Renard series
system of preferred numbers
The Renard series is a system of preferred numbers dividing an interval from 1 to 10 into 5, 10, 20, or 40 steps. The preferred values are rounded forms of a geometric sequence and can be extended above or below this interval by multiplication or division by powers of ten.
The system is named after the French army engineer Colonel Charles Renard, who developed a system of preferred values during the late nineteenth century. It was later adopted for international standardization and became the basis of ISO Recommendation R3, first published in the 1950s and subsequently superseded by ISO 3.
The factor between consecutive Renard numbers is constant: it is the fifth, 10th, 20th, or 40th root of 10 for the R5, R10, R20, and R40 series, respectively. Since their exact theoretical values are all irrational, they are rounded when used in practice.
The resulting spacing allows a range of sizes to be covered while limiting the number of values that must be standardized, manufactured, or stocked. Renard values have been used for engineering dimensions and ratings, including electrical-component ratings such as fuse currents and capacitor voltages.
Base series
The most basic R5 series consists of five rounded values, which approximate successive powers of the fifth root of 10:
R5: 1.00 1.60 2.50 4.00 6.30
Examples
The R5 series may be extended through powers of ten. For example, its values can be expressed as 16, 25, 40, 63, 100, 160, and 250 millimetres when selecting a limited range of nominal product lengths.
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