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

The observer effect basically says that the simple act of measuring something will alter the ‘thing’ being measured, and in electronics this is most certainly the case. This is where 4-wire measurement can help.

To measure the continuity of a cable we need to pass a measurement current through it and Ohms Law tells us that this will create a voltage difference between either end of the cable which is relative to the current flowing and the resistance of the cable. This we can then measure and calculate the cable’s resistance using: R=V/I.


In most applications this is not a problem; however, if the connection wires (test adaptors) to the test point of the cable (the connectors) are relatively long with respect to the cable being tested the voltage difference across the test adaptors will be enough to affect our results.

So, how to avoid this?

  1. Use short test adaptor leads
  2. Use low-resistance test adaptor leads
  3. Minimise the number of series connections in the test adaptors
  4. Employ 4-wire measurement (Kelvin measurement)

1, 2 & 3 (above) should be part of any standard practice in building test adaptors. Cutting corners on building any adaptors is a recipe for unreliable measurements; however, No. 4 (above) requires a different way of thinking and a test instrument capable of 4-wire measurements, such as the CableEye HVX or CableEye M4.

A system that does not use 4-wire measurement cannot automatically eliminate the test adaptor wiring resistance so would either include it in the overall measurement or have the resistance of the test adaptors mathematically deducted before displaying the result.

A 4-wire measurement system, or Kelvin measurement system, uses a set of wires connected in parallel to the test adaptor wires for the specific purpose of electrically eliminating the resistance of the test adaptor wiring. This can be achieved in a number of ways but, basically, it provides a high impedance connection to the test points to allow a voltage offset to be used. In the case of CableEye HVX, this high impedance allows the system to measure the actual voltage at either end of the cable being tested without unduly affecting the test voltage applied.

4-wire measurement schematic

The 4-wire measurement capability is standard on the M4 low voltage tester, and a factory-fit option for CableEye HVX as a single module, regardless of how many test channels your system has, and is enabled/disabled per cable within the CableEye software. This technique means that you have the option of using a higher pin-count or 4-wire measurement on an adaptor-by-adaptor basis. How’s that for flexibility?


Are 4-point measurements the same as 4 wire measurements

Essentially, yes. They are generally used to achieve the same goal, which is to get a more accurate measurement of a particular element, though there are different techniques.

What is the difference between 2 wire and 4 wire measurement?

A 2 wire measurement involves 1 wire from each input on the measuring instrument, often a red and a black terminal on a Digital MultiMeter (DMM), ergo, 2 wires.

What is a 4 wire resistance measurement?

Depending on the application, a 4 wire measurement has two test wires to measure the element (resistance, for example,) and another two wires in parallel to the test wires to ‘offset’ the effect of the test wires on the element being measured. 

Why is a 4-point measurement so called?

Simply put, the test has 2 wires to make the measurement and 2 wires to counteract the effect of the first 2 wires.

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