Total selectivity (discrimination) means the upstream device will clear a downstream fault without the downstream device tripping. Per IEC 60947-2, this requires Irm_upstream > 1.5 × Irm_downstream (current-based) and sufficient time delay in the upstream device so that the downstream clears first on any fault level within its breaking capacity.
Ik_down = c·U_n / (√3·Z_total) per IEC 60909, where Z_total = Z_source + Z_cable. Z_source is derived from the provided supply Ik at the origin. Cable impedance Z_cable = √(R²+X²) using the conductor material resistivity and reactance per unit length. For a 400V / 10kA supply with 50m of 10mm² copper, Ik at the load terminals is approximately 2–3 kA.
Backup protection is required when the downstream device has a breaking capacity (Icu) less than the short-circuit current at its installation point (Ik_downstream). In this case the upstream device must be able to clear the downstream fault if the downstream device fails to interrupt. IEC 60364-4-43 §434.3 requires coordination so that the upstream device provides backup when the downstream is inadequate.
IEC 60947-2 requires that the energy let-through of the upstream device (I²t_up) exceeds that of the downstream device (I²t_down) at the downstream trip current. This ensures the upstream device trips last, protecting the downstream coordination zone. Calculated as I²t = I_r² × t_trip for each device. The upstream must have I²t_up > I²t_down for discriminative coordination.
Per IEC 60364-5-54 §543, the cable must withstand the thermal effects of the short-circuit current without damage. The criterion is I²t_fault ≤ k²·S², where k is a factor depending on conductor material and insulation (k=143 for Cu/PVC, k=115 for Al/PVC, k=76 for Cu/XLPE). If I²t exceeds k²S², the cable cross-section must be increased or a current-limiting device must be installed upstream.
400V system, Ik=10 kA at origin, upstream 250A MCCB Type C Icu=25kA (tripping time 50ms), downstream 63A MCB Type C Icu=10kA (tripping time 20ms), Cu 10mm² 50m cable. Expected result: Ik downstream ≈ 2.07 kA, both breaking capacity PASS, total selectivity ( Irm upstream=1250A > 1.5×315A=472A ), I²t upstream ≈ 78,750 A²s >> I²t downstream ≈ 1,489 A²s → PASS, cable thermal PASS.