Cable short circuit rating calculation provides a tool for evaluating the compliance of cable size in term of fault current. Firstly, the calculation will derive a short circuit at load end (i.e. motor terminal) from supply-side (i.e. MCC) prospective short-circuit and cable parameters. Secondly, the allowable short circuit rating of that given cable size will be determined in accordance with BS 7671 18th Edition.
|System Impedance (Zs)||:||Ohm|
|Cable Impedance (Zc)||:||Ohm|
|Total Impedance (Zs + Zc)||:||Ohm|
|Calculated Short Circuit at Load||:||kA|
|Allowable Cable Short Circuit (BS7671:2018 chapter 54)||:||kA|
Prospective Short Circuit
Prospective Short Circuit is derived from Switchgear Busbar Rating or Power System Short Circuit Study.
Protection Tripping Time
In case accurate tripping time is not at hand, recommended clearing time Table 9-3a/b/c/d/e of IEEE Std 242 can be used.
What is Short Circuit Withstand Rating?
The short circuit current withstand capability is a measure of the short circuit current a device can tolerate without being damaged when a fault let-through the device.
Cable Short circuit current happens in a quick duration, therefore, the heat from I2R losses is almost entirely in the conductors. The cable shall be sized so that the given fault current will not raise the cable conductor temperature over the permissible limiting temperature that will damage the cable.
Cable Short Circuit Rating Formula
The capability of the cables or cable short circuit rating to withstand the short circuit current shall be verified by the following formula:
|Isc||:||Allowable short-circuit of a given cable size (kA)|
|k||:||Coefficient whose value is relevant to type of insulation material and final temperature reached for short circuit|
- k = 143 for copper conductor, XLPE/EPR insulation
|S||:||Cable nominal cross-section (sqmm)|
|t||:||Short circuit duration (s)|
Allowable Short Circuit Currents for Insulated Copper Conductor?
For a copper cable, allowable let-through fault current varies with material of insulation material. Different material will allow and withstand a temperature without damage or deteriorate cable life time. In this aspect, thermosetting has a better performance than thermoplastic.
Applied cable short circuit rating formula and coefficient constant (k) for different insulation type, allowable short circuit currents is a below:
- For thermoplastic insulation: Isc (kA) = 143 x S
- For thermosetting insulation: Isc (kA) = 176 x S