Taking the Heat: IEEE Standard 80 and Bimetallic Conductors

IEEE Standard 80-2013 provides the substation grounding system designer with simple formulae and tabulated data for the estimation of the maximum fault current that can flow through various types and sizes of conductor, for a given duration, before failure due to fusing occurs. Copper conductors are given exhaustive and reasonably accurate treatment. Other types of conductors, however, are given short thrift. Copper-clad steel (CCS) conductors, whose steel cores provide an effective heat sink, appear not to have been studied at all. As a result, the standard provides only an unrealistic simplified methodology based on fixed physical constants to be used for the calculation of CCS current-carrying limits. Computer modeling and lab testing have demonstrated that the highly non-linear heat absorption characteristics of the CCS core, when properly considered, yield considerably different fault current-carrying capacity than IEEE Standard 80-2013 would lead design engineers to expect. A theoretical framework for the calculation of these values is presented. Computed values are compared with those obtained by an independent accredited high voltage test laboratory.