AS 60890:2009 A method of temperature-rise assessment by extrapolation for partially type-test assemblies (PTTA) of low-voltage switchgear and controlgear (IEC 60890, Ed. 1.0 (1987) MOD)
4 Conditions for application
This method of calculation is only applicable if the following conditions are fulfilled:
− there is an approximately even distribution of power losses inside the enclosure;
− the installed equipment is so arranged that air circulation is but little impeded;
− the equipment installed is designed for direct current or alternating current up to and including 60 Hz with the total of supply currents not exceeding 31 50 A;
− conductors carrying high currents and structural parts are so arranged that eddy-current losses are negligible;
− for enclosures with ventilating openings, the cross-section of the air outlet openings is at least 1 .1 times the cross-section of the air inlet openings;
− there are no more than three horizontal partitions in the PTTA or a section of it;
− where enclosures with external ventilating openings have compartments, the surface of the ventilating openings in each horizontal partition shall be at least 50% of the horizontal cross-section of the compartment.
5.1 Necessary information
The following data are needed to calculate the temperature rise of the air inside an enclosure:
− dimensions of the enclosure; height/width/depth;
− the type of installation of the enclosure according to Figure 4,
− design of enclosure, for example, with or without ventilation openings;
− number of internal horizontal partitions;
− effective power loss of equipment installed in the enclosure;
− effective power loss (P n ) of conductors according to Annex B.
NOTE – The effective power losses of the equipment installed in the circuits of the PTTA used for this calculation are the power losses at the rated currents of the various circuits to be taken from information provided by the manufacturer.
Where enclosures without vertical partitions or individual sections have an effective cooling surface greater than 1 1 .5m 2 or a width greater than about 1 .5m, they should be divided for the calculation into fictitious sections, whose dimensions approximate to the foregoing values.