Current rating standards 69-30 Part VI
Sustained current ratings for 70oC thermoplastic insulated cables (ac 50 Hz) in multi-layer groups on trays.
This report, which is Part VI of the series, provides current ratings for 70 °C thermoplastic insulated cables installed touching each other in multilayer groups on trays and ladder supports in air. The group can contain cables of different sizes. Ratings given in this report are based on the cable dimensions in BS 6346:1969. These ratings are also considered to be applicable to cables having a similar construction.
Calculation of ratings for cables installed under such conditions follows rather different lines from those used for other methods of installation. Since it is not usually possible to specify the position of particular cables in the group, the assumption has to be made that cables are laid bundled together at random in a group which is rectangular in shape and of approximately uniform depth, the smaller cables tending to lie in the voids between the larger ones. It is assumed that the width of the group is much greater than the depth. Under these conditions it is appropriate to look at the mass of cables as a whole and to envisage that heat is generated uniformly over its cross sectional area. It then follows that each cable will be able to generate heat losses which are proportional to its own cross sectional area, that is to the square of its overall diameter. The permissible value of heat generation per unit cross sectional area of the group depends on the effective thermal resistivity for heat transfer across the group and the coefficient of heat dissipation from the outer surfaces of the group. An explanation of this method of rating cables and an account of experimental work to establish suitable values for thermal resistivity and surface emission for BS 6346 cables are given in ERA reports 74-0029 and 84-0172. Ratings tabulated in this report have been calculated on the basis of a group thermal resistivity of 3.3 K.m/W and an average surface heat emission coefficient of 4 W/m2 .K1.25.
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