Circuit breaker coordination and selectivity for low-voltage industrial installations. Covers the three coordination strategies — full selectivity, partial selectivity, and cascading — and how to read manufacturer selectivity tables. Includes worked examples for MCB-MCB, MCCB-MCB, and ACB-MCCB combinations at prospective short-circuit currents up to 50 kA.
In a low-voltage distribution board, several breakers stand in series between the incoming supply and the final load. A fault at any point should trip only the nearest upstream breaker, isolating the smallest possible portion of the installation. If instead the upstream breaker trips before the downstream one, the fault has cascaded and every load fed through the upstream breaker goes dark unnecessarily. Coordinated selectivity is the design discipline that prevents cascading.
| Strategy | Meaning | Use when |
|---|---|---|
| Full selectivity | Downstream breaker always trips first, at any current up to its own Icn. Upstream stays closed. | Critical loads, hospital IT panels, data centres |
| Partial (limited) selectivity | Selectivity holds up to a defined limit current (Is). Above Is both breakers trip. | General industrial, where a rare severe fault can be tolerated |
| Cascading (back-up protection) | Downstream breaker has lower Icn than the prospective short-circuit current at its terminals. Upstream breaker helps interrupt, but both trip. | Cost-driven design where selectivity is not required; supply protection to a small sub-panel |
Each major OEM (Schneider, ABB, Siemens, Eaton, Hager) publishes selectivity tables listing every pairing of their own breakers as (Is) selectivity-limit currents. The buyer picks the upstream breaker on the left column and the downstream on the top row; the cell entry is the maximum fault current up to which selectivity is preserved. "T" or "∞" indicates total selectivity to the upstream's Icu.
| Level | Breaker | Rating | Icu (kA) | Setting |
|---|---|---|---|---|
| Main incomer (ACB) | ABB Emax E2.2N | 2000 A | 65 | Ir = 1.0 × In, Isd = 5 × In, tsd = 200 ms |
| Sub-distribution (MCCB) | ABB Tmax XT4 250 | 250 A | 36 | Ir = 0.8 × In, Isd = 8 × Ir, instantaneous |
| Final circuit (MCB) | ABB S203 C63 | 63 A | 10 | C curve (5-10 × In magnetic) |
At a fault of 5 kA on the final circuit: MCB clears in < 10 ms. The MCCB sees 5 kA in its magnetic band but the intentional 40 ms delay from the MCCB's zone-selective interlock (ZSI) with the MCB prevents it tripping. The ACB sees 5 kA well below its 200 ms short-time delay window and holds. Only the S203 trips. Full selectivity achieved.
| Upstream | Downstream | Icu of downstream | Enhanced Icu with cascading |
|---|---|---|---|
| Schneider iC60N 63 A | Schneider iC60H 20 A | 10 kA | 50 kA (back-up) |
| ABB S203 32 A | ABB S201 6 A | 6 kA | 25 kA (back-up) |
| ABB Tmax XT2 160 | ABB S203 63 A | 10 kA | 36 kA (back-up) |