IEEE 118-1978 pdf free.IEEE Standard Test Code for Resistance Measurement.
Inintermediate-value resistance m easuetents of fmoderate accuracy, usual1y no serious lim itations on accuracyresult.from either corntact resistances or insuation resistances. On the other hand, in resistance mneasxremn erts af the tighestaccuracy, both contact resistances and insulation resistances can 1irmit the accuracy.
Cortact and insu ation resistance protblerns can be handl ed sin udtaneously by comn tirning the fe atur es of four-termina1and three-terminal1 measurem erts in the sam e mneasurernent. The equivalent circuit of a resistor whose measurert entcoud be linited by both of these factors is shown in Fig 4.The effects of the contact and insulation resistances arecortr olled as described in 2.1 and2.2.
A measurem ent that cortrols the effects of cantact and insulation resistances simultaneously can be referred to as afive-terminal m easur ernent. Som e standard resistcrs are of five-terminal construction (See Section 3.)
When measuring resistors at high currents, contact resistance effects, dscussed in 2.1，and self-heating efects,discussed in 2.7 , usually must be taken into account. An additional source of uncertainty in the effective resistance ofa high-current resistar af the foux-term inal type can arise from nanunif crm distribution of current resulting from theplacem ernt of leads at different 1ocati ons at the currernt terminals or nonuriform potertia cn the potential terminal:These effects can be nininized by increasingthe1ength-to-widthratio of the conductors between the term ina1s arnd thefour-terminal juncticns. (See also 3.4.)When m easuwing resistors at highn valtage,insulation effects, discussed in 2.2, and sef-he ating effects, discussed in2.7, usually nmust be taken into account. In adlition, some ligh voltage resistars have a voltage coefficient, that is, theirvalue is affected by voltage in additi an to self-heating.In m easxermnents where regions of high electrical stress exist,care mnust be taken to minimze the effects of partial dscharges( cor cna). (See also 3.6.)
The value of most resistors is affected by ervir ormn ental paramheters such as temperature，hmnidity， atm osphericpressuxe， and chern ical and biological corrosion. When measurerm ernts of the highest pr ecision ar e undertaken, all of these factors n ust te taken into acoourt.In m ost practical situations, how ever, tem per ature is the principal concern.Most pure m etals have a temperatuxe coefficient of apr ax.irn ately 0.4 percent per degree C elsius. Several precisionresistor a1loys have ternperature coefficients be1ow 0.0005 percent per degree Celsius over a range of at least 50degees.IEEE 118 pdf download.