ICEA P-32-382-2025

This publication discusses factors for consideration in approximating the operability of insulated and/or covered wire and cable under the influence of uninterrupted short circuit currents encountered as a result of cable or other equipment faults. The duration of such a fault is considered to be up to approximately 2 seconds. Calculation for single short circuits of longer durations will yield increasingly conservative results.

The following items must be considered in order to estimate the short circuit performance of a specific circuit:
a. the magnitude and duration of the fault current including any fault current division due to available conducting paths
b. the capability of joints, terminations, and other accessories in the affected circuit to withstand the thermal and mechanical stresses created by the fault
c. the interaction between the faulting circuit and surrounding equipment, such as supports, ties, and clamps
d. the capability of the affected cable circuit, as installed, to withstand the electromagnetic forces created during the fault
e. the maximum temperature that cable components can withstand without incurring damage due to heating caused by fault current flow
f. damage to adjacent equipment due to arcing at the site of the fault
g. for limitations imposed on the short-circuit capacity of the cable by the fault capacity of the cable metallic sheath/shield, see ICEA Publication P-45-482, Short Circuit Characteristics of Metallic Sheaths and Shields on Insulated Cable

It is noted that the short circuit temperature of the sheath/shield is considerably higher than the conductor (as noted in ANSI/ICEA P-45-482) when both are in contact with thermoset materials. It should be considered that the conductor is the center of the thermal mass of the cable. All heat generated by current loading has to radially dissipate to the cable surface by propagating through all the different cable layers and associated thermal conductivities depending on cable construction. In contrast the metallic shield being usually under the outer covering(s) only has to dissipate heat through said layer. This outer layer having the ability to withstand more heat allows the acceptance of higher temperatures as opposed to the conductor.

An important simplifying assumption in the formula is the adiabatic nature of the heat generated, i.e., the duration of the fault is so short that all the heat developed by the fault current during this time is assumed to be completely contained within the sheath or shield. The amount of heat dissipated from the sheath or shield during continuous, single fault occurrences of relatively short duration is small. A significant amount of heat may be dissipated because of the relatively long cooling periods involved for faults interrupted and reestablished with automatic reclosing of circuit protective devices. A non adiabatic calculation may be more suitable for these situations and for single, uninterrupted short circuits in excess of 2 seconds requiring close accuracy. Non-adiabatic calculation methods are described in several published works listed in Section 4 “Bibliography”.

The formula described in this publication is based on the thermal capacity of the conductor material and the transient temperature limit of the insulation. The quantity of heat contained in the conductor is that created by the fault current and is also a function of the temperature rise in the conductor. The magnitude of the temperature rise is the difference between the limiting transient temperature of the insulation material and the operating temperature of the conductor immediately prior to the initiation of the fault. The limiting transient temperature is that temperature which caused no significant change in any cable component. See Section 3, Table 3-3 for suggested estimated values.