What is micro-ohm measurement and what do you need to know?

A micro-ohm measurement measures resistances between approximately 0.1 µΩ and 5 Ω. That is the range where ordinary multimeters no longer belong. Typical measurement objects:

• contacts in circuit breakers and disconnectors
• busbar connections
• bolts in grounding systems
• shunt resistors and current transformers
• DC resistance of transformer and motor windings

Below the 1 Ω threshold, a multimeter stops being useful. Below about 0.2 Ω, the resistance of your measurement leads is already greater than what you are trying to measure. A micro-ohmmeter (or DLRO, Digital Low Resistance Ohmmeter; Ducter is a Megger brand name) sends a high DC current through the object and measures the voltage drop with separate wires. That gives you an honest resistance value.

Physics is hard

P = I² × R. At 2,000 A through a contact with 50 µΩ resistance, you get 200 W of heat on that one spot. At 200 µΩ, it is already 800 W.

A visual inspection will not catch this. Corrosion is only visible at an advanced stage, and by then the contact has usually already been hot. Measuring is the only way to see early that something is off.

Trending over years predicts failure

One measurement says little. Three consecutive annual measurements creeping from 60 → 120 → 280 µΩ say a lot. That disconnector will not make it to the next maintenance interval. Without baseline measurements, you do not see that pattern coming and you work reactively.

Relevant standards and frameworks

There are a few important references:

• IEC 62271-1, with the measurement of the main circuit resistance (DC, at least 50 A). IEC 62271-100 for HV power circuit breakers refers to it. The manufacturer spec is leading for acceptable values.
• IEEE C37.09-2018, on test procedures for AC HV circuit breakers (DC, at least 100 A).
• IEC 60947 (series), for low-voltage switchgear.
• NEN 1010 and NEN 3140 in the Netherlands, as a framework for installations and operation. They do not prescribe a µΩ measurement themselves.

Without measurement data, an inspection report is incomplete. And without a calibration certificate for the instrument, that measurement data is itself contestable.

This is the crux. If you understand this, you understand why a “cheap micro-ohmmeter” is a contradiction in terms.

Why 2-wire doesn’t work

In a classic resistance measurement, the test current runs through the same cables as the measurement voltage. Measurement cables have 10-50 mΩ per metre of resistance. Contact pins add another 1-10 mΩ per connection. So on a contact of 50 µΩ, you actually measure 50 mΩ of cable plus 50 µΩ of object. That is an error of a factor of 1,000.

What Lord Kelvin came up with

Two cable pairs, each with its own task. One pair carries the test current through the object. The other pair measures the voltage drop directly on the object, just inside the current connections. Because the voltmeter has an input impedance above 10 MΩ, virtually no current flows through those measurement wires, so you do not fall prey to their resistance.

The meter calculates R as U_measured / I_measured. Every modern DLRO works this way.

A small but important detail

The voltage clamps must be inside the current clamps. Reverse them and you also measure the contact resistance of the current clamps. Quality test leads from Megabras and HighTest mechanically enforce this: two insulated contacts per clamp.

Not “more is better”. Too little current and your signal drowns in noise and thermo-EMF. Too much and you bake the object, or roast solder joints. The right answer depends on what you are measuring.

Practical rule of thumb: choose a current at which the voltage drop across your object is at least a few hundred µV to 1 mV. The smaller the signal, the more noise from the electromagnetic field in a substation counts.

This is where micro-ohm measurement becomes interesting for substation maintenance.

During maintenance in an HV installation, a breaker is often earthed on both sides (working earth). If you measure classically, that earth loop short-circuits your current path and you measure the grounding connection instead of the breaker contact. In the past, there was one solution: disconnect one side of the earth. That costs time, creates an unsafe situation (open earth point in an HV environment), and introduces errors on re-connection.

BSG (Megabras) and Dual Ground (HighTest) solve this with an additional current clamp. The principle is elegant: you accept that part of the test current flows away through the earth loop, but you measure that bypassed portion separately.

How it works step by step

1. The 4-wire Kelvin measurement takes place as usual: the current source injects, say, 100 A, and voltage sensors measure the voltage drop across the contact.
2. But because both sides are grounded, part of those 100 A flows away through the earth connection instead of through the contact. Suppose, as an illustration, 30 A goes via earth, 70 A goes through the contact.
3. An additional current clamp is placed around the earth connection. It measures that 30 A return current.
4. The instrument calculates: actual current through the contact = injected current − return current via earth = 100 − 30 = 70 A.
5. Using the measured voltage drop (U) and the actual contact current (I_contact), you correctly compute R = U / I_contact.

Without the earth-current measurement, the instrument would assume 100 A and report too low a resistance (because U/100 < U/70). Measured and compensated with the clamp, you get a value that corresponds to what you would measure if you had disconnected earth.

The function is called BSG at Megabras and Dual Ground at HighTest. The underlying method is similar in both: additional current clamp on the earth connection, plus firmware that computes the compensation. There is no hidden algorithm behind it, just Kirchhoff’s current law applied to a parallel current path.

Which models support this

For HV substation work, you therefore end up with the Megabras MPK-105r or MPK-215r, or the HighTest ARES-200D.

People sometimes ask which of the two is “the best”. That is not a fair question. They are strong in different things, and that is why we carry both.

Megabras

Brazilian brand, active since 1986. Their strong point is the BSG function. The MPK-r line (MPK-105r, MPK-215r) is the workhorse for substation maintenance. The MPK-257 is their lab model, with six selectable test currents up to 10 A and 0.1 µΩ resolution. Comes standard with a built-in printer, Bluetooth, and the MegaLogg 3 reporting software. IP65, 2-year factory warranty.

HighTest Technology

British brand, instruments Made in the UK. Their HARE series is compact (3 kg), has a 4.3″ touchscreen, IP67, and goes up to 300 A test current. The latter is rare in portable instruments. The ARES-200D adds two extra capabilities: Dual Ground mode and dynamic resistance measurement (DRM), where the resistance is recorded during the opening or closing of a breaker contact. That reveals coking (carbon deposition on contacts from arcing), wear, and mis-alignment that a static measurement misses.

How do you choose?

A few handholds:

• BSG for HV maintenance up to 100 or 200 A: MPK-105r or MPK-215r.
• 300 A required, for example for HV power circuit breakers or heavy busbars: HARE-300.
• Dual Ground in a compact form factor with printer: ARES-200D.
• 10 A lab precision for electronics or shunts: MPK-257.
• 100 or 200 A in the field, not grounded, prefer touchscreen and lighter: HARE-100 or HARE-200.
• Complete substation test kit (contact + relay + ratio + timing): SUWI-120.
• Transformer turns ratio and winding resistance: TRAN-203.

If in doubt, schedule a consultation. In 20 minutes we usually figure it out.

Thermo-EMF

Two different metals in contact generate a thermal voltage (Seebeck effect). A copper-iron contact sits around 13 µV per °C, a copper-constantan pair around 43 µV per °C. Sounds small. But at a contact of 50 µΩ and a test current of 10 A, your total voltage drop is 500 µV. Having a few tens of µV of noise on that signal is already a few percent error, quickly increasing if the cables heat unevenly.

Solution: modern micro-ohmmeters automatically measure in both current directions and average the result. Thermo-EMFs then cancel out because they have the same sign in both polarities. Check in the settings that this function is on.

Contact pressure and oxide

A Kelvin clamp on an oxide layer gives readings that jump all over the place. Brush the contact point clean, set the clamp with sufficient pressure (the springs in the clamp are for this), and take three measurements instead of one. If you see 12.1 / 12.3 / 12.0 µΩ, you know your measurement is robust. If you see 11 / 28 / 14, something is wrong with your contact.

Self-heating

At 100 A and above, the measurement object heats up. Copper has a temperature coefficient of about 0.4% per °C, so your measurement drifts upward during the test. Keep the test duration short, under 5 seconds at high currents, wait 30 seconds between repetitions, or use the automatic ramp function if your meter has one.

110 kV disconnector after overhaul

After cleaning and re-greasing, you want to be sure the contacts are within spec. Grab the MPK-215r at 200 A, measure each phase with BSG on, without disconnecting earth. Typically, a contact resistance below 100 µΩ is considered acceptable for a refurbished 110 kV disconnector, but the manufacturer spec remains leading. Real values range from 30 to 150 µΩ depending on type and age.

20 kV busbar system, annual maintenance

Twelve bolted connections on a 20 kV busbar. You want to know which ones have weakened since last year. HARE-200 at 100 A, measure each connection, compare with last time’s baseline. A significant increase (industry practice: 30-50% versus the previous measurement) or a deviation above the manufacturer spec is a signal to re-torque or replace. For absolute comparison between phases, ANSI/NETA MTS-2011 uses as a guideline: the highest phase measurement may not be more than 50% higher than the lowest.

Motor starters in production

Fifty starters, weekly screening desired. MPK-257 at 10 A. Measure the contacts per starter (number depends on configuration), two to five minutes of work per starter, set aside the outliers for the next maintenance round. For small motor starters (contact resistance 0.5-5 mΩ), 10 A gives sufficient voltage drop for a reliable measurement. For large industrial starters (>100 A nominal), 10 A is fine for relative trending, but for a full acceptance test you use a higher test current.

Power transformer, windings

This is NOT a micro-ohmmeter job (see further down why). Grab a WINRES-20B or TRIORES-20. More than 2% deviation between phases is suspicious and reason for further investigation. If you also want to measure turns ratio and magnetising current in addition to winding resistance, the TRAN-203 is more suitable.

Simple decision in two steps.

What is the highest test current you need?

• 10 A and below: MPK-257.
• 100 A: go to the next question.
• 200 A: go to the next question.
• 300 A: HARE-300.
• Combination with ratio / relay / timing: SUWI-120 (120 A contact).

Are you working on grounded HV installations?

• 100 A, yes: MPK-105r.
• 100 A, no: HARE-100.
• 200 A, yes with BSG: MPK-215r.
• 200 A, yes with onboard printer and dynamic resistance measurement (DRM) for breaker diagnostics: ARES-200D.
• 200 A, no: HARE-200.

Measuring transformer windings or doing complete substation tests?

• Winding resistance specifically: WINRES-20B or TRIORES-20.
• Turns ratio + winding resistance: TRAN-203.
• Contact + relay + timing + ratio in one device: SUWI-120.

Really in doubt? We come on-site for a demo. No obligation.

Two pitfalls at the edges of the range.

At the low end: Instruments with a test current of 1 A or less do not, in our view, qualify as micro-ohmmeters. The tolerance and deviation at that current level are too large for the micro-ohm values this article is about. At 1 A on a 50 µΩ contact, your voltage drop is only 50 µV, which is below the noise floor of most instruments.

At the high end: A micro-ohmmeter is NOT a transformer winding tester, no matter how high the test current. DLROs (Digital Low Resistance Ohmmeters) are designed for low-inductance objects: contacts, busbars, shunts. A transformer winding has exactly the opposite characteristics: high inductance, slow stabilisation, a core that must saturate.

Why a DLRO is not suitable for large power transformers

1. The maximum voltage the meter can deliver is too low.
2. The core does not enter saturation.
3. Stabilisation time constant τ = L/R is far too large.
4. Remanent magnetism causes in-rush upon re-energisation.
5. Back-EMF on disconnection.
6. No temperature correction, no three-phase measurement.

In short: it is not a matter of “DLRO not powerful enough”. It is a different class of instrument for a different kind of measurement.

What then?

For both edge cases we offer other instruments. You find them in the substation testers category:

• WINRES-20B / TRIORES-20: dedicated winding resistance testers for transformers. Designed with the specific requirements of transformer measurements: the meter can deliver a high voltage (above 40 V DC, needed to quickly build up current through the winding’s inductance), demagnetisation as a separate test, temperature compensation, three-phase measurement with a single connection on the TRIORES, discharge circuit for safety.
• HighTest TRAN-203: three-phase transformer analyzer for turns ratio (up to 250 V AC), magnetising current and winding resistance (up to 20 A DC).
• HighTest SUWI-120: multi-tester for substations with 120 A DC contact resistance, relay timing, turns ratio, and breaker timing in one unit.

Rule of thumb: measuring only contact resistance → dedicated micro-ohmmeter. Transformer windings → WINRES / TRIORES. Complete substation → SUWI-120.

One thing often forgotten: your meter measures as accurately as its last calibration. Annual calibration is common. A lab working to ISO 17025 must use a defined calibration interval. Always earlier after a drop or damage. After storage outside spec (heat, frost), at least a reference check with a known resistor.

IONIO provides calibration for Megabras and HighTest instruments in the Netherlands, typically within 5 to 10 working days, including certificate. More about our calibration service.

What is the difference between an ohmmeter and a micro-ohmmeter?

A regular ohmmeter works from about 0.1 Ω and uses 2-wire measurement. A micro-ohmmeter works from 0.1 µΩ with 4-wire Kelvin and high DC current. Six orders of magnitude of difference in range. Instruments with 1 A or less of test current do not qualify as micro-ohmmeters, because the tolerance is too large for the micro-ohm values you are looking for.

Is BSG the same as Dual Ground?

Functionally, yes. Megabras calls it BSG (Both Side Grounded), HighTest calls it Dual Ground. Both let you measure on double-grounded contacts without disconnecting earth. For the user it amounts to the same thing.

How much current do I need for a 36 kV breaker?

Typically 100 or 200 A. The manufacturer spec of the breaker is always leading. Rule of thumb: the higher the nominal current of the contact, the more test current you need to get sufficient voltage drop.

Do I really have to disconnect earth for every HV measurement?

No. Not with a BSG or Dual Ground instrument. That is exactly why those modes were developed. With an MPK-105r, MPK-215r, or ARES-200D, you measure safely with both sides grounded.

How often should I calibrate?

Annually is common. Immediately after mechanical damage. In lab environments often required by the quality system. IONIO typically calibrates within 5 to 10 working days, including certificate.

What is an acceptable contact resistance?

Small disconnector: 50-200 µΩ
MV power breaker: 30-150 µΩ per phase
HV power breaker: 20-80 µΩ per phase
Moulded-case LV power breaker: 100-500 µΩ (depending on current rating)
Motor starter (contactor contacts): 500-5000 µΩ per contact
20 kV busbar bolted connection, cleaned: 5-20 µΩ per connection. Light oxide 15-25 µΩ, heavy oxidation 50-200 µΩ: reason to restore the contact first.

Always use the manufacturer spec as guideline, as types, current rating and age vary. As a rule of thumb for relative comparison between phases, ANSI/NETA holds that the highest phase measurement may not be more than 50% higher than the lowest.

Can I also test transformers with a micro-ohmmeter?

As a rule, no. See the section above on why a DLRO is not suitable for that. For transformer windings, use a dedicated tester such as the WINRES-20B or TRIORES-20. For small motor windings (low inductance), you can sometimes get away with a 10 A micro-ohmmeter, but even then we recommend a separate tester for reliable results. For insulation testing you need yet another instrument: an insulation resistance meter up to 5-15 kV.

What is the difference between the HARE series and the ARES-200D?

The ARES-200 (static only) has been discontinued; for static measurements, choose the HighTest HARE series (100, 200 or 300 A). The ARES-200D measures statically and adds two functions: Dual Ground (measuring on double-grounded contacts) and dynamic resistance measurement (DRM), which records the contact resistance while the breaker opens or closes. The latter reveals mechanical wear and coking that a static measurement misses.

What is the difference between a micro-ohmmeter and the SUWI-120?

The SUWI-120 is a multi-tester that combines contact resistance (up to 120 A) with relay timing, turns ratio, and breaker timing. If you are only measuring contact resistance, a dedicated micro-ohmmeter is faster. If you want to do multiple types of tests on the same installation during a maintenance outage, the SUWI-120 saves you lugging around three instruments.

Can IONIO arrange a demo at our site?

Yes. We bring the instrument that matches your use case. No obligation. Request here.

The core:

1. Without 4-wire Kelvin, you mainly measure your cables, not your contact.
2. Choose your test current based on your object, not on what happens to be on the shelf. With 1 A or less of test current you are no longer doing micro-ohm measurement. For transformer windings, use a winding resistance tester.
3. BSG and Dual Ground save you a lot of time and safety hassle in substations.
4. Thermo-EMF compensation and sufficient voltage drop are the two knobs that determine accuracy. For windings, wait for stabilisation.
5. Calibrate at a fixed interval, usually annually, if you want to be able to back up your reports.

At IONIO you find both lines side by side, including advice with no agenda. Call +31 85 401 6045 or email info@ionio.nl, or schedule a consultation directly. And if you want to see whether Megabras or HighTest fits your work better, we’ll come by.

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