4-wire Kelvin measurement to increase your accuracy, even with long adaptor cables.

The observer effect states that the act of measuring a system inevitably alters the state of the ‘thing’ being measured. In precision electronics, this is a tangible obstacle to accuracy. This is where 4-wire measurement becomes essential.

To determine the continuity of a cable, we must pass a known measurement current through it. Ohm’s Law dictates that this current creates a voltage differential between either end of the cable, relative to both the current flow and the cable’s resistance. We then calculate the resistance using the formula: R = ^V⁄_I

In a standard 2-wire setup, the instrument measures the resistance of the test leads and interface adaptors alongside the cable itself. 4-wire Kelvin measurement eliminates these parasitic resistances, ensuring the reported value reflects only the component under test.

CableEye^® provides 4-wire measurement as standard on the M4 low-voltage tester, and as a factory-fit single module option for the HVX series. Crucially, this single module supports the entire system regardless of channel count. Within the CableEye software, Kelvin testing can be enabled or disabled on a per-cable or per-adaptor basis, allowing you to seamlessly mix high pin-count continuity testing with precision low-resistance measurements on the same system.

In most cable testing applications, lead resistance is negligible. However, if the interface adaptors are long relative to the assembly under test, the voltage drop across those adaptors will skew the results.

To maintain measurement integrity, engineers typically follow three standard practices:

1. Utilise short test adaptor leads.
2. Specify low-resistance wiring for all interface adaptors.
3. Minimise the number of series connections within the test path.

While these are essential for any robust test interface, they cannot physically eliminate the electrical presence of the adaptors. To achieve that, 4-wire (Kelvin) measurement is required.

Standard systems that lack 4-wire capability must either include the adaptor resistance in the final result or attempt to mathematically “tare” it out. Neither method is as reliable as the CableEye HVX or M4 approach. These systems use a dedicated sensing pair in parallel with the source leads. Because the sensing circuit is high-impedance, it carries negligible current, allowing the system to measure the precise voltage at the component terminals. This physically eliminates the test adaptor resistance from the calculation, ensuring the displayed result is the true resistance of the cable.

CableEye testers provide 4-wire measurement as standard on the M4 low-voltage tester, and as a factory-fit single module option for the HVX series. Crucially, this single module supports the entire system regardless of channel count. Within the CableEye software, Kelvin testing can be enabled or disabled on a per-cable or per-adapter basis, allowing you to seamlessly mix high pin-count continuity testing with precision low-resistance measurements on the same system.

FAQ:

Q: Why is it called a 4-point measurement?

A: The term refers to the four distinct connection points at the component under test. Two points provide the source current, while the other two points measure the voltage drop directly across the resistance. By separating these paths, the measurement excludes the resistance of the test leads themselves.

Q: Is a 4-point measurement the same as a 4-wire measurement?

A: In the context of cable testing, yes. Both terms refer to the Kelvin technique used to achieve milliohm-level accuracy. While the physical implementation can vary between benchtop DMMs and automated ATE, like CableEye systems, the engineering objective remains the same: the removal of parasitic lead resistance.

Q: What is the difference between 2-wire and 4-wire measurement?

A: In a 2-wire measurement, the test leads are part of the circuit being measured, meaning their own resistance is added to the result. In a 4-wire measurement, the voltage is sensed through a secondary pair of wires that carry no current, ensuring the measurement reflects only the resistance of the item being tested.

Q: What is a 4-wire resistance measurement?

A: It is a precision testing method that uses two wires to supply a known current and a parallel pair of wires to sense the voltage drop. Because the sensing wires have high impedance and negligible current flow, they do not suffer from voltage drop, allowing for a precise calculation of resistance according to Ohm’s Law.