Micro Ohmmeter --- Why 4-Wire Kelvin Measurement Is Essential for Low Resistance Testing

Posted by Billy 29/05/2026 0 Comment(s)

When measuring very small resistance values, a normal 2-wire resistance measurement is often not accurate enough. This is especially true when the resistance being measured is in the milliohm or micro-ohm range. In these applications, the resistance of the test leads, probe contact points, connectors, and surface contamination can become large enough to distort the measurement result.

That is why 4-Wire Kelvin Measurement, also called the 4-terminal resistance measurement method, is commonly used for accurate low resistance testing in industrial maintenance, quality control, EV service, battery connection testing, motor winding inspection, transformer testing, busbar verification, and connector contact resistance measurement.

How 4-Wire Kelvin Measurement Removes Lead Resistance Error

This animation compares a normal 2-wire resistance test with a 4-wire Kelvin method for accurate milliohm and micro-ohm measurements.

Why it matters: When the DUT resistance is only a few milliohms or micro-ohms, lead resistance and contact resistance can be larger than the real change you are trying to detect. 4-wire Kelvin measurement separates the current path from the voltage sensing path, so the meter calculates the resistance at the actual test points.

Measurement Principle and Formulas

The Kelvin method is based on Ohm's Law, but it controls where the voltage is measured. The meter sends a known test current through the DUT, then measures the voltage drop only between the two sense points. The resistance is calculated from that voltage drop.

2-Wire Method: error is added into the reading

V_meas = I × (R_DUT + R_leads + R_contacts) R_meas = V_meas / I R_meas = R_DUT + R_error R_error = R_leads + R_contacts

In a 2-wire test, the same wires carry current and sense voltage. Any voltage drop on the leads or probe contacts becomes part of the measured resistance.

4-Wire Kelvin Method: sense leads remove the lead drop

I_source flows through the SOURCE leads and the DUT I_sense ≈ 0 because the voltage input impedance is very high V_sense-leads = I_sense × R_sense-leads ≈ 0 V_sense ≈ I_source × R_DUT R_DUT = V_sense / I_source

Because almost no current flows in the sense leads, their resistance creates almost no voltage drop. The meter therefore calculates the resistance across the actual sense points.

R_DUT = resistance of the device under test I_source = known test current from the meter V_sense = voltage measured directly across the DUT R_error = unwanted lead and contact resistance

Simple example: If the real DUT resistance is 5 mΩ and the total lead/contact resistance is 20 mΩ, a 2-wire meter may read 25 mΩ. The error is (25 - 5) / 5 × 100% = 400%. With a proper 4-wire Kelvin connection, the sense leads measure only the voltage across the DUT, so the result is much closer to 5 mΩ.

The key point is that the sense probe positions define the exact section being measured. For repeatable low-resistance testing, place the SENSE tips at the two points you want to evaluate, keep the contact surfaces clean, and use suitable Kelvin clips, pin probes, or fixtures.