Apply 2 or 4-wire measurements?

Most automatic wiring harness testing systems offer 2- or 4-wire resistance measurements.  Some systems also offer double-wire measurements, offering the flexibility to choose which of the two methods is most suitable for the specifications of the item under test, normally called the Unit Under Test (UUT).  A UUT can be a single wire, a component, a sub-assembly or the finished product.

To determine whether the 2-wire or 4-wire test method is most appropriate for your UUT, you need to understand how both methods work.

2-wire resistance measurement

The 2-wire test method simply means that two wires are used to connect the measuring equipment to the UUT.

To measure the electrical resistance of a component or UUT, an electric current is passed through it.  The current flowing through it is measured in Amps (A) and the voltage across it is measured in Volts (V). Since the voltmeter (shown in green in diagram 1) has a very high resistance, only a negligible current flows through it, so the current measured by the ammeter (shown in red) can be considered equal to the current of the UUT (shown in blue).

The resistance can then be calculated in Ohms (Ω) using Ohm’s law:

In the real world, the wires (or interface) used for the connections also each have a small electrical resistance, so the actual circuit is as shown below in Diagram 2.  Note: The Ohmmeter represents the test system, such as MK Test’s Automeg®. MK Test.

The measured voltage is the sum of the voltages across the UUT and the two wires supplying the current. This is shown in blue in diagram 2 above. So the calculated resistance is the resistance of the UUT, plus the resistance of the + wire, plus the resistance of the – wire.

In an Automeg, 2-wire testing would require two connection points at two test points. MK Test’s MKAT® software can take into account the extra resistance of the interface wires by subtracting an offset value from the measured resistance and reporting the result.

For this reason, the 2-wire method is suitable for most applications where resistance needs to be measured with an accuracy in the order of Ohm.

4-wire resistance measurement

To measure resistance with greater accuracy, the 4-wire method can be used.  This requires connecting four wires to the UUT, as shown in diagram 3 below in red and green.  This means that the current follows the same path through the same resistors, but the voltage is measured directly across the UUT.

Since the measured voltage is the voltage across the UUT, the calculated resistance is the exact resistance of the UUT.  Therefore, this method can be used to measure resistances on the order of milli-ohms or micro-ohms.

Summary

In summary, 2-wire resistance testing is a simpler form of measurement than 4-wire testing.  It offers measurements that are accurate to the nearest Ohm.  In contrast, 4-wire testing uses twice as many test points, but the accuracy is much higher.

The Automeg range of test systems offers models for 2-wire or 4-wire testing, but the most popular models are those with both modes. Many test applications can use the 2-wire method for the majority of joints to be measured, and 4-wire for certain joints that require higher accuracy.