## Overview of Four-wire Kelvin Testing

Testing has come a long way since the days when the only option for testing wires was checking continuity with a handheld meter. Harnesses that used to take hours to test can now be analyzed in seconds. High voltage testing can find errors that might go unnoticed otherwise. Even four-wire Kelvin testing is easier than ever.

Example of four-wire testing setup.

If you are unfamiliar with four-wire Kelvin testing, you are not alone. Many manufacturers may not learn about this method of testing until they need more accurate results. Four-wire testing eliminates the resistance of your fixturing and can measure resistance down to .001 ohm. Reasons for using four-wire Kelvin testing may include:

• Needing a more accurate resistance measurement. A normal two-wire test will be able to read .1ohm. A four-wire Kelvin test can read down to .001ohm.
• Needing precise results for a components test without the fixturing interfering.
• Needing to test without the resistance of the fixturing interfering with the test results With normal two-wire testing, the tester sends a current through a wire and measures the amount the voltage drops. It then calculates the resistance using Ohm’s Law.

Since the current is sent through the same fixturing wires that measures the resistance, any resistance caused by fixturing is included within that measurement. Normally the resistance from the fixturing is not enough to cause any distress, but occasionally cable and harness manufacturers need to test without having this extra resistance added into the results. That is where Four-Wire Kelvin testing can be used.

Four-wire testing can measure the device-under-test (DUT) without measuring the fixturing. While normal testing has one wire connected to each end of the cable, each wire sending a current and recording the result, four-wire has two wires for each side of the cable. That means that for each end of the cable, one wire sends a current and a different wire measures the result. The wire that measures the result can be connected directly to the DUT, eliminating the extra resistance in the fixturing.

Setting up a four-wire kelvin test is not difficult when using Cirris equipment. Cirris has four-wire capabilities programmed into the software. Building the fixturing is a matter of knowing which pins will send the current and which will measure the resistance. Cirris has several resources to help with this. See this more in-depth article, your tester’s manual, your Cirris representative, or search for 4-wire on cirris.com.