Thermal derating

Note This operating condition is, however, not specific for it does not stipulate the frequency of occurrence of such a contingency. It may be assumed that this condition will not occur more than once before thermal stability is reached. See also Section 3.7. In normal practice a motor meeting the other operating conditions noted above in all likelihood will satisfy this requirement also without needing yet another derating. See also Example 7,1. 

Temperature can ultimately be viewed as a thermal stress — one that causes an increase in failure rate (and life if applicable). But how severe a stress really is, must naturally be judged relative to the ratings of the device. For example, most semiconductors are rated for a maximum junction temperature of 150°C. Therefore, keeping the junction no higher than 105°C in a given application represents a stress reduction factor, or alternately — a temperature derating factor equal to 105/150 = 70%. 

A cable may experience varions different environments along its ronte. For example it may start at a switchboard, run through the switch room in a trench with a lid or steel flooring, pass throngh a dnct in a wall and under a roadway, mn a long way directly bnried and finish on a ladder rack at the consumer. At each of these environments the thermal resistivity and ambient temperatnre will be different. The environment that canses the most derating of the rated current should be taken and nsed for the whole cable. 

Small thermal-spectmm reactors of long refuelling interval using uranium dioxide pellet type fuel derate their core power density (kW(th)/litre) and specific power (kW(th)/kg compared to commercial LWRs, thereby increasing fissile specific inventory ... 

Where cables are run in thermally insulated spaces or in hot locations, ensure that the conductor size is adequate when derated. 

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