A connection can still conduct electricity and yet have enough resistance to create wasted energy, abnormal heating or unreliable equipment operation.

Low-resistance testing helps identify small changes in electrical joints, conductors, contacts, bonds and windings before those changes become serious failures.

A digital low resistance ohmmeter, commonly called a DLRO or micro-ohmmeter, is designed to measure resistance values that are too small for an ordinary digital multimeter to assess reliably.

Typical applications include:

• Busbar joints
• Circuit-breaker contacts
• Switches and relays
• Motor and generator windings
• Transformer taps
• Battery straps
• Cable joints
• Grounding and bonding connections
• Rail bonds
• Lightning-protection conductors
• Welded electrical connections
• Wind-turbine bonding paths

Megger’s current low-resistance range includes portable 2 A and 10 A instruments, heavy-duty DLRO models and higher-current equipment for applications such as circuit-breaker contact testing. The correct instrument depends on the resistance range, required test current, asset type and applicable procedure.

For Canadian utilities, mines, rail operators, manufacturers, renewable-energy facilities and industrial maintenance teams, periodic low-resistance testing can support earlier fault detection and more dependable asset maintenance.

What Is a Low-Resistance Measurement?

A low-resistance measurement is generally a measurement below approximately 1 ohm.

At this level, ordinary test-lead resistance, probe contact resistance and standing voltages can significantly distort the result. Thermal electromotive forces may also be present where dissimilar metals meet.

A standard multimeter may be useful for basic continuity checks, but continuity alone does not show whether a connection has increased from, for example:

• 20 micro-ohms to 80 micro-ohms
• 0.2 milliohms to 1 milliohm
• 10 milliohms to 50 milliohms

Those changes may still look like a short circuit to a conventional multimeter, but they can be operationally significant when large currents pass through the connection.

A DLRO is designed to measure these small values with higher accuracy, resolution and test current.

Why Small Resistance Changes Matter

Electrical power is converted into heat at a resistance according to the relationship:

Power = Current² × Resistance

Because current is squared, a small increase in resistance can cause a much larger increase in heat when the circuit carries substantial current.

For example, a loose busbar joint, deteriorated battery connection or corroded switch contact may still allow the equipment to operate. However, the increased resistance can generate localized heating, accelerate deterioration and eventually cause failure.

Megger identifies corrosion, loose joints, contamination, vibration, fatigue, temperature cycling and reduced contact area as common causes of increasing electrical resistance.

Low-resistance testing can help identify:

• Loose bolted connections
• Corroded contact surfaces
• Degraded welds
• Broken or frayed conductors
• Poor bonding
• Damaged switch contacts
• Uneven motor or transformer winding resistance
• Deteriorating battery interconnections
• Changes caused by vibration or thermal cycling

The purpose is not simply to find a connection that is completely open. It is to identify deterioration while current can still flow through the component.

How a Digital Low Resistance Ohmmeter Works

A DLRO applies a known DC test current through the item under test and measures the resulting voltage drop.

The instrument then calculates the resistance.

For accurate low-resistance measurements, the instrument commonly uses a four-terminal Kelvin connection.

Four-Terminal Kelvin Measurement

A four-terminal system uses two separate lead pairs:

• Current leads apply the test current.
• Potential leads measure the voltage drop across the component.

Because almost no current flows through the potential leads, their resistance has very little effect on the measured result.

This helps remove the resistance of the test leads and lead contacts from the final measurement. Megger describes true four-wire Kelvin testing as essential for stable low-resistance measurements and for reducing the effects of lead resistance, contamination and external influences.

The potential probes should normally make contact inside the current-injection points so the instrument measures the voltage drop across the intended section.

A poor lead arrangement may include additional joints, conductor length or probe-contact resistance in the result.

Forward and Reverse Test Current

Standing voltages and thermal EMFs can introduce errors into very small resistance measurements.

Some DLRO modes apply current in one direction and then reverse it. The instrument averages the results to reduce the effect of those standing voltages.

Megger’s DLRO10, DLRO10X, DLRO10HD and DLRO10HDX ranges include modes that use current reversal for this purpose.

A unidirectional test may be faster, but it can be more affected by thermal EMFs in connections involving dissimilar metals.

Why Use a DLRO Instead of a Digital Multimeter?

A digital multimeter is useful for:

• Basic continuity tests
• General resistance measurements
• Checking whether a conductor is open
• Electrical troubleshooting over ordinary resistance ranges

A DLRO is better when the technician must measure very small resistance changes accurately.

Key differences can include:

Megger specifically warns that simple continuity testing is sometimes used for applications that require the greater accuracy, current and four-terminal configuration of a DLRO.

A low reading on a multimeter may show that continuity exists. It may not show whether the connection has degraded enough to overheat under operating load.

Common Low-Resistance Testing Applications

Busbar and Cable Connections

Busbars and high-current cable joints rely on clean, secure contact surfaces.

Resistance may increase because of:

• Loose bolts
• Incorrect torque
• Oxidation
• Corrosion
• Contamination
• Vibration
• Thermal cycling
• Reduced contact area

Comparing equivalent joints can help identify one connection with an abnormal reading.

Measurements should be taken consistently across the same physical section. Moving the potential probes can change the conductor length included in the result.

Circuit Breakers and Disconnect Switches

Contact-resistance testing can reveal deteriorated or contaminated contacts.

However, many circuit-breaker procedures call for test currents greater than the 10 A supplied by a DLRO10HD. Megger identifies higher-current instruments such as the DLRO100, MOM and MJÖLNER ranges for demanding circuit-breaker contact applications.

Do not assume that a 10 A instrument is suitable for every breaker simply because it can display a micro-ohm result.

Follow:

• The breaker manufacturer’s procedure
• The applicable maintenance standard
• The required test current
• The asset owner’s acceptance criteria

Motors and Generators

A DLRO can be used to measure winding resistance and compare phases or coils.

Differences may indicate:

• Loose connections
• Open conductors
• Poor joints
• Winding damage
• Unequal conductor paths
• Tap or termination problems

Temperature strongly affects copper winding resistance. Readings taken from a warm motor cannot be compared directly with cold readings unless temperature is considered.

For inductive assets, the reading may take time to stabilize as current builds in the winding. Megger includes an inductive mode on the DLRO10HD and DLRO10HDX for measurements on assets such as motors and generators.

Megger also warns that the DLRO10HD and DLRO10HDX are not intended for every large inductive circuit, including large power transformers.

Transformers and Tap Changers

Low-resistance measurement may help evaluate:

• Primary windings
• Secondary windings
• Tap positions
• Tap-changer contacts
• Internal connections
• Phase-to-phase consistency

Transformer testing can involve significant inductance and stored energy. Instrument selection and procedure must match the transformer size and winding configuration.

The DLRO10HD may be suitable for some smaller inductive measurements, but large power-transformer winding testing may require dedicated transformer winding-resistance equipment.

Battery and UPS Connections

Battery straps and terminal connections must carry substantial current with minimal voltage drop.

High resistance can develop because of:

• Loose connections
• Corrosion
• Acid vapour
• Contaminated surfaces
• Poor welds
• Thermal expansion and contraction

A DLRO can help compare strap and terminal resistance across a battery system. Megger lists UPS battery straps and battery welds among common low-resistance applications.

Testing must follow the battery manufacturer’s procedure and account for the risk of high available fault current.

Grounding and Bonding Connections

Low-resistance testing can be used to verify conductive continuity through:

• Equipment bonding conductors
• Structural bonds
• Ground-grid connections
• Lightning-protection bonds
• Rail bonds
• Wind-turbine bonding paths

This is not the same as measuring resistance to earth.

A DLRO measures the resistance of a conductor, joint or bond. An earth-resistance tester measures the performance of the grounding electrode system relative to the surrounding soil.

The two tests answer different questions and should not be treated as interchangeable.

Rail Systems

Low-resistance testing is used on rail bonds, return-current paths, cable joints and grounding connections.

Poor rail or structural bonds can contribute to:

• Signal-system problems
• Power loss
• Lightning-protection issues
• Increased step-and-touch voltage risk
• Unreliable traction-current return paths

Megger specifically identifies rail bonds, cable joints and structural grounding connections as railway applications for low-resistance testing.

Wind Turbines

Wind turbines contain numerous bonding and lightning-protection paths.

Potential test locations can include:

• Tower sections
• Nacelle-to-tower connections
• Nacelle-to-hub connections
• Control panels
• Lightning-protection conductors
• Machine supports
• Weather mast connections

Megger lists wind-turbine bonding and lightning-protection paths among common DLRO applications.

For Canadian wind sites, portable battery operation and rugged outdoor construction may be important because tests can be performed in remote locations and difficult environmental conditions.

Megger DLRO10HD and DLRO10HDX

The original JM Test article focuses on the Megger DLRO10HD.

Megger’s current product range includes both:

• DLRO10HD
• DLRO10HDX

Both are heavy-duty 10 A digital low resistance ohmmeters designed for workshop, laboratory and field use.

Main Capabilities

Megger currently lists the following core capabilities:

• Up to 10 A test current for circuits up to 250 mΩ
• 1 A for measurements up to 2.5 Ω
• Test durations up to 60 seconds
• High- and low-output-power selection
• Rechargeable battery or mains operation
• Five test modes
• Backlit display
• IP54 protection while operational
• IP65 protection with the lid closed
• Protection against inadvertent connection to voltages up to 600 V without blowing a fuse
• Controls designed for use while wearing gloves

The current official page describes the DLRO10HDX as adding memory for up to 200 results and PowerDB download capability. It also states that its CAT III 300 V rating applies when the optional terminal cover is fitted.

High- and Low-Power Testing

The DLRO10HD and DLRO10HDX offer selectable high- and low-power outputs.

Megger explains that this can help reveal different types of condition.

Low-Power Testing

A lower-power test may be more effective for identifying:

• Surface contamination
• Corrosion
• Weak low-current contacts
• Unreliable connections that a higher current might temporarily break through

In some contaminated connections, a high test current may heat or electrically break through the contaminant and produce a reading that appears better than the connection’s normal low-current performance.

High-Power Testing

A higher-power test can help reveal:

• Small contact points
• Frayed conductors
• Rough mating surfaces
• Weak conductive paths
• Resistance changes caused by localized heating

If only a small part of the connection is carrying the current, a higher-power test may heat that path and cause the displayed resistance to change.

The chosen power level should reflect the asset and the condition being investigated.

Megger DLRO10HD Test Modes

Bidirectional Mode

The instrument applies current in both directions and averages the readings.

This helps reduce error from standing voltages and thermal EMFs.

Use this mode where maximum measurement confidence is more important than obtaining the fastest possible result.

Unidirectional Mode

Current is applied in one direction only.

This mode is faster, but it may be more affected by thermal EMFs or standing voltages, particularly where dissimilar metals are involved.

Automatic Mode

Testing begins automatically when suitable contact is made.

This can speed repetitive testing where many similar joints or components must be measured.

Continuous Mode

The instrument repeatedly updates the measurement until the test circuit is broken or the test is stopped.

This can be helpful when:

• Adjusting a joint
• Watching a changing resistance
• Checking contact stability
• Monitoring a component during movement or heating

Inductive Mode

Inductive mode is used for assets such as motors and generators where the current and voltage require time to stabilize.

The operator waits until the result becomes stable before ending the test.

Inductive equipment can retain stored energy. Follow the instrument’s discharge indication and the approved asset-testing procedure before disconnecting leads.

DLRO10HD vs DLRO10HDX

The two models share the same fundamental measurement capability. The DLRO10HDX is the more appropriate option when result storage and data download are important.

DLRO10HD vs DLRO10 and DLRO10X

These instrument names are similar but refer to different product designs.

DLRO10 and DLRO10X

Megger’s DLRO10 and DLRO10X are lighter portable micro-ohmmeters that:

• Supply up to 10 A DC
• Measure from 0.1 µΩ to 2,000 Ω
• Automatically select suitable test current
• Use forward and reverse current
• Are intended for portable field and laboratory use

The DLRO10X adds manual current selection, onboard result storage and real-time PC downloads.

DLRO10HD and DLRO10HDX

The heavy-duty versions emphasize:

• Rugged field construction
• Higher noise immunity
• Stable bench or ground operation
• High- and low-power modes
• Long-duration 10 A tests
• Battery or mains operation
• Weather-resistant housing

Megger recommends the DLRO10 or DLRO10X for many general continuity and bonding measurements and the DLRO10HD for more demanding field environments.

The best model depends on whether portability, broad measurement range, ruggedness, output power or result storage is the main priority.

How to Obtain Repeatable Low-Resistance Results

The exact test procedure depends on the asset, but reliable measurements generally require consistency.

1. De-Energize and Isolate the Asset

Low-resistance tests are normally performed on de-energized equipment.

Apply the approved lockout procedure and verify the absence of voltage before connecting the instrument.

Although the DLRO10HD range includes protection against accidental voltage connection, that protection is not permission to test energized equipment.

2. Inspect the Test Surface

Remove or account for:

• Dirt
• Oxidation
• Paint
• Oil
• Loose hardware
• Corrosion
• Unstable probe contact

The goal is not necessarily to alter the asset before every condition test. However, the technician must understand whether the measurement includes surface contamination or the intended metal-to-metal connection.

3. Use the Correct Lead Configuration

Connect the current and potential leads in the correct Kelvin arrangement.

Keep potential connections inside the current connections so the measurement covers only the intended section.

4. Select the Appropriate Test Mode

Choose bidirectional, unidirectional, continuous, automatic or inductive mode according to the asset and test objective.

5. Use Comparable Test Current and Power

Changing the current or power level can change the observed condition, particularly where contamination, heating or weak contact points are present.

Trend tests should use the same relevant settings whenever possible.

6. Allow Inductive Measurements to Stabilize

Motor, generator and coil measurements may take time to reach a stable value.

Do not record the first number displayed unless the procedure specifically requires it.

7. Record Temperature

Conductor resistance changes with temperature.

Record the asset and ambient temperature, particularly when comparing:

• Motor windings
• Generator windings
• Transformer windings
• Long conductors
• Seasonal outdoor measurements

8. Record the Physical Test Points

A small change in probe location can add or remove conductor length or additional joints.

Photographs, diagrams or labelled test points can improve repeatability.

How to Interpret DLRO Results

There is no single universal resistance value that applies to every joint, winding or contact.

Results should be compared with:

• Manufacturer data
• Commissioning measurements
• Previous maintenance readings
• Equivalent phases
• Similar components
• Project specifications
• Applicable standards
• Temperature-corrected expectations

Megger recommends paying close attention to repeatability and trending, because changes from the initial or previous measurements can provide early evidence of deterioration.

Comparative Testing

Comparative testing is useful where several components should be similar.

Examples include:

• Three motor phases
• Three breaker poles
• Equivalent busbar joints
• Multiple battery straps
• Matching rail bonds
• Repeated tap positions

One value that is significantly different deserves investigation even when no universal pass/fail limit is available.

Trend Testing

A gradual increase over time may indicate:

• Corrosion
• Loosening
• Fatigue
• Thermal damage
• Reduced contact area
• Contamination
• Conductor deterioration

Seasonal temperature changes can also affect readings, particularly on outdoor assets. The records should include enough context to distinguish environmental variation from actual degradation.

Sudden Change

A large unexpected increase may indicate:

• Loose hardware
• Broken strands
• Failed welds
• Damaged contacts
• Corrosion
• An incorrect probe position
• Poor test-lead contact

Repeat the measurement and check the connections before declaring the asset defective.

Canadian Safety Considerations

Low-resistance measurement is generally performed on de-energized equipment, but the work can still involve:

• Stored inductive energy
• Battery fault current
• Multiple power sources
• Backfeed
• Adjacent energized equipment
• Large conductive systems
• Incorrect isolation
• Accidental test-lead contact

Canadian work should follow:

• The employer’s electrical safety programme
• Approved hazardous-energy control procedures
• Absence-of-voltage verification
• Manufacturer instructions
• Site-specific risk assessment
• Applicable provincial, territorial or federal requirements
• CSA Z462 where relevant

CSA Z462:24 is the current Canadian workplace electrical safety standard and is harmonized with the Canadian Electrical Code and CSA Z460 hazardous-energy control principles.

The instrument’s input-protection specification does not replace de-energization, isolation or safe-work procedures.

Choosing the Right Low-Resistance Ohmmeter

Before selecting a DLRO, consider:

Required Test Current

A 2 A or 10 A instrument may be suitable for:

• Bonds
• Cable joints
• Windings
• Battery straps
• Relays
• Switches
• Smaller contacts

Some breaker and high-current contact applications may require 50 A, 100 A, 200 A or more, depending on the governing procedure.

Resistance Range

Confirm the expected resistance and required resolution.

An instrument intended for micro-ohm contact tests may differ from one needed for winding measurements extending into ohms or kilohms.

Inductive Capability

Motors, generators, coils and transformers require the instrument to charge the inductance and manage stored energy safely.

Portability

Consider whether the tester will be used:

• On a production bench
• Inside machinery
• In a substation
• On rail infrastructure
• Inside a wind turbine
• At remote Canadian sites

Environmental Protection

IP-rated construction may be important for field, utility and outdoor work.

Data Storage

Choose a model with memory and reporting capability when the maintenance programme requires:

• Digital asset records
• Trend graphs
• Formal reports
• Work-order integration
• Traceable result history

Calibration

Confirm that the instrument’s calibration is current and suitable for the customer’s quality system.

Do not copy the US article’s calibration and accreditation promises to the Canadian page unless JM Test Systems Canada confirms the applicable Canadian laboratory, scope and certificate terms.

Practical Takeaway

Low-resistance testing identifies electrical deterioration that a basic continuity test may miss.

A Megger DLRO applies a controlled DC current and uses a four-terminal Kelvin connection to measure very small resistance values while minimizing the influence of test leads and contact resistance.

Testing can help detect:

• Loose connections
• Corroded joints
• Damaged contacts
• Poor bonds
• Winding problems
• Weak battery straps
• Deteriorating busbar connections
• Changes caused by vibration, fatigue and thermal cycling

The Megger DLRO10HD and DLRO10HDX deliver up to 10 A, support high- and low-power testing and include five test modes for field, workshop and laboratory applications. The DLRO10HDX adds result storage and PowerDB download capability.

The reading should not be judged in isolation. The strongest maintenance decisions come from repeatable test points, recorded temperature, comparable settings and historical trend data.

JM Test Systems Canada can support maintenance teams with digital low resistance ohmmeters, Megger DLRO equipment, Kelvin test leads, equipment rentals and calibration services. Exact models, accessories, calibration documents and Canadian availability should be confirmed before ordering.