# Measuring small Resistances

A 2-wire tester such as a digital multimeter has one problem. When you measure a resistance with a 2-wire tester you also measure the resistance of the test leads. This will be in the range 0.01 to 1 Ohms and is typically 100 milliohms for a Digital Multimeter. If you want to measure resistances of less than 10 ohms you should consider using a 4-wire tester.

Resistance is usually measured by passing a current through the test piece and then measuring the voltage drop across it. The current source and the voltmeter use the same leads. This is why a 2-wire tester is inaccurate for measuring low resistances (Fig 1).

 Two wire or Four wire (Kelvin) testing.

This is why a 2-wire tester is inaccurate for measuring low resistances (Fig 1).

RS : -Resistance of sample (e.g. Cable or Harness)
VM : - Measured voltage

VM = (RS + RL + RL)I

The alternative is to use a 4 wire tester. i.e. One pair of wires for the current source and another pair of wires for the voltage measurement (Fig 2).

The current due to the voltage reading Im will be so small that it is negligible, so VM = RS * I .

It is important that current and voltage wires are connected as close to the sample as possible. If you are testing very small resistances > 50 mOhms, this may mean using specially designed connectors with two separate points making contact with each pin of the sample being tested. This approach can also help when the resistance of connectors becomes significant due to wear.

A four-wire test system can appropriate when testing very large harnesses as the length of wire from the tester to the harness can become significant.

When the junction of 2 dissimilar metals is heated (e.g. by the testing current) a voltage can be generated leading to an error in the resistance being measured. It is possible to remove this effect by reversing the polarity of measurement. This phenomenon is unlikely to be encountered during testing of most cables.

Banair has been manufacturing electronic test equipment for over 20 years. 4-wire testing is one of the topics that we get asked about again and again so we have produced this card explaining how it works. Look out for other cards explaining test methods.