Branch Circuits, Feeders, and Overcurrent Protection
Coordination, not just rating
Branch Circuits, Feeders, and Overcurrent Protection tests whether you can coordinate the load, the conductor and the protective device so that overload and fault current are cleared without damaging the cable or losing selectivity. The basic rule is Ib is less than or equal to In is less than or equal to Iz, but the device must also have adequate breaking capacity and the correct tripping characteristic.
Core rules
- Type B MCBs trip magnetically at 3 to 5 times In, Type C at 5 to 10 times In and Type D at 10 to 20 times In. Higher magnetic thresholds demand lower earth fault loop impedance.
- The rated short-circuit capacity of the device must exceed the prospective fault current at its terminals. A domestic MCB is not automatically suitable for a heavy industrial busbar.
- Discrimination means the downstream device clears a fault before the upstream device. Time-current curves,current limitation and cascading data must be checked; identical ratings in series do not guarantee discrimination.
- RCDs, AFDDs and SPDs are additional protective measures, not replacements for overcurrent protection. Feeders need their own overload and short-circuit protection unless the upstream device is proven to protect the feeder.
- Diversity may reduce feeder size but must be documented. Continuous industrial loads should be assessed on realistic duty cycle, not optimistic assumptions.
Worked example
A feeder has a design current of 28 A. After ambient and grouping correction, the chosen cable has Iz = 34 A. A 32 A MCB satisfies 28 is less than or equal to 32 is less than or equal to 34. The prospective fault current at the feeder origin is 6 kA, so the MCB must have a 6 kA or higher breaking capacity. For a Type B32 device, the maximum Zs for 0.4 s is 1.37 ohm; a measured Zs of 1.5 ohm would fail despite the current rating being correct. The reasoning is that current rating, breaking capacity and disconnection time are three separate tests.
Traps that cost marks
Choosing a breaker to match the load but ignoring the corrected cable capacity; installing a Type C or D device without rechecking Zs; assuming an upstream 63 A breaker protects a 32 A feeder; and treating an RCD as if it provided short-circuit protection.