IEC (International Electrotechnical Commission) Standards of a circuit breaker - Working Principle

IEC circuit breakers are electromechanical safety devices governing under standards like IEC 60898 (residential) and IEC 60947-2 (industrial) that automatically interrupt abnormal current (overloads/short-circuits) using thermal-magnetic or electronic trip units. They protect circuits by separating contacts to break the current, extinguishing the arc to prevent damage.

Key IEC Standards for Circuit Breakers

• IEC 60898-1: Covers miniature circuit breakers (MCBs) for household and similar installations (a.c. only) BEAMA.
• IEC 60947-2: Covers low-voltage circuit breakers for industrial, commercial, and similar applications.
• IEC 62271-100: Focuses on high-voltage alternating-current circuit breakers (above 1 kV).
  Functionality: Standards define requirements for performance, such as rated current, breaking capacity (I_cu, I_cs), and endurance.

Note:

•  I_cu = Service Short-Circuit Breaking Capacity
• I_cs = Rated Service Short-Circuit Breaking

Working Principle

• Overload Protection (Thermal): A bimetallic strip heats up and bends when current exceeds the rated capacity over an extended period, triggering the mechanism to trip.
• Short-Circuit Protection (Magnetic): A solenoid/electromagnet coil detects an immediate, massive surge in current, pulling a plunger to instantaneously break the contacts.
• Arc Extinguishing: Upon separation of contacts, an electrical arc is formed. The breaker uses mechanisms (e.g., arc chutes, vacuum, or SF6 gas) to extinguish the arc safely.

Main Components

1. Fixed and Moving Contacts: Carry the current and separate to break the circuit.
2. Trip Mechanism: A combination of a bimetallic strip (thermal) and an electromagnet (magnetic).
3. Arc Chute/Extinguisher: Cools and splits the arc to extinguish it.
4. Operating Handle: Used to manually reset the breaker after a trip.

• M1/M2 Classes: Defines mechanical endurance (number of operations).
• Short-Circuit Categories: Type C1 (non-current limiting) and C2 (current limiting).
• Tripping Characteristics (IEC 60898):
  • Type B: Trips at 3-5 times rated current (fast).
  • Type C: Trips at 5-10 times rated current (general purpose).
  • Type D: Trips at 10-20 times rated current (high inrush loads).

Circuit breakers operate as automatic electrical switches designed to protect circuits from damage caused by overcurrent (overload) and short circuits. The International Electrotechnical Commission (IEC) defines specific standards for their design, testing, and performance across different voltage levels.

1. Key IEC Standards

• IEC 60898-1: Applies to low-voltage circuit breakers for household and similar installations (e.g., shops, schools).
• IEC 60947-2: Governs circuit breakers for industrial applications, handled by skilled personnel.
• IEC 62271-100: Applies to high-voltage AC circuit breakers (above 1 kV) used in transmission and distribution.

2. Working Principle

• Mechanism: Uses a bimetallic strip made of two metals with different thermal expansion coefficients.
• Process: As current exceeds the rated limit, the strip heats up and bends.
• Result: Once the bend reaches a set point, it releases a mechanical latch that trips the breaker and opens the contacts.

• Mechanism: Uses an electromagnetic coil (solenoid).
• Process: A sudden, high-magnitude current surge (short circuit) generates a powerful magnetic field.
• Result: This field instantly pulls a plunger or armature to release the trip mechanism, disconnecting the circuit within milliseconds.

C. Arc Extinction
When contacts separate under load, an electric arc forms. To prevent damage or fire, circuit breakers use various mediums to extinguish this arc:

• Air: Uses arc chutes to split and cool the arc (common in Air Circuit Breakers).
• Vacuum: The lack of gas prevents arc maintenance (used in VCBs).
• SF6 Gas: Sulfur hexafluoride has high dielectric strength to rapidly quench arcs.

• Icu (Ultimate Breaking Capacity): The maximum fault current a breaker can interrupt at least once without being destroyed.
• Ics (Service Breaking Capacity): The maximum current it can break three times and still remain operational.