ISO 3:1973 PREFERRED NUMBERS – SERIES OF PREFERRED NUMBERS

Renard series is a system of preferred numbers dividing an interval from 1 to 10 into 5, 10, 20, or 40 steps. This set of preferred numbers was proposed in 1877 by French army engineer Colonel Charles Renard. His system was adopted by the ISO in 1949 to form the ISO recommendation R3, first published in 1953 or 1954, which evolved into the international standard ISO 3. Renard's system of preferred numbers divides the interval from 1 to 10 into 5, 10, 20, or 40 steps. The factor between two consecutive numbers in a Renard series is approximately constant (before rounding), namely the 5th, 10th, 20th, or 40th root of 10 (approximately 1.58, 1.26, 1.12, and 1.06, respectively), which leads to a geometric sequence. This way, the maximum relative error is minimized if an arbitrary number is replaced by the nearest Renard number multiplied by the appropriate power of 10. One application of the Renard series of numbers is to the current rating of electric fuses.

The most basic R5 series consists of these five rounded numbers, which are powers of the fifth the root of 10, rounded to two digits. Note that the Renard numbers are not always rounded to the closest three-digit number to the theoretical geometric sequence:

R5: 1.00 1.60 2.50 4.00 6.30