Why Does the Circuit Resistance Tester Use Four Wires? What Are the Requirements for Wiring Sequence?

Many people use a circuit resistance tester for the first time and see that the instrument has four wiring terminals - I+, I -, U+, U -, and is equipped with two sets of test wires of different thicknesses. They are a bit confused why it is so complicated. Can't two wires work?

What problem does the four wire system solve

The typical value of circuit resistance is in the range of tens to hundreds of micro ohms. If measured using a regular two-wire system, the wire resistance and contact resistance of the test line itself (each group may also have a few milliohms) will be much greater than the measured resistance, directly adding them to the measurement result, resulting in severe numerical distortion and making it impossible to determine the contact state.

The four wire system (Kelvin connection) divides the circuit into two independent circuits: the current circuit (I+/I -) and the voltage sampling circuit (U+/U -). High current flows into the tested contact from I+and flows out from I -; At the same time, U+/U - collects voltage drop at both ends of the tested contact. Due to the extremely high input impedance and almost no current flowing through the voltage sampling circuit, the contact resistance on the U+/U - test line cannot generate a voltage drop, so the measured voltage is completely the pure voltage drop across the tested contact. The true contact resistance of the contact is obtained by dividing this voltage drop by the precisely known current.

Specific requirements for wiring

There are two types of test wires for HZ-5100S: thick wire (plug-in type) connected to current terminal I+/I -, and thin wire (spring rod type) inserted into voltage terminal U+/U -.

The key sequence of wiring: the current clamp (thick wire) is clamped on the outside of the contact point of the tested contact, and the voltage clamp (thin wire) is clamped on the inside of the current clamp, as close as possible to the contact point itself. This is the only way to exclude the resistance of the connecting leads from the voltage sampling range. If the order is reversed - voltage clamped outside, current clamped inside - some lead resistances will mix into the measurement results, resulting in higher values.

The terminal must be tightened

The current terminal I+/I - must be tightened and cannot be loosely connected. Virtual connection at the current end can increase the contact resistance and prevent the current from reaching the set value. The instrument will automatically stop testing and sound an alarm; On the other hand, it also wastes battery power as the instrument repeatedly attempts to establish current. The instruction manual specifically reminds that it is necessary to use the matching dedicated low resistance test wire, tighten the terminal post, and clamp the test clamp to reduce the lead resistance.

The resistance of the supporting test line is less than 10m Ω, which is a prerequisite for ensuring the overall error control of the measurement system.