Kelvin Four-Wire Measurement (Low Resistance)

A measurement technique for accurately determining very low resistances (typically below 1 Ω, such as contact resistances of terminal clamps, resistance of busbar joints or transformer winding resistance) in which, unlike an ordinary two-wire measurement, current injection and voltage measurement are carried out via separate conductor pairs, each attached at their own connection point: two 'current wires' carry a known, fixed test current through the item under test, while two separate 'voltage wires', connected closer to the actual measured object, exclusively register the voltage drop across that object without themselves carrying current. Because the voltage measuring wires are practically current-free, the resistance of the measuring leads themselves and of the contact transitions at the connection terminals (which, in a low-resistance measurement using an ordinary two-wire method, can quickly become as large as or larger than the resistance being measured) falls outside the measured value, giving much higher accuracy. Kelvin four-wire measurement is used, among other things, for continuity measurement of protective conductors under NEN 1010 annex 6A, for measuring weld-joint resistances, and for assessing contact resistances of circuit-breaker contacts during NEN 3140 inspections.