Why Do DC Resistance Testers Need Four Wires?

People who come into contact with a DC resistance tester for the first time often have this question - even if it's just measuring resistance, using two multimeter probes is enough? In fact, a multimeter may have an error of tens or even hundreds of milliohms when measuring small resistors in the milliohm range, which is completely insufficient. That's why we use a four wire measurement system.

What are the issues with the two-line system

Using a regular two-wire system for measurement, both contact resistance and wire resistance are connected in series in the measurement circuit. The DC resistance of transformer windings is already very small, with some low-voltage high-capacity transformer windings having a resistance of only a few milliohms, or even a few tenths of milliohms. If the contact resistance or wire resistance itself is a few milliohms, more than half of the measured results are "contact errors", which cannot reflect the true state of the winding.

How to solve this problem with a four wire system

The four wire system (Kelvin connection) divides the circuit into two parts: the current circuit and the voltage measurement circuit are independent of each other.

HZ-3110B has four terminals: I+and I - are current output terminals, and V+and V - are voltage sampling terminals. During measurement, a large current flows into the winding from I+and out from I -, forming a complete current circuit; At the same time, V+and V - are clamped at both ends of the tested winding to collect voltage.

The key is that there is almost no current flowing through the voltage sampling terminal (V+/V -) (with extremely high input impedance), so there is almost no voltage drop on the contact resistance of the clamp. The voltmeter measures the pure voltage at both ends of the winding - without any contact resistance error. Then divide this pure voltage by the precisely known current to obtain the true DC resistance of the winding.

What should be paid attention to in actual wiring

The correct connection method is to clamp the current clamp (I+/I -, usually a thick red and black wire) on the outside of the tested terminal, and clamp the voltage clamp (V+/V -, usually a thin yellow green wire) on the inside of the current clamp, as close as possible to the tested winding terminal.

If the connection is reversed - voltage clamped outside and current clamped inside - the contact resistance will re-enter the voltage measurement circuit, increasing the error. This sequence is easily overlooked during on-site operations, and when the results are too high, the first thing to check is often the wiring sequence.

The HZ-3110B test line comes standard with a length of 10 meters and can be customized to meet the wiring distance requirements of large transformers on site.