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Permissible peak cyclic load current

The permissible peak cyclic load current is the highest current over an indefinitely repeating load pattern, which is called cyclic rating.

Different analytical methods exist to calculate cyclic rating:

  1. IEC 60853: The method requires knowledge of the full shape of the load variation over the last 6 h immediately preceding the time of maximum temperature and an average value for a 24 h load cycle which keeps repeating.The cyclic rating factor is designated by the letter $M$, and represents the factor by which the permissible continuous current rating (100 % load factor) can be multiplied to obtain the peak current during a daily cycle, such that during this cycle, the conductor reaches, but does not exceed, the standard maximum permissible temperature. The loss factor $\mu$ is calculated and used to obtain the average losses.
  2. Neher-McGrath: The method and its derivations and extensions by L. Heinhold (Siemens) and E. Dorison (EdF) consider the long thermal time constant of the earth surrounding the power cable system and recognize that the extent of heating that occurs in the soil will effectively be the average of the heat generated by the power cable during the load cycle. A characteristic diameter $D_x$ is calculated up to which the load varies and beyond which the reduced losses $\mu W_{tot}$ are acting on other objects. In between the surface of the cable at $D_e$ or of the duct at $D_{de}$ we reduce the losses between $W_{tot}$ and $\mu W_{tot}$ using a quadratic equation depending on the amount of soil.
  3. IEC 60092-352: This standard for electrical installations in ships provides an exponential function to calculate a correction factor for short time duty or intermittant duty cycles of medium voltage cables, depending on their size.

The equation below is relevant for IEC 60853 method only. When using Neher-McGrath, Heinhold, or Dorison, the result of the calculation of $I_c$ is in fact already the peak current $I_{c_{peak}}$.

Symbol
$I_{c,peak}$
Unit
A
Formulas
$M I_{c,1}$
Related
$I_{c,1}$
Steady-state current before transient [A]
$M$
Cyclic rating factor [p.u.]