3p 4p 630A 800A 1000A 1250A 1600A 2000A 4000A 3200A 6300A Acb Air Circuit Breaker

An air circuit breaker is a low-voltage power circuit breaker that uses air as the arc-quenching medium, designed for the highest current levels in low-voltage distribution. It is typically used as the main incoming breaker or bus-tie breaker in a main switchboard. It combines a robust operating mechanism, a magnetic-blowout/arc-chute system, and a microprocessor-based trip unit.

This series spans 630 A, 800 A, 1000 A, 1250 A, 1600 A, 2000 A, 3200 A, 4000 A, and 6300 A, covering the full low-voltage main-switchgear band. The rated current (In) is chosen to match the installed load and transformer rating, while the frame size determines the maximum current and breaking capacity. ACBs are the standard protection for incoming feeds from large transformers (typically 1000 kVA up to several MVA).

A 3-pole ACB switches the three line conductors (L1, L2, L3), while a 4-pole ACB also switches the neutral conductor. Three-pole units are standard for three-phase main incomers; four-pole units are used where neutral isolation is required, for changeover/bus-tie arrangements, or where neutral switching is mandated by the installation rules. The pole count must match the system earthing and switching arrangement.

When the contacts open under fault current, an arc forms and is driven by a magnetic field into a stack of splitter plates in the arc chute. The plates split the arc into many short arcs, and the hot ionized air is forced out through exhaust vents, cooling and de-ionizing the plasma until the arc cannot re-strike. Air is the interrupting medium, as opposed to oil, vacuum, or SF6 used at medium voltage.

Most high-current ACBs are drawout units: the breaker can be racked OUT on its cassette to a disconnected, isolated position while the primary and secondary contacts remain connected to the switchboard. This allows safe withdrawal, maintenance, and replacement without de-energizing the whole board, and provides a visible isolation gap. Fixed (withdrawable-less) versions are also available for smaller ratings and simpler layouts.

A microprocessor trip unit is an electronic control that measures current (and often voltage and power) via current transformers and calculates the trip time according to programmable protection curves. It replaces the older thermal-magnetic block and offers adjustable long-time, short-time, instantaneous, and earth-fault protection, plus metering, event logging, and communication (Modbus, etc.). It provides far more precise and selective coordination than a thermal-magnetic MCCB.

LSI stands for Long-time, Short-time, and Instantaneous protection. Long-time (L) gives inverse-time overload protection; Short-time (S) gives a time-delayed short-circuit trip with a selectable delay and I²t character, enabling downstream MCCBs to clear faults first (selectivity); Instantaneous (I) trips immediately on very high faults. A fourth element, Ground-fault (G), is added on LSIG units for earth-leakage/earth-fault detection.

ACBs are installed in main low-voltage switchgear (PCC/MCC rooms) as the main incomer from the distribution transformer, as bus-tie breakers between two sections of busbar, or as generator/utility intertie breakers. They are mounted vertically in drawout cassettes, busbar-connected, and usually equipped with shunt/tripping releases and under-voltage releases for remote and emergency tripping.

ACBs are built and type-tested to IEC 60947-2, the same low-voltage circuit-breaker standard as MCCBs, but they typically fall under utilization Category B — meaning they have a defined short-time withstand current (Icw) and can withstand (and coordinate through) short-time faults rather than tripping instantaneously. This short-time rating is what enables selective grading with downstream MCCBs.

An ACB handles much higher currents (630–6300 A vs. typically ≤1600 A for an MCCB), uses drawout construction for maintenance, and always includes a microprocessor trip unit with adjustable LSI/LSIG curves and short-time withstand (Icw). An MCCB is usually Category A (instantaneous only, no Icw), fixed-mounted, and cheaper for feeder duty. Rule of thumb: use an MCCB for feeders and an ACB for the main incomer/bus-tie.