Cyclic rating factor

The cyclic rating factor M is the factor by which the permissible steady-state rated current (100% load factor) may be multiplied to obtain the permissible peak value of current during a daily (24 h) or longer cycle such that the conductor attains, but does not exceed, the standard permissible maximum temperature during this cycle.

A factor defined in this way has the steady-state temperature, which is usually the permitted maximum temperature, as its reference. The cyclic rating factor depends only on the shape of the daily cycle and is thus independent of the actual magnitudes of the current.

Detail of the load cycle is needed over a period of only 6 h before the time of maximum temperature, and earlier values can be represented with sufficient accuracy by using an average, provided by the loss factor $\mu$. Location of the time of maximum temperature is done by inspection, bearing in mind that although it usually occurs at the end of the period of maximum current this may not always be the case.

$1/ \sqrt {\sum\limits_{i=0}^5 Y_i \left[ \frac{\Delta\theta_R(i+1)}{\Delta\theta_{R_\infty}} - \frac{\Delta\theta_R(i)}{\Delta\theta_{R_\infty}} \right]+ \mu\left[1-\frac{\Delta\theta_R(6)}{\Delta\theta_{R_\infty}}\right]}$