Protection for Safety: Thermal Effects, Overcurrent and Surge
Overcurrent: overload vs short circuit
Overload is a current in an otherwise healthy circuit; short circuit is a fault between live conductors (or to earth). Protection must operate before thermal damage occurs. The core coordination rule is:
Ib ≤ In ≤ Iz and I2 ≤ 1.45 × Iz
where Ib is design current, In the device nominal rating, Iz the cable current-carrying capacity, and I2 the current ensuring effective operation. For standard MCBs and fuses, I2 ≤ 1.45 Iz is met by the device standard, so the practical check is Ib ≤ In ≤ Iz. A frequent trap is choosing In to match Ib but ignoring derating factors (Iz after correction must still exceed In).
Short-circuit protection and breaking capacity
The device's rated short-circuit breaking capacity must exceed the prospective fault current at its point of installation. Also check the energy let-through (I²t) does not exceed the cable's withstand. Questions ask whether a device with adequate In but low breaking capacity is acceptable - it is not.
Thermal effects beyond overcurrent
Consider ambient temperature, grouping, thermal insulation, and the effect of harmonics on neutral current. Derating factors multiply; apply all that apply.
Surge protection
Surge protective devices (SPDs) limit transient overvoltages. They are selected by Type (impulse waveform), voltage protection level (Up) and location (origin vs sub-distribution). SPDs do not replace overcurrent protection; they need their own short-circuit/backup protection.
Worked example
A circuit with Ib = 20 A uses a cable with tabulated Iz = 27 A. Ambient derating 0.87 and grouping 0.8 give corrected Iz = 27 × 0.87 × 0.8 ≈ 18.8 A. Even a 20 A device now violates Ib ≤ In ≤ Iz - you must upsize the cable.