4-Wire Measurement

Description

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 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.

two-wire-measurement

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 measurement, such as the CableEye HVX.

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 option for CableEye HVX is 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?

To find out more about CableEye HVX 4-wire measurement, or any of our other products just drop us a line.

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