00 Preface

When discussing the tripping modes of low-voltage circuit breakers, many professionals feel both familiar and unfamiliar with the topic. While low-voltage system diagrams often emphasize specifications like "main incoming switches with long/short delays, instantaneous tripping, and grounding protection," these brief descriptions conceal significant technical depth. For instance:

· What exactly is tripping, and what tripping modes exist?

· What defines the three-stage protection, and how does it relate to tripping?

· What are short-circuit breaking capacities (Ics/Icu)?

This article demystifies these concepts to deepen your understanding of low-voltage circuit breaker operations.

01 What Is Tripping? Tripping Modes of Low-Voltage Circuit Breakers

1.Tripping Mechanism

A trip device releases the holding mechanism to open/close the circuit breaker. It consists of a force-reduction linkage and a releasable fulcrum, protecting distribution lines, equipment, and motors from overloads and short circuits. Common trip types include:

· Overcurrent trip devices

· Undervoltage trip devices

· Shunt trip devices

2.Overcurrent Trip Devices

Short-Circuit Tripping: Instantaneous or short-delay electromagnetic trips. Short-circuit currents cause severe electrodynamic/thermal stress, requiring immediate (instantaneous) tripping. Short-delay trips (<1s) ensure selectivity between upstream/downstream breakers, minimizing outage scope.

Overload Tripping (Thermal/Long-Delay): Delays action to verify sustained overloads. Thermal releases respond to prolonged overcurrents, while thermal-magnetic (compound) releases combine electromagnetic and thermal mechanisms.

3.Undervoltage Trip Devices

Trips at 35%-70% of rated voltage, prevents closure below 35%, and allows closure at 85%-110%. Protects equipment from voltage instability by disconnecting during severe fluctuations.

4.Shunt Trip Devices

A trip coil triggered by external signals (e.g., 24V DC in fire alarms). Used for remote shutdowns, such as cutting non-fire loads during emergencies.

5.Electronic Trip Devices

Monitors leakage current via zero-sequence transformers. Triggers within 0.1s for electric shock or ground fault protection.

02 Three-Stage Protection and Its Relationship with Tripping

The three-stage protection of low-voltage circuit breakers includes:

1. Instantaneous Short-Circuit Protection (electromagnetic tripping).

2. Short-Delay Short-Circuit Protection(electromagnetic tripping with selectivity).

3. Long-Delay Overload Protection (thermal tripping).

Thermal-magnetic releases often combine these functions for comprehensive protection.

03 Short-Circuit Breaking Capacities

1.Rated Service Short-Circuit Breaking Capacity (Ics):

Maximum fault current a breaker can interrupt and remain operational post-fault. Example: A 50kA Ics-rated breaker can safely disconnect and reuse after a 50kA fault.

2.Rated Ultimate Short-Circuit Breaking Capacity (Icu):

Maximum fault current a breaker can interrupt once without failure. Post-operation, the breaker may require replacement. Example: A 50kA Icu-rated device cannot reliably operate after interrupting a 50kA fault.

Selection Principle:

· Ensure Icu > line’s expected short-circuit current.

· Prioritize safety (higher breaking capacity) while balancing cost. For instance, a 35kA Icu breaker safely handles 20kA faults but fails at 35kA.

Conclusion

Understanding tripping modes, three-stage protection, and breaking capacities (Ics/Icu) ensures optimal circuit breaker selection for safety and reliability. While higher breaking capacities enhance safety, cost-effectiveness remains critical. Always align specifications with application requirements and industry standards.