ABB MCB Trip Curves & Selectivity Guide (System pro M™ S200)
How to choose B/C/D curves and coordinate downstream MCBs with upstream protection. Miniature circuit breakers (MCBs) must clear faults quickly without tripping on normal inrush. ABB’s System pro M™ S200 range offers different instantaneous trip curves—B, C, and D—to match load behavior. This guide explains what those curves mean, how to size breaking capacity against your site’s prospective fault current (AFC), and how to approach selectivity/cascading with upstream MCCBs. It also includes quick visuals (rendered with Chart.js) that you can reuse on similar projects.
1) What B, C, D trip curves mean
Trip curves define the multiple of the breaker’s rated current (In) at which the magnetic (instantaneous) element operates. Typical bands for ABB S200–family MCBs:
- B curve: trips at roughly 3–5 × In — best for purely resistive loads and very low inrush.
- C curve: trips at roughly 5–10 × In — mixed loads and small/standard motors.
- D curve: trips at roughly 10–20 × In — high inrush (large motors, transformers, capacitive banks).
Figure 1 — Typical instantaneous bands used for quick curve selection. Always verify against the specific ABB datasheet for the exact part number.
2) Breaking capacity vs prospective fault current (AFC)
The MCB’s breaking capacity (per IEC 60898-1 use Icn; per IEC 60947-2 use Icu/Ics) must be equal to or greater than the AFC at the installation point. A safe field workflow:
- Estimate AFC at each board (utility short-circuit + cable impedance + transformer data, or measurements where appropriate).
- Choose Icn/Icu ≥ AFC, including margin and coordination with upstream protection (backup protection where permitted).
- Re-check selectivity/cascading with the upstream MCCB using ABB’s coordination tables.
Figure 2 — Example check that the chosen Icn at each board exceeds local AFC. If AFC is close to the breaker limit, consider a higher-capacity device or upstream current-limiting.
3) Selectivity & cascading — basic workflow
- Verify downstream In and protective settings first (thermal sizing per conductor, ambient, grouping).
- Check manufacturer selectivity/cascading tables for the exact device pair and rating (MCB ⟷ MCCB, voltage, prospective short-circuit).
- Use backup protection (where certified by ABB) if full selectivity cannot be achieved with the chosen pair.
- Record torque, accessories, and settings (trip units, shunt releases) for handover and future maintenance.
Tip: ABB provides certified coordination files; always match the exact catalog codes, voltage, and installation category when you apply the tables.
4) Quick selection table (rule-of-thumb)
| Load type | Typical inrush / behavior | Suggested curve | Notes |
|---|---|---|---|
| Resistive (heating, incandescent) | Very low inrush | B | Fast nuisance-free operation on short-circuits |
| Mixed receptacles / lighting (LED drivers vary) | Low–moderate inrush | C | Default for general purpose final circuits |
| Small motors, office AHU, pumps | Moderate inrush | C | Verify manufacturer motor LRA/FLA |
| Large motors, transformers, capacitor banks | High inrush / magnetizing | D | Check start method; consider soft-start/VFD and upstream current-limiting |
Table 1 — Heuristics only; always verify against ABB curve data for the exact part and the site’s measured inrush.
5) Practical checklist
- Confirm In, conductor size, and derating (ambient, grouping, enclosure).
- Choose B/C/D curve based on measured or specified inrush characteristics.
- For each board, ensure Icn/Icu/Ics ≥ AFC with margin.
- Use ABB selectivity/cascading tables for the exact pair (MCB↔MCCB) and voltage.
- Record part numbers, torque values, accessories, settings for O&M.
6) Helpful references
- ABB Circuit Breakers — shop catalog
- IEC 60898-1 (MCBs for household/analogous installations): definitions of Icn.
- IEC 60947-2 (industrial circuit-breakers): definitions of Icu/Ics and coordination rules.
- ABB System pro M™ S200 series datasheets and selectivity/backup-protection tables.
Disclaimer. The values and visuals herein are educational. Always check ABB’s latest datasheets, certificates, and coordination files for the exact catalog number, voltage, and installation category before final selection.