Micro-ohmmeters in the power and industrial sectors – measuring low resistances and preventing failures

Micro-ohmmeters are essential instruments for measuring very low resistances in electrical power systems. They enable the early detection of loose connections, overheated contacts and hidden defects before these issues result in failures or fire hazards. This article explains how the four-wire Kelvin measurement method works, why a high test current is essential, and how Sonel MMR-6500 and MMR-6700 micro-ohmmeters support diagnostics of circuit breakers, busbars, cables, earthing systems, transformers and motors.

1. Introduction

Modern electrical power systems, both in industry and in public infrastructure, depend on the reliability of electrical connections. Even minor variations in resistance at these connection points can lead to serious consequences, including energy losses, overheating and, in extreme cases, failures or fires. For this reason, low-resistance measurements are critical, and micro-ohmmeters are indispensable tools for such applications.

What is a micro-ohmmeter? Advantages over a standard ohmmeter

A micro-ohmmeter is a dedicated measuring instrument designed for the precise determination of very small resistance values, typically ranging from micro-ohms (µΩ) to milliohms (mΩ). Unlike conventional ohmmeters, which are adequate for measuring higher resistance values, micro-ohmmeters employ advanced measurement techniques to meet the challenges inherent in ultra-low resistance measurements.

The primary advantage of these instruments is their ability to inject a high test current for a short duration, enabling highly accurate readings. For instance, the Sonel MMR-6700 micro-ohmmeter can perform measurements with test currents of up to 200 A.

Image 1. Sonel MMR-6700 micro-ohmmeter

The repeated emphasis on “high current” reflects not merely a design choice but a core engineering necessity. According to Ohm’s law (U = I × R), for extremely low resistance values (in the µΩ or mΩ range), a sufficiently high test current (I) is essential to generate a voltage drop (U) large enough to be measured accurately. If the current is too low, the resulting voltage drop becomes negligible and highly susceptible to electrical interference, thermal electromotive forces (EMFs ) and limitations of the measurement circuit itself.

The use of high current addresses these issues by ensuring a high signal-to-noise ratio and adequate resolution in micro-ohm measurements. The wide range of applications – from busbar measurements, through sensitive transformer windings, to mobile field testing –imposes differing requirements in terms of current range, measurement algorithms and instrument design. This flexibility enables manufacturers to align the instrument’s functionality with specific tasks. Sonel S.A. focuses on the versatility of its solutions, a feature that distinguishes the MMR-6500 and MMR-6700 micro-ohmmeters on the market.

How does it work? Understanding the four-wire Kelvin method and its role in measurement accuracy

Micro-ohmmeters achieve high measurement accuracy by using the four-wire Kelvin method. This technique effectively eliminates the effects of lead resistance and contact resistance, which would otherwise cause significant measurement errors.

In the Kelvin method, the test current is applied to the tested object via one pair of current leads, while the voltage drop is sensed by a separate pair of voltage-sensing leads connected directly at the test points. Because the voltage-sensing leads carry virtually no current and their effect can be ignored, the resistance of the leads and contacts does not affect the measurement result. The resistance value is determined using the following relationship:

R = U / I

Figure 1. Four-wire method for low-resistance measurement

The superiority of the four-wire measurement method over the two-wire technique is particularly apparent in very low resistance measurements, where even small lead resistances can cause substantial errors.

Figure 2. Traditional method. Measurement error due to lead resistance: 8.7%

Moreover, achieving high measurement accuracy also requires the elimination of other interference factors. Direct current (DC) measurements are sensitive to thermoelectric voltages generated at junctions of different metals, which appear as offset voltages. To eliminate this effect, Sonel MMR-6500 and MMR-6700 micro-ohmmeters perform measurements with current flowing in both directions.

In addition, these instruments employ advanced compensation algorithms and combine signal-processing techniques, ensuring stable and reliable results even under demanding industrial conditions.

The selection of test current type (DC or AC) depends on the properties of the tested object. DC instruments are used for testing windings, contacts and resistors, whereas AC methods are applied when measuring the impedance of electromagnets, capacitors or batteries. Differences between AC and DC measurement results reflect the distinct physical properties of the tested objects and must be interpreted accordingly.

2. Why low-resistance measurements are essential

Low-resistance measurements performed using micro-ohmmeters go far beyond a standard technical procedure. They play a crucial role in ensuring the safety, energy efficiency and durability of electrical power systems. Crucially, these measurements make it possible to:

identify hidden defects,
prevent failures and fires,
verify the quality of mechanical connections.

Figure 3. Resistance measurement result shown on the Sonel MMR-6700 display

Identification of hidden defects and degradation of electrical connections

Micro-ohmmeters make it possible to identify minor defects that are invisible to the naked eye, including micro-discontinuities, insufficient crimping, corrosion, microcracks in solder joints and progressive deterioration of contact quality.

Increased resistance at such connections causes power losses in the form of heat (the thermal power dissipated in a resistance is expressed as P = I²R). This, in turn, may cause overheating, insulation damage and, in extreme cases, fires. This phenomenon is particularly critical in direct current installations, such as photovoltaic systems, for example at MC4 connectors.

The underlying cause-and-effect relationship is simple:

a slight increase in resistance → higher losses → higher temperature → material degradation → risk of failure or fire.

Early detection of irregularities enables prompt corrective action before equipment damage or unplanned downtime occurs.

Prevention of hazards: overheating, energy losses, failures and fires

Regular micro-ohmmeter measurements provide an effective, proactive approach to mitigating electrical risks. Loose electrical connections are a common issue and may lead to:

overheating of conductors and terminals,
sparking and electric arcing,
short circuits,
fires.

Data from the United States show that between 2014 and 2018 electrical failures were the second leading cause of residential fires, accounting for 13% of all fires and 18% of fire-related fatalities. Loose electrical connections were identified as the primary contributing factor.

By carrying out systematic measurements, it is possible to identify gradual increases in resistance that signal an impending failure. This enables maintenance and repair activities to be planned in advance, before a critical condition occurs, minimising the risk of fire, equipment damage and downtime.

Verification of mechanical connection integrity through resistance measurement

Electrical resistance is a reliable indicator of mechanical connection quality: the lower and more stable the resistance, the better the connection. Micro-ohmmeters are used for:

testing cable joints and busbars,
assessing the quality of welds and contacts,
verifying equipotential bonding connections.

An increase in resistance may indicate corrosion, microcracks, inadequate crimping or insufficient contact pressure, even when the connection appears visually sound.

This measurement is a non-destructive method, allowing mechanical integrity to be assessed without interfering with the component. This is especially important for equipment operating in the field or in sensitive systems.

Image 2. Inspection of the resistance of a bolted connection

Contribution to system safety and reliability

Micro-ohmmeters contribute to the overall safety of electrical power systems across the entire power chain, from generation to final consumption. They help to:

detect and prevent overheating, short circuits and fires,
ensure compliance with safety standards and regulatory requirements,
collect data confirming the correct technical condition of equipment.

In many cases, micro-ohm measurements are not only technically recommended but are also formally required, as part of quality documentation, compliance procedures or audit strategies.

3. Applications of micro-ohmmeters

Micro-ohmmeters are applied across various fields, depending on the type of tested object:

resistive objects, including contacts, cables, connectors, earthing conductors and welds,
inductive objects, including transformer windings, motor windings and instrument transformers.

Each of these categories requires a slightly different measurement approach.

Resistive objects

These components exhibit negligible frequency dependence, with resistance being the dominant part of the impedance. Such objects are typically measured using direct current (DC).

Switchgear and circuit breakers (contact resistance)

Measurement of contact resistance in circuit breakers and switchgear is critical for:

ensuring correct operation,
minimising power losses,
preventing localised overheating.

Instruments such as the Sonel MMR-6700 enable measurements with test currents of up to 200 A, including on high-voltage (HV) circuit breakers, even when they are earthed on both sides.

Excessive contact resistance can lead to:

local hot spots,
degradation of contact materials,
reduced short-circuit interruption capability,
increased risk of electric arcing and system damage.

Consequently, micro-ohmmeters are an essential tool for assessing the reliability of short-circuit current interruption systems.

Cable joints and busbars

The quality of cable joints and busbar connections has an impact on:

energy efficiency,
prevention of hot spots,
operational safety.

Loose or corroded joints lead to increased resistance and localised heating, accelerating insulation degradation and increasing the risk of failure. Specific acceptance limits apply to tested components; for example, busbar resistance should not exceed 0.1 Ω.

Regularne pomiary pozwalają identyfikować nieprawidłowości, zanim te doprowadzą do nieodwracalnych uszkodzeń.

Wires and cables

Micro-ohmmeters are used for:

verifying conductor continuity,
assessing workmanship quality and detecting damage,
determining conductor length based on its unit resistance.

Deviations from theoretical values may indicate:

changes in cross-sectional area,
mechanical damage,
material defects.

Such tests are valuable during installation, fault diagnosis and throughout the production process.

Earthing systems and equipotential bonding

The purpose is to ensure low impedance for fault currents and to protect personnel by limiting touch and step voltages.

Typical limit values include:

Busbar earthing connection: < 0.1 Ω
Individual equipotential bonding connection: < 1.0 Ω
Artificial earth electrode (I > 500 A): < 5 Ω
Earthing resistance in a TN system: < 30 Ω

Excessive resistance may prevent the safe dissipation of fault current, posing a risk of electric shock.

Welded and soldered connections

High resistance in such joints may indicate:

cracking,
improper material bonding,
internal voids,
corrosion.

Micro-ohm testing provides a non-destructive way to assess the durability and quality of these connections, both in production and during service.

Inductive objects

Components such as windings of power transformers, motors and instrument transformers exhibit inductance, meaning that their impedance is frequency-dependent. Measuring their resistance using direct current requires specialised procedures that consider core saturation phenomena, transient effects and the energy stored in the magnetic field.

Transformer windings

Measuring transformer winding resistance makes it possible to detect:

inter-turn short circuits,
loose connections,
winding deformation.

Instruments such as the Sonel MMR-6500 and MMR-6700 are well suited to these measurements, including applications involving amorphous-core transformers.

Exceeding permissible resistance differences between phases may indicate:

winding faults,
mechanical or thermal issues,
transformer operating asymmetry.

Even minor deviations provide important diagnostic insight and can prevent serious faults.

Electric motors and generators

Similar to transformers, electric motors and generators are exposed to insulation degradation and winding overheating. An increase in resistance may indicate:

localised hot spots,
inter-turn short circuits,
mechanical damage,
reduced efficiency.

Regular measurements combined with trend analysis help optimise maintenance planning and minimise the risk of unplanned downtime. Such measurements are also essential following motor overhauls, for example after rewinding.

Measuring and protection transformers

Current transformers (CTs) and voltage transformers (VTs/PTs) must maintain precise parameters in order to ensure:

accurate revenue metering,
correct operation of protection systems.

Even subtle changes in winding resistance can affect transformation ratios and phase angles, potentially leading to:

metering inaccuracies,
false alarms,
failure of protective devices to operate during short circuits.

In this area, micro-ohmmeters provide an indispensable means of quality control.

Specific aspects of inductive object measurements (core saturation, safety)

Measuring inductive loads using DC current requires:

saturation of the magnetic core at the start of the test,
maintaining a constant test current throughout the measurement,
safe discharge of the stored energy once the test is complete.

The Sonel MMR-6500 and MMR-6700 instruments provide:

controlled current sources,
fast charge/discharge algorithms,
on-screen visualisation of the measurement process,
protection against discharge arcing.

Important measurement guidelines:

The test current should not exceed 10% of the rated current of the tested object.
The measurement should be initiated only after the current has stabilised.
Inductive coils accumulate substantial energy, which can pose a safety risk if the circuit is opened suddenly; consequently, the meter must provide controlled internal discharge.

Image 3. Measurement of motor winding resistance using the Sonel MMR-6700

4. Example limit values and standards

Understanding resistance limit values and applicable standards is crucial for proper result interpretation, although comparative assessment methods can also be used. These requirements vary depending on the type of component, its application and operating conditions.

Circuit breaker contact resistance

Circuit breaker manufacturers specify maximum permissible contact resistance values to ensure safe and efficient operation.

Tabela 1. Wymagania rezystancji styku na przykładzie wyłącznika Schneider Electric Compact NSX

Rated current of the circuit breaker (A)	Maximum permissible contact resistance (µΩ)
100	1800
160	1000
250	500
400	250
630	140

The higher the rated current, the lower the permissible resistance, due to power loss effects (P = I²R). Even a small increase in resistance (R) at high current (I) levels can result in significant heating, potentially leading to failure.

Resistance of busbar connections and earthing systems

Typical values:

Busbars: <0,1 Ω
Artificial earth electrodes (I > 500 A): <5 Ω (including the influence of connections on the permissible value)
Systems with low fault current: <10 Ω (including the influence of connections on the permissible value)
Earthing in TN systems: <30 Ω (including the influence of connections on the permissible value)
High soil resistivity (≥ 500 Ω m): values may be corrected using normative formulae (e.g. ρ/16, ρ/100).

These values vary depending on:

the nature of the load,
the fault current level,
ground conditions.

Permissible resistance values must always be assessed in relation to the specific application; they are not universal.

Transformer winding resistance – permissible differences

For transformer windings, differences between phases are more critical than absolute resistance values.

Table 2. Normative requirements for transformer winding resistance

Parameter	Requirement	Reference
Phase resistance differences	≤ 5% of the average value	PN-E-04700
MV winding voltage differences	≤ 2%	IEC 60076-1
LV winding voltage differences	≤ 4%	IEC 60076-1

Exceeding these limits may indicate defects such as:

winding irregularities,
partial short circuits,
asymmetry,
deformation.

Example: for a transformer rated at 2 W, the primary winding resistance may be approximately 2.2 kΩ, whereas for a 70 W transformer it may be around 23 Ω.

Resistance of equipotential bonding connections

Applicable standards specify a maximum resistance of 1.0 Ω for a single equipotential bonding connection.

This is a critical parameter ensuring:

equipotentialisation,
protection against touch voltage,
personnel safety in the event of a short circuit.

Values above this threshold indicate ineffective equipotential bonding and a serious risk of electric shock.

5. Result interpretation and trend analysis

Interpreting micro-ohmmeter measurements requires more than knowledge of limit values; it also demands an understanding of the measurement context and environmental conditions, combined with the ability to analyse data over time.

Importance of measurement repeatability and temperature compensation

Since electrical resistance varies with temperature:

measurements must be repeatable and within specified accuracy limits (e.g. ±0.2% ±2 digits),
temperature compensation, manual or automatic, should be applied.

Modern micro-ohmmeters (e.g. Sonel MMR-6700, MMR-6500) provide automatic temperature compensation of the tested object, enabling:

comparability of results over time,
reliable assessment of technical condition,
elimination of errors caused by changes in ambient or component temperature.

Comparison with manufacturer data and industry standards

Measured values should be compared against:

manufacturer datasheets,
previous results (e.g. factory reference values or data supplied by the manufacturer),
data obtained from comparable equipment.

Without reference values, the diagnostic value of a measurement is significantly reduced. Even a result that remains “within limits” may indicate early-stage damage if it deviates from the reference value specific to the device!

Use of data for predictive maintenance

Analysis of resistance measurement trends over time makes it possible to:

predict failures,
plan maintenance activities in advance,
minimise unplanned downtime.

Key principles include:

recording and archiving measurement data (e.g. in a database),
performing measurements consistently at the same points on the tested object,
using trend visualisation tools (e.g. time-based charts).

This approach shifts maintenance philosophy from reactive to predictive, helping to avoid costly failures and increase overall system reliability.

Image 4. The large display of the Sonel MMR-6700 ensures fast and clear reading of results

6. Summary

The key role of micro-ohmmeters in modern power engineering and industry

Micro-ohmmeters are essential tools for:

diagnosing the condition of electrical connections,
verifying production quality,
preventing operational hazards.

They enable the detection of hidden defects before these become a risk to personnel, equipment or entire systems. Micro-ohmmeters are used in:

testing of circuit breakers, busbars, cables, earthing systems, transformers and motors,
railway and aviation sectors (earthing connections, control circuits),
maintenance operations and quality control.

Development prospects and the importance of continuous measurement

Modern micro-ohmmeters offer:

touchscreen displays,
Wi-Fi, USB and LAN communication,
integration with data management and documentation systems.

This trend is driving full digitalisation of measurements and their integration with maintenance management systems. As a result, it becomes possible to:

plan service activities,
detect degradation in real time,
document compliance with regulations and standards.

In manufacturing companies, integrating micro-ohmmeters into the production process is a key factor in maintaining manufacturing quality and avoiding product rejects.

Author:
Roman Domański, Sonel S.A.

Bibliografia:

Campbell, R. (2021). Home fires caused by electrical failure or malfunction. National Fire Protection Association. Retrieved November 22, 2025, from https://www.nfpa.org/education-and-research/research/nfpa-research/fire-statistical-reports/home-fires-caused-by-electrical-failure-or-malfunction
Electrical Safety Foundation International. (n.d.). Fire prevention. Electrical Safety Foundation International. Retrieved November 22, 2025.