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Short Circuit Ratings

The conductor size in a cable for an installation is also governed by its ability to carry short circuit current of the system. Short circuit ratings are based on the assumption that the duration of short circuit is so small and apparently there is no transmission of heat, produced during short circuit through the insulation and the whole of it is absorbed by the conductor.

The short circuit current rating (r.m.s. values) of XLPE / PVC insulated cables with Aluminum and copper conductor for one second duration are given as under. These ratings have been calculated on the following assumptions.

1)	Temperature of conductor just prior to short circuit. a) With general purpose insulation b) With heat resisting insulation c) With XLPE insulation	UNIT	ALUMINUM	COPPER
		^o C	70	70
		^o C	85	85
		^o C	90	90

2)	Max. Permissible temperature of conductor during short circuit. a) With PVC insulation b) With XLPE insulation	^o C	160	150
2)		^o C	250	260
3)	Volumetric specific heat of conductor at 2O°c	J/^oCmm³	2.5 x 10³	3.45x10³
4)	Resistively of conductor at 20° c	ohm-mm	28.264x10³	17.24x10³
5)	Reciprocal of temperature co-efficient of resistance of conductor at 0°C	^o C	228	234.5



With the above assumptions the short circuit rating “Ish is given by the following formula.

	Where K= Constant (depends upon the type of insulation and conductor material) A=Nominal cross-sectional area of conductor in mm²




	SHORT CIRCUIT RATING OF 1100 VOLTS GRADE PVC INSULATED HEAVY DUTY CABLES (FOR DURATION OF ONE SECOND)		SHORT CIRCUIT RATING FOR XLPE INSULATED CABLES (FOR DURATION OF ONE SECOND)

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