Moulded Case Circuit Breaker (MCCB)

A Moulded Case Circuit Breaker (MCCB) is a type of circuit breaker that provides overcurrent protection for higher‑current circuits, typically from 16A up to 1600A or more. Unlike MCBs (Miniature Circuit Breakers), which are limited to lower ratings (up to 125A) and have fixed trip characteristics, MCCBs offer:

• Adjustable Trip Settings: The overload (thermal) and short‑circuit (magnetic) trip settings can be adjusted to match the specific circuit requirements.
• Higher Breaking Capacity: MCCBs are capable of interrupting fault currents up to 100kA or more, making them suitable for high‑fault‑current environments like main distribution boards and CCR inputs.
• Higher Current Ratings: Available in a wide range of current ratings from 16A to 1600A, covering all AGL power distribution needs.
• Electronic Trip Units: Many MCCBs are equipped with microprocessor‑based trip units that provide sophisticated protection curves (LSI – Long‑time, Short‑time, Instantaneous) and communication capabilities.
• Robust Enclosure: The moulded case provides excellent insulation and mechanical protection, making MCCBs suitable for harsh environments.

In AGL systems, MCCBs are used as the main circuit protection devices in substations, CCR rooms, and distribution panels, protecting the incoming power supply and major feeder cables.

The MCCB's purpose is to provide the same fault-clearing protection as an MCB but at the higher currents and fault levels found at an airfield's main distribution and CCR input points, where a fault could otherwise release far more energy than a miniature breaker is designed to interrupt safely. Its adjustable trip settings let the protection be tuned precisely to the cable and equipment it protects rather than accepting a fixed, one-size-fits-all threshold, and its higher breaking capacity is what allows it to safely clear a short circuit on a high-current feeder without the breaker itself failing catastrophically in the process. In systems with electronic LSI trip units, this purpose extends to selective coordination — ensuring that when a fault does occur, only the breaker closest to it trips, rather than an upstream breaker unnecessarily taking out a much larger section of the airfield.

MCCBs are used in the higher‑power sections of AGL electrical infrastructure:

• Incoming Main Distribution: Protecting the main power supply to the AGL substation from faults in the utility feed or internal distribution.
• CCR Input Protection: Protecting the input side of each Constant Current Regulator from overloads or short circuits in the power supply.
• Sub‑Distribution Panels: Protecting feeder cables that supply power to various airfield sectors (e.g., runway 09, runway 27, taxiway lighting, apron lighting).
• Generator and UPS Changeover Panels: Protecting the connection between the backup power sources (generators, UPS) and the AGL distribution system.
• Large‑Capacity Series Circuits: In some applications, MCCBs are used to protect high‑current series circuits (e.g., 20A isolating transformer circuits).

MCCBs are panel-mounted at the higher-power points in the network — incoming main distribution, CCR input protection, sub-distribution feeders, and generator or UPS changeover panels — fixed to a rigid mounting plate or chassis rail rated for the breaker's frame size and fault current. Given their larger physical size and higher current ratings compared with MCBs, MCCBs are positioned with generous bus bar and cable bend clearances, and enclosures around them are sized to manage the heat generated under sustained load. Their location within a switchboard reflects the distribution hierarchy — mains incomers first, then feeder MCCBs to each sector of the airfield — so isolation for maintenance follows a clear, traceable sequence back to the main supply.

An MCCB operates using a combination of thermal, magnetic, and (in electronic versions) microprocessor‑based trip mechanisms:

• Thermal Overload Protection: A bimetallic element heats up and bends under sustained overcurrent, triggering the trip mechanism after a time delay. The delay is inversely proportional to the square of the current (I²t curve), providing true thermal protection for cables and equipment.
• Magnetic Short‑Circuit Protection: An electromagnetic coil provides instantaneous tripping under severe fault currents (short circuits). The trip threshold is adjustable in many MCCBs.
• Electronic Trip Units (LSI): Microprocessor‑based trip units offer three distinct protection functions:
  • L (Long‑time): Adjustable overload protection with inverse‑time characteristic.
  • S (Short‑time): Adjustable short‑time protection with definite‑time or inverse‑time characteristic, providing selective coordination with downstream devices.
  • I (Instantaneous): Adjustable instantaneous protection for high‑magnitude faults.
• Arc Extinction: MCCBs use arc chutes with de‑ionizing plates to extinguish the arc quickly and efficiently, ensuring safe interruption of high fault currents.
• Communication: Modern MCCBs can be equipped with communication modules (Modbus, Profibus, Ethernet) for remote monitoring, control, and integration with the ALCMS.

The MCCB typically sits one tier above the MCBs it protects, at the main incomer or a major sub-distribution point, and its role in the architecture is defined as much by coordination as by protection: its long-time, short-time, and instantaneous (LSI) settings are deliberately staggered against the trip curves of the MCBs downstream so that a fault on a final circuit is cleared by the MCB first, with the MCCB acting as backup protection only if the MCB fails to clear it. Where the MCCB is fitted with an electronic trip unit and a communication module (Modbus, Profibus, or Ethernet), its status, trip history, and even its protection settings can be read and adjusted remotely by the ALCMS or a SCADA system, turning it from a purely local protection device into a monitored node in the wider control network — a capability that basic thermal-magnetic MCCBs, which report nothing beyond the position of their handle, do not have.

The MCCB's moulded case is formed from high-dielectric-strength, flame-retardant thermoset material engineered to contain internal arcing and withstand the mechanical and thermal stresses of interrupting large fault currents without case rupture. Heavy-duty silver-alloy contacts, sized for the breaker's higher current rating, are operated by a stored-energy spring mechanism that ensures rapid, contact-welding-resistant opening regardless of manual operating speed, backed by a larger arc chamber with multiple splitter plates to quench higher-energy arcs. The trip unit, whether a fixed thermal-magnetic assembly or a field-adjustable electronic (LSI) unit, is mounted in a separate compartment accessible for setting adjustment without exposing the main contact chamber. Line and load terminals are designed for larger cable lugs or bus bar connections and are shrouded per the enclosure rating, with the complete unit available in DIN-rail, panel-mount, or draw-out chassis configurations to suit switchgear integration.