Applicable more particularly to instrumentation for gas-cooled graphite-moderated reactors, Type 1: reactors having metallic natural uranium fuel clad with magnesium alloy, and Type 2: reactors having enriched uranium oxide fuel clad with stainless steel. The IEC 60231 series of standards is no longer up-to-date and will not be revised by IEC. The standards are maintained for the time being for bibliographical purposes and because they were used in the past for the design of nuclear power plants which are still in operation.