What Is a Micro-Ohmmeter? Types and Selection Criteria
A micro-ohmmeter measures very low resistances in the micro-ohm and milli-ohm range with the four-wire (Kelvin) method to check joints, contacts and windings.
A micro-ohmmeter is a test instrument that measures very low resistances in the micro-ohm and milli-ohm range using the four-wire (Kelvin) method. It is used to measure contact resistance at busbar and cable lug joints and on circuit breaker and contactor contacts, and winding resistance in motors and transformers. The instrument passes a known DC current through the part and measures the voltage drop across it with separate leads. Because the current and voltage leads are separate, the resistance of the test leads and contact points is not added to the result. The two-wire ohm range of a multimeter measures these resistances too, so it does not replace a micro-ohmmeter.
Types of micro-ohmmeters
| Type | Feature | Typical use | Models in the catalogue |
|---|---|---|---|
| Portable | Battery powered, handheld | Checking busbar, lug and earthing joints on site | UNI-T UT620A, UT620C+ |
| Benchtop | Used on the bench in laboratories and production lines | Measuring the resistance of components, cables, coils and joints | UNI-T UT3513, UT3513+, UT3516, UT3516+ |
| Multi-channel scanning | Measures several points in sequence | Multi-point production tests | UNI-T UT3515-S10, UT3515-S20 |
| High-current low resistance ohmmeter | DC test current up to 200 A | Circuit breaker and disconnector contact resistance, busbar joints | UNI-T UT625B |
| Transformer DC resistance tester | Four-wire, 10 A test current | Transformer and motor windings | UNI-T UT620T |
Selection criteria
- Measuring range and resolution: The instrument's smallest resolution should be clearly smaller than the value you will measure. In the catalogue, the ranges are 1 µΩ–20 kΩ on the UT3513, 1 µΩ–2 MΩ on the UT3516 and UT3515-S10/S20, 0.001 mΩ–300 kΩ on the UT620C+ and 0.1 µΩ–2000 mΩ on the UT625B.
- Test current: A higher current produces a larger, more stable voltage drop across a micro-ohm resistance. High-current instruments are chosen for circuit breaker contacts and busbar joints; for electronic components and thin wires, a low current does not heat the part. The maximum test current is 1 A on the UT620C+ and 200 A on the UT625B.
- Accuracy: The accuracy class is decisive for sorting in production and for comparative measurements. According to the product data, it is 0.05% on the UT3516 and UT3515 models and 0.1% on the UT3513.
- Place of use: On site, a battery-powered model with a protected housing (UT620C+, IP54) does the job. Production lines use benchtop or multi-channel models, and circuit breaker maintenance uses a mains-powered high-current model.
- Temperature: The resistance of copper changes by about 0.4% per °C. For measurements to be compared over time, record the temperature or choose a model with temperature compensation.
Common mistakes
- Connecting the leads in the wrong place: The voltage leads must sit on the two points being measured, inside the current leads; otherwise the contact resistance is added to the result.
- Measuring without cleaning the contact surface: Paint, oxide and grease raise the reading.
- Measuring a live circuit: The part being measured must be disconnected from the circuit and de-energised.
- Removing the leads too early in a winding test: The energy stored in an inductive winding produces a high voltage when the leads are removed. Do not disconnect until the instrument has discharged the winding.
Micro-ohmmeters in the catalogue
All 10 active products in the Micro-Ohmmeters category are Unit and sold per piece; all models except the UT3513+ are sold on quotation. For insulation measurements at the high-resistance end, see Insulation Resistance Testers, and for general-purpose measurements see Digital Multimeters.
Frequently asked questions
What is the difference between a micro-ohmmeter and an insulation tester (megger)?
A micro-ohmmeter measures the very low resistance of conductive joints with a low voltage and a high current. An insulation tester measures insulation resistance in the megohm range with a high DC voltage. One shows whether a joint is good, the other whether the insulation is sound.
Why is four-wire measurement needed?
At micro-ohm level, the resistance of the test leads and probe contacts can be larger than the value being measured. With a four-wire connection the voltage is measured directly on the part with separate leads, so these resistances are not added to the result.
Which instrument measures circuit breaker contact resistance?
Circuit breaker manufacturers usually call for a DC test current of tens to hundreds of amperes for this measurement; take the exact value from the breaker's maintenance instructions. The catalogue offers the UT625B, which provides a test current of up to 200 A, for this job.
Can transformer winding resistance be measured with a micro-ohmmeter?
Small windings can be measured with a four-wire micro-ohmmeter. On transformer windings with a large inductance the current takes a long time to settle, so instruments made for this job are used; the catalogue offers the 10 A UT620T for this purpose.