance and small thermal voltages can distort the result.

Low resistance ohmmeters use a four-wire Kelvin measurement method. Megger explains that four-wire DC measurement removes errors caused by probe lead resistance and contact resistance by moving the voltage measurement points directly to the test piece. Megger also notes that current reversal can help cancel standing EMF effects in the circuit. 

In plain English, the instrument sends current through the test object and separately measures voltage drop across the exact points of interest.

That makes the result more useful when testing very low resistance connections.

Why Low Resistance Testing Matters

High resistance in a power system can create heat.

That heat can damage:

• Bus joints
• Breaker contacts
• Switch contacts
• Cable terminations
• Motor windings
• Transformer tap connections
• Ground bonds
• Welded joints
• Lightning protection paths
• Battery connections
• High-current connectors

The source article explains that poor bond connections can reduce the effectiveness of ground beds and create power quality or surge-related problems. It also notes that degraded bolted connections in power systems can lead to excessive heating or loss of power if left uncorrected. 

Low resistance testing helps find these issues before they become visible failures.

Common Applications for Low Resistance Testing

Bus Bars and High-Current Joints

Bus bars carry large amounts of current.

A small increase in resistance at a bus joint can create serious heating when current is high.

The source JM Test article explains that bus bars and lap joints can degrade from vibration and corrosion, and that measuring resistance across the joint is one of the quickest ways to evaluate the connection quality. 

Low resistance testing can help identify:

• Loose bolted joints
• Corrosion
• Oxidation
• Poor contact pressure
• Damaged surfaces
• Installation problems
• Degradation since the last shutdown

A good test programme should compare readings against baseline data, similar joints and manufacturer or project requirements.

Circuit Breakers

Circuit breaker contacts can degrade over time.

Arcing, pitting and carbon buildup can reduce the live contact area and increase resistance.

The source article notes that arcing at breaker pads can create carbonized layers, reduce contact area, increase resistance and create heating that can reduce breaker efficiency or contribute to failure. It also mentions IEC 62271-100 and ANSI C37.09 as standards to consider for test current requirements when planning breaker tests. 

Low resistance testing can help check:

• Main contacts
• Pole-to-pole consistency
• Contact wear
• Contact contamination
• Poor mechanical alignment
• Maintenance effectiveness after cleaning or repair

For Canadian use, the exact test current, method and acceptance criteria should come from the breaker manufacturer, owner procedure, project specification, applicable standards and qualified test personnel.

Transformer Windings and Tap Connections

Transformer winding resistance testing can help identify problems with windings and tap connections.

The source article notes that transformer winding tests are performed in the factory and periodically in the field, and that tap connections can corrode when not used and may overheat under high current. 

Low resistance testing may help identify:

• Poor tap changer contacts
• Winding connection problems
• High-resistance joints
• Imbalance between phases
• Loose connections
• Corrosion or wear
• Heat-related changes over time

Transformer testing must be done carefully because windings are inductive. The right instrument, procedure, discharge practice and safety controls matter.

Motors and Generators

Low resistance testing can be used on motor and generator windings to identify abnormal resistance patterns.

The source article explains that motor armature testing can help identify shorting between adjacent coils or conductors, and that bar-to-bar testing on DC motor rotors can help identify open or shorted coils. 

Applications may include:

• Armature testing
• Commutator segment checks
• Winding resistance testing
• Squirrel-cage rotor troubleshooting
• Generator winding checks
• Maintenance baselines during shutdowns
• Comparison between phases or coils

Resistance readings are most useful when compared to historical data, phase-to-phase comparisons, temperature-corrected values and manufacturer expectations.

Ground Bonds and Grounding Connections

Grounding and bonding connections must be reliable.

A connection that looks mechanically complete may still have too much resistance for the intended purpose.

The original source article notes that poor connections can reduce the effectiveness of ground beds and cause power quality-related problems or failure during a major surge. 

Low resistance testing can support checks on:

• Ground bonds
• Equipment bonding jumpers
• Ground grid connections
• Lightning protection conductors
• Structural bonds
• Utility and industrial grounding connections
• Critical bonding paths in electrical rooms

In Canada, grounding and bonding checks should be aligned with the project design, CSA C22.1 as adopted in the applicable jurisdiction, owner requirements and AHJ expectations. CSA C22.1:24 applies to electrical work and equipment in installations for buildings, structures and premises, with listed exceptions such as utility installations under regulatory authority and Transport Canada-regulated ship electrical systems.

Cable Reels and Conductors

Low resistance testing can also help estimate remaining conductor length on a reel.

The source JM Test article explains that if a reel tag gives ohms per unit length, the remaining length can be estimated by measuring resistance. If the tag is missing, a known sample length can be measured to establish ohms per length, then used to estimate the remaining reel length. 

This method can be useful for:

• Inventory checks
• Construction planning
• Cable management
• Maintenance shops
• Utility work
• Estimating remaining insulated conductor length

Temperature matters because conductor resistance changes with temperature. For more accurate work, temperature correction may be required.

Low Resistance Testing vs Continuity Testing

Continuity testing only confirms that an electrical path exists.

Low resistance testing evaluates the quality of that path.

A connection can pass continuity and still be a poor connection.

For example:

• A loose bus joint may show continuity.
• A corroded bond may show continuity.
• A breaker contact may show continuity.
• A damaged joint may show continuity.

But under load, that same connection can overheat.

Low resistance testing gives a more useful reading for high-current connections where even a small resistance increase can matter.

Why Four-Wire Kelvin Testing Matters

When measuring very small resistance values, test lead resistance can be larger than the resistance being measured.

That is why a four-wire method is used.

Two leads supply current.

Two separate leads measure voltage drop.

Megger explains that four-wire DC measurement removes errors from probe lead resistance and contact resistance because the high-impedance voltage measurement is taken at the actual test piece. 

This is especially important for:

• Bus joints
• Breaker contacts
• Ground bonds
• Battery straps
• Motor windings
• Transformer windings
• Low-ohm components
• High-current joints

If the connection of the probes is poor, the result can still be unreliable. Surface preparation, probe pressure and test point consistency matter.

Why Current Reversal Matters

Low resistance measurements can be affected by small voltages already present in the test object.

These are sometimes called standing EMFs or thermal EMFs.

Megger explains that reversing the polarity of the test leads and averaging the results can help overcome these effects, and that some DLRO instruments include automatic current reversal. 

This helps improve measurement reliability, especially when testing low resistance values where small voltage errors can affect the result.

Megger DLRO10HD Overview

The Megger DLRO10HD is a 10 A digital low resistance ohmmeter designed for field, workshop and laboratory low resistance measurements.

Megger says the DLRO10HD and DLRO10HDX can deliver 10 A into circuits up to 250 mΩ and 1 A into circuits up to 2.5 Ω. Megger also states that tests can run up to 60 seconds, and that the instruments include high and low output power selection for condition diagnosis. 

Useful features include:

• 10 A maximum DC output current
• 1 A range for higher resistance measurements
• High and low output power modes
• Battery or mains operation
• Rugged case
• IP54 operational rating
• IP65 rating with lid closed
• Current reversal with averaging
• Five test modes
• Large backlit LCD
• Rotary controls that can be used with gloves
• Protection up to 600 V for inadvertent connection to mains
• Live voltage warning light

Megger also notes that the DLRO10HDX adds onboard storage for up to 200 test results and PowerDB download capability. 

High-Power vs Low-Power Testing

The DLRO10HD supports both high and low output power.

This matters because different faults show up in different ways.

Megger explains that low power can help identify problems such as contamination and corrosion, while high power can show weaknesses related to heating. Megger also notes that high current can sometimes “blast” through contamination and produce a good reading even when the connection is unreliable in use, while low current may reveal the issue more readily. 

This is important for Canadian maintenance teams because the “best” test setting depends on what you are trying to find.

Use the manufacturer’s instructions, test plan and asset-specific procedure.

Testing Inductive Loads

Some assets are inductive.

Examples include:

• Transformer windings
• Motor windings
• Generator windings
• Coils
• CT circuits
• Large inductive assemblies

The source JM Test article warns that high DC currents across CTs may saturate CTs and reduce sensitivity to potential faults. It also notes that ripple in the test current can cause circuit breakers to trip and that careful positioning and lead separation can help. 

Megger states that DLRO10HD/HDX instruments can supply 25 W continuously for at least 60 seconds, which can help when measuring resistance with inductance, but it also says these models are unsuitable for testing large inductive circuits such as power transformers. 

This is a key point for the Canadian article:

Do not assume one DLRO is suitable for every transformer, winding or inductive test.

Safety Considerations for Low Resistance Testing

Low resistance testing is often performed on de-energized equipment.

That does not make it automatically low risk.

Before testing, confirm:

• Equipment is properly isolated.
• Lockout is complete.
• Absence of voltage has been verified.
• Stored energy has been discharged.
• The test instrument is suitable for the task.
• The test leads are rated and in good condition.
• The test current will not damage the equipment.
• CTs, relays or controls will not be affected.
• The test plan is approved.
• The worker is qualified for the task.

CSA Z462:24 reorganized requirements around establishing an electrically safe work condition and added that absence of voltage must be verified at each point of work. 

For Canadian content, keep this framing:

Low resistance testing should be planned and performed by qualified workers under the employer’s electrical safety programme, CSA Z462-informed safe work practices, manufacturer instructions and site procedure.

Canadian Code and Standards Context

Low resistance testing can support Canadian electrical maintenance and commissioning, but it does not replace code compliance.

Relevant references may include:

• CSA C22.1 Canadian Electrical Code, as adopted in the jurisdiction
• CSA Z462 workplace electrical safety practices
• Manufacturer test procedures
• Owner maintenance standards
• Utility requirements
• Project commissioning specifications
• IEC or ANSI standards where specified
• NETA standards where specified by the project
• AHJ expectations

CSA C22.1:24 applies broadly to electrical work and electrical equipment in buildings, structures and premises, subject to listed exceptions. 

For Canadian pages, avoid writing the article as if NEC, OSHA or NFPA 70E are the governing Canadian requirements. They may be relevant to US customers or multinational standards programmes, but Canadian implementation should be checked against Canadian code, CSA Z462, the AHJ and the owner’s procedure.

What to Record During Low Resistance Testing

A useful low resistance test record may include:

• Asset ID
• Location
• Equipment type
• Manufacturer
• Model or rating
• Test date
• Technician
• Instrument model
• Instrument serial number
• Instrument calibration status
• Test current
• Test mode
• Test lead setup
• Ambient temperature
• Equipment temperature, where relevant
• Test points
• Measured resistance
• Phase or pole comparison
• Baseline value
• Previous reading
• Pass/fail limit, where applicable
• Notes about cleaning, tightening or repair
• As-found and as-left values, where applicable

The most valuable low resistance results are usually comparative.

One reading by itself is useful.

A trend over time is better.

When to Perform Low Resistance Testing

Low resistance testing may be useful:

• During startup and commissioning
• Before energization
• After installation
• After a shutdown
• After maintenance
• After breaker service
• After bus joint work
• After transformer tap work
• After grounding changes
• After a fault event
• After overheating is found
• During preventive maintenance
• During troubleshooting
• When thermography shows a hot spot
• When power quality problems suggest bonding issues
• When equipment is losing performance

The source article also notes that tests can be made during periodic or annual shutdowns, and that periodic review of test data can identify overheating effects and connection degradation. 

Low Resistance Testing and Thermography

Low resistance testing and thermography work well together.

Thermography can show where heating is happening under load.

Low resistance testing can help confirm whether a connection has higher resistance than expected when equipment is de-energized and isolated.

For example:

• A hot bus joint found by infrared scan may be checked with a DLRO during shutdown.
• A breaker with high contact heating may be checked with contact resistance testing.
• A ground bond issue may be investigated with low resistance testing.
• A repaired connection can be re-tested before return to service.

Thermal images show heat.

Low resistance readings help quantify the connection problem.

Low Resistance Testing and Commissioning

During commissioning, low resistance testing can help confirm installation quality before equipment is placed into regular operation.

It can help identify:

• Loose bus joints
• Poor bolted connections
• Bad breaker contacts
• Improperly made bonds
• Grounding continuity issues
• Poor crimp or compression connections
• Manufacturing or assembly problems
• Shipping or installation damage

This creates a baseline for future maintenance.

If the same equipment is tested later, the team can compare readings and see whether resistance has changed.

Choosing a DLRO for the Job

Before renting or buying a digital low resistance ohmmeter, confirm:

• Resistance range required
• Test current required
• Whether 10 A is enough
• Whether higher current is required
• Whether the asset is inductive
• Whether current reversal is required
• Whether data storage is required
• Whether software reporting is required
• Whether mains power is needed
• Whether battery operation is needed
• Required IP rating
• Required safety rating
• Lead length and probe type
• Kelvin clip or hand probe requirement
• Calibration certificate requirement
• Rental duration
• Canadian availability
• Accessories included

Megger’s DLRO10HD/HDX page lists maximum output current, high/low output power, battery and mains power, CAT III 300 V safety features and product document availability. 

For some applications, a 10 A DLRO is the right tool.

For other applications, a higher-current micro-ohmmeter or specialized transformer winding resistance tester may be required.

JM Test Systems Canada Support

JM Test Canada’s calibration services page lists electrical test equipment calibration capabilities including low ohm resistance testers, insulation testers, hipot testers, earth ground resistance testers, circuit breaker test sets, relay test equipment and transformer test equipment.The same page also describes onsite and mobile calibration lab services and says JM Test, formerly BHD, provides precision test, measurement and calibration solutions for industries across Canada. 

For the Canadian article, avoid copying US-only contact, distributor, rental or A2LA service claims from the source page unless they are confirmed for Canada.

Canadian customers should confirm:

• DLRO model availability
• Purchase or rental availability
• Test lead and probe options
• Calibration certificate details
• Low-ohm tester calibration availability
• Onsite or mobile support
• Turnaround time
• Shipping timeline
• Current Canadian stock
• Whether the exact instrument is suitable for the asset being tested

Common Mistakes to Avoid

Using a Standard Multimeter for Low-Ohm Work

A standard DMM is usually not suitable for micro-ohm or milli-ohm connection testing because lead resistance and contact effects can dominate the reading.

Testing Without a Baseline

Low resistance readings are most useful when compared with previous readings, similar phases, similar poles or manufacturer expectations.

Ignoring Temperature

Conductor resistance changes with temperature. Temperature should be considered when comparing results over time.

Using the Wrong Test Current

Some assets require specific test current values. Circuit breaker testing, for example, may have test current expectations based on the breaker type and applicable test method.

Testing Large Inductive Circuits With the Wrong Instrument

Megger states that the DLRO10HD/HDX is not suitable for testing large inductive circuits such as power transformers.

Forgetting About CT Saturation

The source article warns that high DC currents across a CT may saturate it and reduce sensitivity to faults. 

Not Verifying Absence of Voltage

CSA Z462:24 adds that absence of voltage must be verified at each point of work when establishing an electrically safe work condition. 

Comparing Results Taken at Different Points

Use consistent probe placement, test points, test current and setup if readings will be trended over time.

Not Documenting Repairs

If a joint is cleaned, tightened or repaired, record both as-found and as-left values.

Practical Takeaway

Low resistance testing helps identify connection problems that visual inspection may miss.

It is useful for bus bars, breaker contacts, motor and generator windings, transformer connections, tap changers, ground bonds, cable reels and other low-resistance paths.

The original JM Test article explains that high resistance can cause unwanted heating, energy loss and equipment problems, and that low resistance testing helps identify issues before they lead to apparatus failure. The Megger DLRO10HD is a practical 10 A digital low resistance ohmmeter for many field and workshop applications. Megger states that it can deliver 10 A into circuits up to 250 mΩ, supports high and low output power, includes current reversal modes and can operate from battery or mains power. 

For Canadian teams, the best approach is to match the instrument and test method to the asset, follow CSA Z462-informed safety practices, confirm applicable CSA C22.1 / AHJ context where relevant, document results clearly and keep test equipment calibration current.

JM Test Systems Canada can support low resistance tester calibration and electrical test equipment needs where available. Confirm exact DLRO availability, accessories, calibration certificate, rental terms and service details before booking.