Contents
Standards Referenced:
- IEC 60947-2
- IEC 60947-1 (General Rules)
- SBC 401 (Saudi Building Code – Electrical)
📘 1.0 What is an MCCB?
MCCB (Molded Case Circuit Breaker) is a protective device used for medium-level power distribution (up to 1000V AC) with higher current capacity (up to 2500A). Unlike MCBs, MCCBs offer adjustable trip settings, selective coordination, and high breaking capacities, making them ideal for industrial, commercial, and utility-grade panels.
🧱 2.0 MCCB Construction (With Diagram)
Main Components:
- Thermal Trip Unit (Overload protection)
- Magnetic Trip Unit (Short-circuit protection)
- Arc Chute (Interrupts arc during opening)
- Contacts (Fixed & Moving)
- Operating Mechanism (Toggle, handle, motorized)
- Trip Mechanism (Connects detection to contacts)
MCCBs are enclosed in a high-strength molded plastic case that insulates and protects internal components.
⚡ 3.0 Working Principle
| Fault Type | Detection | Trip Mechanism |
|---|---|---|
| Overload | Thermal bimetal (Ir) | Bends, releases trip latch |
| Short Circuit | Electromagnetic coil (Ii) | Instantaneous magnetic action |
| Ground Fault (optional) | Electronic trip unit | Logic-based detection |
| Manual OFF | Toggle/Remote | Mechanically actuated |
MCCBs are thermo-magnetic or electronic in trip unit type.
🧠 4.0 IEC 60947-2 Clauses (Key Sections)
| Clause | Description |
|---|---|
| 2.1 | Definitions: Icu, Ics, Icm |
| 4.3 | Test conditions for short-circuit rating |
| 4.4.1 | Tripping characteristic – Overload (long delay, short delay, instantaneous) |
| 4.5 | Coordination & selectivity rules |
| 7.1.2 | Terminal marking and identification |
SBC 401 requires conformance to IEC 60947-2 for main circuit protection above 100A.
🔧 5.0 MCCB Ratings & Settings
🔹 5.1 Frame Size (Inm) vs Rated Current (Ir)
| MCCB Frame (Inm) | Adjustable Current Range (Ir) |
|---|---|
| 100 A | 63–100 A |
| 250 A | 100–250 A |
| 400 A | 250–400 A |
| 630 A | 400–630 A |
| 800 A | 500–800 A |
| 1000 A | 600–1000 A |
| 1600 A | 1000–1600 A |
| 2500 A | 1600–2500 A |
Ir is the long-time pickup current. Adjustable using dials or electronic UI.
🔹 5.2 Trip Settings
| Function | Symbol | Description |
|---|---|---|
| Long Time Delay | Ir | Adjustable (current & time) – for overloads |
| Short Time Delay | Isd | Adjustable – for short circuits |
| Instantaneous | Ii | Fixed or adjustable – for high faults |
| Ground Fault | Ig | Optional (electronic trip unit only) |
Curve types:
- LSI (Long Short Instantaneous)
- LSIG (with Ground fault)
⚙️ 6.0 MCCB vs MCB
| Feature | MCB | MCCB |
|---|---|---|
| Max Current | Up to 100 A | Up to 2500 A |
| Adjustability | Fixed | Adjustable (Ir, Ii, etc.) |
| Trip Curve | B, C, D | LSI, LSIG |
| Breaking Capacity | 6–15 kA | 25–100 kA |
| Use | Final circuits | Main/submain panels |
| Standard | IEC 60898-1 | IEC 60947-2 |
🧰 7.0 Where to Use MCCBs
| Application | MCCB Size | Curve Type | Notes |
|---|---|---|---|
| Panel Main Incomer | 400–1600 A | LSI | With short-time delay for selectivity |
| Motor Feeders | 100–400 A | D/K (or LSIG) | Use adjustable Ii |
| HVAC Systems | 250–630 A | LSI | With thermal-magnetic units |
| UPS Outgoing | 125–400 A | LSI | Must avoid nuisance trips |
| Generator Output | 400–800 A | LSIG | Allow back-feed current |
📎 8.0 Selection Tips
✅ Always match:
- Inm ≥ Load Current
- Icu ≥ Fault level at installation point
- Trip settings adjusted after coordination study
✅ MCCBs are required where:
- Adjustable protection is needed
- Fault levels exceed 10–15kA
- Panelboard has multiple tiered breakers
- Remote operation or metering is needed
📚 References
- IEC 60947-2: Low-voltage switchgear & controlgear – CBs
- SBC 401: Electrical installations – Saudi Code
- NEMA AB1: Molded-case circuit breakers
- IEEE 1015: Applying low-voltage power CBs
- Schneider, ABB, Siemens application handbooks