This standard applies to on-line in-core neutron detectors, together with associated components and instrumentation, designed for purposes important to reactor safety: protection, information or control. The detector types usually used are direct current ionization chambers, fission ion chambers and self-powered neutron detectors. This standard provides guidance for the design of in-core instrumentation for neutron fluence rate measurements in thermal neutron reactors designed for power production. The major emphasis in this standard is on the general design aspects of on-line systems. For the principles of overall system design and for the purpose of neutron flux measurements, reference should be made to IEC 61513.