Parameters of circuit breakers in level three distribution boxes

Circuit breakers in level three distribution boxes are selected based on rated current, breaking capacity, voltage rating, number of poles, trip settings, and coordination with upstream and downstream devices.

Key Parameters

1. Rated Current (In): The rated current is the maximum continuous current the breaker can carry without tripping, typically defined for an ambient temperature of 40°C. Adjustable trip units allow setting the rated current within a range, e.g., 0.4–1.0 × In for electronic trip units or 0.7–1.0 × In for thermal-magnetic units. The breaker must handle the maximum load current (Ib) but remain below the cable's permissible current (Iz) to prevent overheating ( ). 2. Rated Voltage (Ue): This is the maximum RMS voltage the breaker can safely interrupt. For low-voltage systems (<1000 V), the rated voltage is usually 1.1 times the system voltage. Three-phase breakers are typically 3-pole or 4-pole, with 4-pole breakers also interrupting the neutral ( ). 3. Breaking Capacity (Icu) and Making Capacity (Icm):

• Breaking capacity (Icu): Maximum fault current the breaker can safely interrupt.
• Making capacity (Icm): Maximum current the breaker can close onto without damage.
• Short-time withstand current (Icw): Current the breaker can carry for a specified duration without thermal damage ( ). 4. Trip Settings:
• Long-time (overload) trip: Protects against sustained overloads.
• Short-time trip: Protects against short circuits with a slight delay for selectivity.
• Instantaneous trip: Immediate disconnection for high fault currents. Proper coordination ensures upstream breakers trip after downstream breakers to maintain selectivity ( ). 5. Selectivity and Cascading: Level three distribution boxes often use cascading protection, where upstream breakers limit fault current to downstream breakers. Selective breakers allow staged protection, maintaining power to healthy circuits while isolating faults ( ). 6. Environmental and Derating Considerations: Ambient temperature affects trip settings. Breakers in hot enclosures or high-temperature locations must be derated. For example, a breaker rated for 40°C may trip at lower currents if installed in a 60°C environment ( ). 7. Number of Poles: Three-phase systems typically use 3-pole breakers; 4-pole breakers include the neutral. The number of poles ensures simultaneous interruption of all phases to prevent unbalanced faults ( ). 8. Coordination with Transformer and Load: For installations fed by a transformer, the main breaker must handle the maximum short-circuit current from the transformer. Outgoing feeders are selected based on load and fault current, e.g., NSX400 H, NSX250 H, and NSX100 H with Icu ratings of 70 kA for typical LV distribution ( ). By carefully evaluating these parameters, engineers can ensure safe, reliable, and selective protection in level three distribution boxes, accommodating both normal operation and fault conditions.