Short Circuits

On power systems with particularly high KVA capacity, the available short circuit current must be considered in the selection of the conductor size and cable shield design. The graphs in this section show the maximum currents that various cables and shields can carry for different time durations without degrading the insulation system and jackets.

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Fault Currents

When calculating how long a conductor can carry a particular fault current, or determining the fault current a conductor can withstand for a specific time, it is assumed that the total heat generated is stored in the conductor. This assumption is valid for the brief duration of the short circuit, without any heat dissipation to the environment.

To analyze fault currents, one of the following three parameters must be known:
- The allowable fault current (I),
- The allowable duration of time (t),
- The necessary cross-sectional area (A) of the metal to sustain a particular fault.

The fault current is calculated using the following equation:

I = sqrt((k × A²) / t)

Where:
- A = Total cross-sectional area of conductor or shield material (circular mils).
- I = Fault current (amperes).
- t = Duration of fault (seconds).
- k = Material constant for conductor or shield with fixed initial and final temperatures.

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Material Constants for Short Circuit Calculations

The k value in the above equation depends on the material of the conductor or shield, as shown in the table below:

Shield Material	Conductor Material
Copper (k = 6.258 × 10⁻³)	Copper (HV) (k = 5.215 × 10⁻³)
Cupro-Nickel (k = 0.560 × 10⁻³)	Aluminum (HV) (k = 2.341 × 10⁻³)
N/A	Copper (MV) (k = 4.627 × 10⁻³)
N/A	Aluminum (MV) (k = 2.077 × 10⁻³)

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Starting and Maximum Final Temperatures for Conductors

Starting Temperature	65°C	90°C	105°C
Maximum Final Temperature	250°C	250°C	250°C

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Key Considerations:
- The duration of the fault is crucial in determining how much current a conductor can carry.
- If the fault duration is too long, excessive heating can damage insulation and lead to system failure.
- The k value varies depending on the material and its thermal properties.
- Engineers must ensure that cables and shields meet the required specifications to handle potential fault currents.

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This structured explanation allows for a clear understanding of short circuit and fault current calculations while making the information easy to read and reference.