How To Coordinate Mccbs With Other Protection Devices

It’s crucial to ensure your electrical systems are both safe and reliable, which is why proper coordination between protective devices like Molded Case Circuit Breakers (MCCBs) is a non-negotiable step for any professional operation. By mastering this process, you protect your assets from unnecessary downtime and enhance your overall operational continuity. This guide provides the practical information your engineering and procurement teams need to confidently select and coordinate the right devices for your specific industrial and commercial needs.

The Critical Importance of Selective Coordination

Selective coordination, often simply called “selectivity,” is the strategic arrangement of protective devices to ensure that only the device immediately upstream of a fault trips, isolating the issue without causing a total system shutdown. This hierarchy of operation, where the closest device clears the fault first, is vital for maintaining power to healthy parts of your installation. For large-scale industrial and commercial environments, maximizing uptime is a direct contribution to the bottom line.

Understanding Your MCCB’s Role

A Molded Case Circuit Breaker (MCCB) is a robust and highly versatile protective device that safeguards circuits from overloads and short circuits. Our NUOMAK MCCBs are designed with features that make coordination studies simpler and implementation more reliable. Your selection of the correct MCCB for a specific application—considering factors like trip units and interrupting capacity—forms the foundation for an effective coordination scheme.

Coordination Techniques: Matching MCCBs and Other Devices

Effective coordination relies on the time-current characteristics (TCCs) of all devices in series. You must ensure that the TCC curve of the downstream device always falls below and to the left of the upstream device’s curve.

1. Analyzing Time-Current Characteristics (TCCs)

TCCs are the graphical representation of how long a protective device takes to interrupt a fault current. For successful selectivity, you must compare the TCCs of the upstream and downstream devices. Electronic trip units on larger MCCBs offer the adjustable settings (long-time, short-time, instantaneous) that make fine-tuning coordination possible.

2. Achieving Overload Coordination (Long-Time Delay)

Overload coordination focuses on the low-current region of the TCC. You should set the long-time delay and pick-up of the upstream device higher than the downstream device to ensure the downstream MCCB trips first on a sustained overcurrent condition.

3. Mastering Short-Circuit Coordination (Short-Time Delay)

For moderate fault currents, the short-time delay feature allows the downstream device a brief moment to operate before the upstream breaker even begins to trip. This delay must be sufficient to allow the load-side device to clear the fault fully.

4. Navigating Instantaneous Coordination

High-level short circuits trigger the instantaneous trip function. You can often set the instantaneous trip setting of the upstream MCCB higher than the maximum available fault current at the downstream breaker, ensuring that the downstream device remains the sole protector in this scenario. Manufacturer-published coordination tables for specific pairs of devices are often required for verification in the instantaneous region.

5. Coordinating MCCBs with Fuses

When an MCCB protects a circuit where a fuse is also present (often downstream for specific equipment protection), the fuse is generally faster acting. Coordination here involves checking that the fuse’s melting time curve is lower than the MCCB’s tripping curve across the fault current range, especially at high short-circuit levels.

6. Integrating with Upstream Air Circuit Breakers (ACBs)

In systems with an ACB as the main incoming device, the ACB’s advanced electronic trip unit and adjustable zones should be utilized. You typically set the ACB with a selective time delay to allow all downstream MCCBs to clear the fault before it considers tripping.

7. Utilizing Zone Selective Interlocking (ZSI)

For high-current, complex systems, ZSI is a powerful tool. This method allows communicating trip units (like those in our premium NUOMAK MCCBs) to “talk” to each other. If a fault is detected, the downstream breaker sends a signal to the upstream breaker to delay its trip. If the fault persists or is upstream, the upstream breaker trips without delay, vastly improving speed and selectivity.

Coordination Summary Table

Protection Device Pair	Key Coordination Focus	Key Action for Upstream Device
MCCB (Upstream) – MCCB (Downstream)	All overcurrent and short circuit zones (TCCs)	Ensure time/current settings are higher and slower
ACB (Upstream) – MCCB (Downstream)	Short-time and instantaneous delay	Utilize ACB’s longest time delays or ZSI
MCCB (Upstream) – Fuse (Downstream)	High short-circuit current limit	Check that MCCB’s trip curve is above the fuse’s clearing curve

To maintain system integrity, you need a coordinated approach where every protective device plays its part without disrupting the entire system. Investing in reliable, high-quality NUOMAK MCCBs and conducting thorough coordination studies are your best defenses against costly outages. Our dedicated support team can help you navigate the complexities of selective coordination to design a resilient power distribution network tailored to your operational needs.

FAQs

What is the primary difference between total and partial selective coordination?

Total selectivity means coordination is achieved across the entire range of fault currents, from overload up to the maximum available short-circuit current at the downstream device. Partial selectivity is only achieved up to a certain, lower value of fault current.

What is the benefit of using electronic trip MCCBs over thermal-magnetic ones for coordination?

Electronic trip units offer superior adjustability for the long-time, short-time, and instantaneous trip settings, allowing for much finer shaping of the TCC curve and easier, more precise coordination.

Does selective coordination prevent arc flash?

While selective coordination’s primary goal is service continuity, it indirectly helps minimize arc flash hazards by quickly isolating the fault, limiting the fault energy. However, it does not replace a dedicated arc flash study.