1. Determine the Application Type
- IEC 60898: For residential and commercial installations.
- IEC 60947-2: For industrial and complex electrical networks.
2. Define System Parameters
- Voltage Rating (Ue): Must match or exceed the system’s nominal voltage.
- Frequency: Typically 50/60 Hz.
- Installation Category: TN-S, TN-C, TT, IT networks.
3. Select the Rated Current (In)
- Based on the continuous load current of the circuit.
- Must consider ambient temperature derating and cable capacity.
4. Short-Circuit Breaking Capacity (Icu / Ics)
- Icu (Ultimate Breaking Capacity): Maximum fault current the breaker can interrupt without damage.
- Ics (Service Breaking Capacity): Breaker’s ability to operate safely under fault conditions multiple times.
According to IEC 60947-2:
- Ics ≥ 25% to 100% of Icu depending on classification.
Match the Icu/Ics to the prospective short-circuit current (Isc) at the installation point, determined by fault level analysis.
5. Trip Curve Selection
Defines how fast the breaker reacts under overcurrent. Common curves:
- B Curve: Trips at 3–5 × In – suitable for residential loads
- C Curve: Trips at 5–10 × In – ideal for general purpose & small motors
- D Curve: Trips at 10–20 × In – for heavy inductive loads like large motors
6. Coordination with Downstream Devices
- Ensure selective coordination to prevent nuisance tripping.
- Apply discrimination and back-up protection principles per IEC guidelines.
7. Breaking Technique
- Thermal-magnetic: Standard for MCBs
- Electronic trip units: Preferred for MCCBs in industrial settings, offering adjustable protection settings
8. Environmental and Mounting Considerations
- Altitude and temperature affect derating.
- Mounting position, enclosure type (IP rating), and pollution degree must comply with IEC 60664-1 and IEC 60529.
9. Accessories and Integration
- Remote trip units, auxiliary contacts, undervoltage releases, etc., must be selected as per control requirements.
- Ensure compatibility with Building Management Systems (BMS) or SCADA if integration is required.
Conclusion
Proper breaker selection under IEC standards is not just about current and voltage—it involves coordination, fault level analysis, protection class, and system design integration. For mission-critical or industrial environments, partner with a certified electrical consultant or integrator to ensure full compliance and operational safety.