BS EN IEC 62812:2019 pdf free.Low resistance measurements -Methods and guidance.
3.4 four-wire sensing Kelvin sensing four-terminal sensing four-point sensing electrical impedance measuring technique using separate pairs of wires for carrying the measuring current and for sensing the potential difference in order to eliminate the impedance contribution of wiring and contact resistances
3.5 two-wire sensing conventional electrical impedance measuring technique using one pair of wires for carrying the measuring current and for sensing the potential difference on the same wires 4 Resistance measurement phenomena 4.1 General The measurement of a low resistance usually relies on the measurement of a low voltage, which requires a number of precautions against typical detrimental phenomena such as offset voltages, radio frequency interference, electromagnetic interference, electrical noise, or nonohmic contacts. However， these phenomena are not discussed here as they are not
specifically related to the measurement of resistance. The voltage to be measured increases with an increase of the measuring current, which may
also result in effects which are adverse to the measurement. Such phenomena are discussed in Clause 4. 4.2 Lead and contact resistance A conventional method for measuring a resistance is to use a constant current source with a known (or measured) output current and a voltmeter for measuring the voltage across the unknown resistor, while the connection is built with a single pair of test leads, as shown in Figure 1.The raised temperature on the unknown resistor due to self-heating not only affects the specimen, but also spreads the heat to the test assembly or mounting and affects those parts of the measurement circuit as well. Therefore, the raised temperature will be root cause of the variation of resistance with temperature, as discussed in 4.4, and of the thermoelectric e.m.f., as discussed in 4.5. Self-heating is decreased by reducing the measuring current IM as much as possible while still providing the required voltage for a measurement with the desired accuracy. However, setting the measuring current is not a common feature with resistance meters. Other options to reduce the self-heating are to activate the measuring current for a short period only, as discussed in Clause 5, and of course to enhance the heat flow from the specimen and the test fixture. BS EN IEC 62812 pdf download.