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|Equations have been established for short circuit calculations
for conductors made of copper or aluminum. The coverings and
insulations, which determine the maximum allowable short circuit
temperatures, are paper, varnished cloth and several thermoplastic
and thermosetting materials presently appearing in ICEA standards.
Temperature limits, considered safe, were established for the
various covering and insulation materials.
The equations may be used to determine:
• The maximum short circuit permitted for a specific conductor
and short circuit duration.
• The conductor size necessary to carry a specific short circuit
current for a given duration.
• The maximum duration a specific conductor can carry a specific
short circuit current.
An equation has been established for short circuit calculations
with conductors of copper or aluminum. The insulations, which
determine the maximum allowed short circuit temperatures, are
described in the ICEA Standards. The equation is based on the heat
content of the conductor material and the temperature limit of the
insulation with the assumption that the time interval is so short
that the heat developed during the short circuit is contained in
the conductor. At the time this document was originally published
there was no standard mathematical method available to calculate
heat flow from the conductor through the insulation at the
cessation of the short circuit load. It was necessary to enlist the
aid and facilities of member laboratories and Massachusetts
Institute of Technology to obtain in cooperation a solution to this
problem so that safe temperature limits could be established for
the various types of insulations. The solution is still a viable,
conservative approach to the calculation of short circuit
Results are sufficiently conservative to neglect conductor skin
effect except for very large conductors. Skin effect can be taken
into account by dividing the right-hand member of the equations
shown by the appropriate conductor ac/dc resistance ratio.
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