Contents
Standards Covered:
- IEC 60898-1 (MCBs)
- IEC 60947-2 (MCCBs, ACBs)
- SBC 401 (Saudi Building Code – Electrical)
- NEC Article 240, IEEE 1015
📘 1.0 Introduction
Circuit breakers are automatic protection devices that safeguard electrical installations from faults such as overloads, short circuits, and sometimes earth faults. They operate by automatically disconnecting the power supply when dangerous current levels are detected.
Key factors in selecting a circuit breaker include:
- Rated current (In)
- Voltage rating (Ue)
- Short-circuit breaking capacity (Icu/Ics)
- Tripping curve (B, C, D, K)
- Application environment
This article explores Miniature Circuit Breakers (MCBs) in depth — including their construction, working principle, IEC tripping curves, and where to use each type in compliance with international and Saudi SBC 401 standards.
📚 2.0 Classification of Circuit Breakers
| Breaker Type | Voltage Range | Typical Applications |
|---|---|---|
| MCB | ≤ 415 V AC | Lighting, small loads, final circuits |
| MCCB | ≤ 690 V AC | Panel feeders, high-load circuits |
| ACB | ≤ 1000 V AC | Main LV incomer, large equipment |
| VCB | 3.3 – 33 kV | Distribution substations |
| SF₆/Hybrid | 66 – 400 kV | High-voltage transmission systems |
⚙️ 3.0 Miniature Circuit Breakers (MCB)
🔹 Standard: IEC 60898-1
| Clause | Description |
|---|---|
| 5.3.6 | Overcurrent protection method |
| 5.4 | Time-current tripping characteristics |
| 6.3 | Testing short-circuit breaking capacity |
| Annex A | Reference tripping curves B, C, D |
SBC 401 (Electrical) requires the use of MCBs that comply with IEC 60898-1 for residential and commercial electrical protection.
🧱 3.1 MCB Construction
- Bimetallic strip – detects overloads (thermal trip)
- Solenoid coil – detects short circuits (magnetic trip)
- Arc chute – safely extinguishes arc during interruption
- Toggle lever – for manual operation
- Trip latch – common actuator for both thermal/magnetic trips
📌 Standard MCB construction diagram is recommended for visual reference in training documents.
⚡ 3.2 Working Principle
| Fault Type | Detection Method | Trip Mechanism |
|---|---|---|
| Overload | Bimetal heats & bends | Releases latch |
| Short Circuit | Solenoid generates magnetic force | Trips latch instantly |
| Manual Operation | Toggle switch | Directly opens contacts |
📈 3.3 MCB Tripping Curves (B, C, D, K)
📊 Tripping Curve Definitions
| Curve | Instant Trip Range (×In) | Application |
|---|---|---|
| B | 3–5 × In | Residential lighting, resistive loads |
| C | 5–10 × In | General use, power circuits |
| D | 10–20 × In | Inductive loads, motors |
| K | 8–12 × In | Heavy inrush loads, HVAC |
All MCBs have both thermal (inverse time) and magnetic (instantaneous) trip units.
📉 Graph: Time vs Multiple of Rated Current
This graph shows how faster tripping occurs as the fault current increases.
- Curve B is the most sensitive
- Curve D allows high inrush before trip (good for motors)
📏 3.4 MCB Current Ratings
| Frame Size | Standard Ratings (A) | Breaking Capacity |
|---|---|---|
| 6kA | 6, 10, 16, 20, 25, 32, 40, 63 | Residential |
| 10kA | 6–63 A | Commercial, LV panels |
| 15kA | 16–63 A (C/D curves) | High fault areas |
- Voltage rating: 230 V (1P), 400–415 V (3P, 3P+N)
- Frequency: 50/60 Hz
- Breaking capacity: min 6kA (residential), ≥10kA preferred in commercial use
📌 3.5 Where to Use Each Curve
| Application | Preferred Curve |
|---|---|
| Residential lighting | B |
| Socket outlets | C |
| AC units / HVAC | D |
| Water pumps | D or K |
| Panel feeders | C |
| IT/UPS supply | C (sensitive load) |
| Machine with inrush | K |
✅ Never use B-curve for inductive loads — it may trip unnecessarily.
📎 Summary
- MCBs are critical for final circuit protection under 1000 V.
- Choose the correct curve type (B, C, D, K) based on load behavior.
- Confirm breaking capacity is suitable for the expected fault level.
- Follow IEC 60898-1 and SBC 401 for compliance in KSA projects.
📚 References
- IEC 60898-1: Circuit-breakers for overcurrent protection
- IEC 60947-2: Low-voltage switchgear and controlgear – Circuit breakers
- SBC 401: Saudi Building Code – Electrical Requirements
- IEEE 1015: Guide for application of low-voltage CBs
- NEC Article 240: Overcurrent Protection