Cable Protection in Electrical Panels

Improper short-circuit calculations in the design and construction of electrical installations have led to faulty implementations. This results in damages to fuses, cables, and other equipment, causing fires and accidents.

According to IEC 60364-5-6, when a short circuit occurs in any circuit, the fault current must be interrupted before the cable insulation reaches its maximum permissible temperature.

Another important factor in cable selection is determining the short-circuit current capacity. The correct cable cross-section must be selected based on this capacity.

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Short-Circuit Current and Cable Selection

During a short circuit, the energy generated by the fault current is stored in the conductor. Under these conditions, the conductor should not heat beyond the permissible temperature limits. This means the initial conductor temperature and the short-circuit duration must be considered.

To determine the short-circuit current carrying capacity of cables, the following formulas are used:

Sn = (Ith / Jthr) × √(tk / tkr)

Where:

• Sn: Required cable cross-section for the short-circuit duration
• Ith: Effective short-circuit current flowing during the fault
• Jthr: Nominal short-time current density
• tk: Short-circuit duration
• tkr: Conductor short-circuit endurance time

Permissible Disconnection Time Calculation:

tkz = k × (Sn / Ith)²

Where:

• tkz: Allowable disconnection time
• k: Coefficient

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Example Calculation:

Using the formula:

Sn = (Ith / Jthr) × √(tk / tkr)
Sn = (720 / 103) × √(0.01 / 1)
Sn = 0.69 mm² cable

Based on this result, the following minimum cable sizes are selected for different circuit breakers:

• 100A compact breaker → 1 mm² cable
• 250A breaker → 2.5 mm² cable
• 400A breaker → 4 mm² cable
• 630A breaker → 6 mm² cable
• 800A breaker → 6 mm² cable
• 1000A breaker → 10 mm² cable
• 2500A breaker → 50 mm² cable
• 4000A breaker → 70 mm² cable

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Thermal Short-Circuit Capacity Calculation:

Using the following formula, we calculate the thermal short-circuit capacity of a 16mm² cable:

Ithz = qd × Jthr × √(tkr / tk)

Where:

• Ithz: Thermal short-circuit capacity
• qd: Selected cable cross-section
• Jthr: Nominal short-time current density
• tk: Short-circuit duration
• tkr: Conductor short-circuit endurance time

For a 16mm² cable:

Ithz = 16.103 × √(1 / 0.05)
Ithz = 7.36 kA


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Simulation Analysis of Circuit Breakers

Simulations using Siemens SIMARIS Curves provide insight into how different breakers respond to short-circuit conditions:

• 100A thermal-magnetic breaker: Trips in 0.001 seconds at 7.2 times nominal current.
• 1000A open-type breaker: Trips in 0.034 seconds at 7.2 times nominal current.
• 10A automatic fuse (C-type): Trips in 1.784 seconds at 7.2 times nominal current.

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Final Thoughts:

• Circuit breakers should be selected according to short-circuit current calculations to ensure cable protection.
• Incorrect calculations may lead to overheating, insulation damage, and potential fires.
• Using compact breakers instead of open-type breakers in panels enhances safety.

These calculations ensure that electrical panels are protected against short circuits, improving system reliability and preventing hazards.