This part of IEC 62610 which deals with thermal management for cabinets in accordance with IEC 60297 and IEC 60917 series, provides for compatible measurement methods of recirculation ratio and bypass ratio which are indicators for defining quality of airflow in the forced air cooling that can be commonly applied to indoor cabinets for mounting subrack and/or chassis-based equipment. NOTE 1 Both recirculation and bypass represent leakage airflows, i.e. detrimental phenomena in terms of cooling efficiency; their measurement is obviously aimed at their mitigation. This document contains the following: a) the definition of recirculation and bypass flow rates in the cooling of the cabinet, b) the levels of the recirculation ratio RC, c) the definition of the formula for the recirculation ratio RCs of forced air cooling subrack and/or chassis-based equipment installed in the cabinet, d) the definition formula of recirculation ratio RCr and bypass rate BPr of the entire cabinet, e) the requirements of the measuring method of each temperature necessary for calculating the recirculation ratio RCs, RCr and bypass ratio BPr. NOTE 2 This document includes the definition of measuring bypass ratio, but excludes the definition of levels of bypass ratio. The drawings used are not intended to indicate product design. They are only for explanatory indications for defining forced air cooling airflows. The recirculation and bypass measurement methods dealt with in this document are assumed to be applied to a cabinet installed indoors. The cooling air inlet is at the front or the bottom of the cabinet and the heated air is exhausted to the rear or the top. These methods are also applicable to a cabinet that is installed outdoors and has a cooling device such as a heat exchanger or an air conditioner on the front or the back (see Annex C). The recirculation ratio of a subrack or a cabinet is defined for each individual subrack or chassis-based equipment mounted in the cabinet or for the entire cabinet. The bypass ratio of a cabinet is defined for the entire cabinet.