Interrupting Capacity Considerations for MCCBs

Molded case circuit breakers (MCCBs) provide overload and short-circuit protection in electrical distribution systems. A key rating for proper MCCB selection is interrupting capacity, also called short circuit current rating. This specifies the maximum fault current the device can safely interrupt. 

Choosing an appropriate rating prevents catastrophic failures during fault conditions. Several factors determine the required interrupting capacity for a given MCCB application.

MCCB’s Interrupting Capacity – Factors to Consider

Here are a few key factors to consider when it comes to interrupting capacity for MCCBs:

1. Available Fault Currents

The first step is calculating the prospective short circuit currents at the MCCB location. This depends on the utility transformer kVA, conductor lengths and sizes, motor contributions, and other system parameters. 

A thorough fault study using modeling software determines the single-phase, three-phase, and ground fault currents. The maximum value establishes the minimum interrupting rating needed.

Higher available fault currents require breakers with increased interrupting capacities. If the rating is too low for the calculated currents, the MCCB will be unable to clear faults without extensive damage. This could endanger equipment and personnel. 

Carefully comparing the ratings to fault study results prevents such dangerous mismatches.

2. Coordination Needs

The role of the MCCB in the overall system protection scheme influences the required rating. MCCBs feeding motor circuits may need a rating high enough to coordinate with larger main breakers. Units on branch circuits with multiple downstream devices should match those loads.

Proper coordination ensures backup protection if an upstream breaker fails to open. The cascading trip settings isolate only the faulted section. An MCCB with inadequate interrupting capacity cannot participate in selective tripping schemes. This limits options for sequencing protection devices.

3. Available MCCB Options

Standard MCCBs have interrupting capacities of 10 kA, 18 kA, 25 kA, or 65 kA. 

The series ratings available depend on the manufacturer. Higher kA classes allow coordination in systems with high short-circuit currents. They also cost more due to heavier-duty components to withstand and interrupt the currents.

If no standard rating meets the system needs, custom order units are an option. Interrupting capacities of 80 kA, 100 kA, or 150 kA may be achievable. However, the lead time and expense increase significantly. The manufacturer evaluates each application to determine viability.

4. Location Influences

MCCBs closer to large utility transformers generally require higher interrupting ratings. Without sufficient impedance upstream, the available fault current entering the facility will be very high. Longer, smaller branch circuits present lower fault currents than basic MCCBs can handle.

The same MCCB model may be suitable in some areas and inadequate in others. This emphasizes the need to derive ratings from fault studies rather than assumptions. Minor system changes can alter short circuit currents enough to warrant re-evaluating breaker ratings.

5. Series Rating

If no single MCCB has an interrupting rating to match the fault study, series rating is an option. This uses a main circuit breaker with current-limiting fuses sized to bring fault currents within the MCCB’s capacity. The combination achieves an equivalent higher rating through coordinated operation. Careful testing and certification are needed.

Conclusion

Adequate interrupting capacity for MCCBs starts with modeling the electrical system under short circuit conditions. The ratings must cover the highest available currents with a margin. Series rating, higher classes, and custom designs provide alternatives when standard options fall short. MCCBs tailored to safely interrupt prospective fault currents are key to reliable protection.

For expert guidance and tailored solutions to meet your interrupting capacity needs, consider consulting with iALLway.