Definition: The making capacity of a circuit breaker is the maximum current the circuit breaker can handle upon closing under an electrical fault. This is a critical specification for circuit breakers because it indicates the maximum current the breaker can safely handle in a circuit under fault conditions without getting damaged.
Key Points about Making Capacity:
- The making capacity is called the “peak making current” and is typically expressed in kiloamperes (kA). It represents the highest current the breaker can safely close on without damage.
- This capacity is significant during short-circuit conditions, where the current can surge to high values. The circuit breaker must be able to close and establish the circuit under these extreme conditions.
- According to the IEC 60947-2 standard, the making capacity is usually 2.55 times the rated short-circuit breaking capacity (Isc). For instance, if a circuit breaker has a breaking capacity of 50 kA, its making capacity would typically be 2.55X 50 = 127.5 kA.
- The making capacity is verified through rigorous testing to ensure the breaker can handle the peak currents without mechanical or thermal damage.
- It is commonly denoted by the symbol Im and expressed in kA(peak).
Formula for Circuit Breaker Making Capacity
The standard formula used per IEC (International Electrotechnical Commission) standards is:
Where:
- Im = Making capacity of the circuit breaker
- Is = Rated short-circuit breaking capacity of the circuit breaker
- k = Multiplying factor, which is often taken as 2.55 according to IEC 60947-2 standards
Practical Example:
Consider a circuit breaker with the following specifications:
- Breaking Capacity (Isc): 21kA
- Making Capacity (Im):
Making capacity = 2.55 * symmetrical breaking capacity.
= 2.55 × 21 kA = 53.55 kA(Peak)
In this case, the circuit breaker can safely close on a fault current of up to 53.55 kA without getting damaged, ensuring reliable performance even under severe fault conditions.
Importance of Making Capacity of Circuit Breaker in Electrical Systems:
- Protection: This ensures the breaker can safely handle the maximum possible current during fault conditions, protecting the breaker and the connected equipment.
- Reliability: Helps maintain system reliability by ensuring the breaker can close and establish the circuit even under extreme conditions.
- Safety: Prevents damage and potential hazards by ensuring the breaker operates correctly under high current conditions.
Understanding and selecting a circuit breaker with an appropriate making capacity is crucial for the safety and reliability of electrical systems.
Author- Satyadeo Vyas