This part of IEC 60951 provides general guidance on the design principles and performance criteria for equipment for continuous off-line monitoring of radioactivity in gaseous effluents and ventilation air used in nuclear power plants for accident and post-accident conditions. General requirements for technical characteristics, test procedures, radiation characteristics, electrical, mechanical, and environmental characteristics are given in IEC 60951-1. These requirements are applicable in this part unless otherwise stated. This standard is applicable to: • noble gas activity monitors intended to measure the volumetric activity of radioactive noble gases in gaseous effluents at the discharge point and the variation of volumetric activity with time during accident and post-accident conditions. The monitor may also be used for the determination of the total discharge of noble gas activity over a given period; • noble gas, aerosol and specific nuclide (commonly iodine, in its different forms: inorganic iodine, organic iodine and iodine sticking on dust) monitors intended to measure the volumetric activity in air or gas systems (control room ventilation, reactor leakage collection, drywell ventilation exhaust, fuel handling building ventilation exhaust, reactor building ventilation purge exhaust) and detect any significant increase of radioactivity during or after an accident. This standard is only applicable to continuous off-line measurement, i.e. monitors whose detector measures a representative proportion of the main effluent or ventilation stream at some remote location (sampling assembly). It does not apply to monitors with the detector positioned in or adjacent to the effluent or ventilation stream, which are within the scope of IEC 60951-4. Sample extraction and laboratory analysis, which are essential to a complete programme of effluent monitoring, are not within the scope of this standard.