Protection, switching, cables and fault current calculations
Domain 5: Protection, switching, cables and fault current calculations
This domain tests coordination of protective devices with cable capacity and fault energy. You must calculate prospective fault current, select breaking capacity, check thermal withstand and prove discrimination.
Core rules
- Overload protection: Ib ≤ In ≤ Iz, where Ib is design current, In the device rating and Iz the cable capacity after derating.
- Short-circuit protection: device breaking capacity must exceed prospective fault current at its terminals.
- Cable thermal withstand: S ≥ √(I²t)/k, using the fault current, disconnection time and conductor k factor.
- Check earth-fault loop impedance so disconnection occurs within the required time.
- Verify discrimination between upstream and downstream devices using time-current curves, not current rating alone.
- Account for ambient temperature, grouping, insulation, installation method and harmonics in cable derating.
Worked example
A cable must withstand a 6 kA fault cleared in 0.10 s; k = 115 for the conductor/insulation combination. I²t = (6000)² × 0.10 = 3.6 × 10⁶ A²s. √(I²t) = 1897 A√s. Minimum area = 1897/115 = 16.5 mm². A 16 mm² conductor is therefore inadequate; select at least 25 mm², then recheck voltage drop and protection settings.
Traps
- Using normal load current instead of fault current.
- Forgetting the k factor or disconnection time.
- Selecting cable from current rating only.
- Assuming discrimination from dissimilar ratings.