Open Type Power Circuit Breakers (ACB) Selection Guide
When selecting an open type power circuit breaker (ACB), verify 630 A–6300 A rated current, Icw short-time withstand current, fixed vs withdrawable mounting and electronic protection.
This guide explains what to look for when choosing products in the Open Type Power Circuit Breakers category for transformer incomers, bus couplers and high-current main low-voltage switchboards.
Rated current (In) and short-time withstand current (Icw)
Open type air circuit breakers (ACBs) operate under IEC/EN 60947-2 between 630 A and 6300 A. Unlike standard molded case breakers, ACBs are utilization category B devices designed to withstand prospective short-circuit current for a defined intentional delay (typically 0.5 s, 1 s or 3 s) so downstream protective devices can clear the fault first. This rating is expressed as Icw (short-time withstand current) and is the foundation of total time-current selectivity.
Fixed vs withdrawable (draw-out) construction
In data centers, hospitals and continuous industrial plants where downtime must be minimized, withdrawable (cassette-mounted) ACBs are preferred because the breaker body can be racked into Connected, Test and Disconnected positions or swapped without unbolting main busbars. Fixed-pattern ACBs offer reduced cubicle depth and lower initial cost.
Open type power circuit breaker (ACB) comparison table
| Selection Criterion | Fixed Pattern ACB | Withdrawable (Draw-Out) ACB | Bus Coupler / Generator ACB |
|---|---|---|---|
| Rated Current (In) | 630 A – 4000 A | 630 A – 6300 A | 800 A – 6300 A |
| Breaking / Icw (1 s) | 50 kA – 65 kA | 65 kA – 150 kA | Icw matched to main busbar fault level |
| Mounting & Maintenance | Bolted directly to busbars | Cassette with Service / Test / Disconnected positions | Mechanical cable or rod interlock ready |
| Control Accessories | Charging motor + closing & shunt coils | Motor + MX + MN + ready-to-close contact | Dual shunt trip + ATS/synchronization interface |
Frequently asked questions
Why is the Icw (short-time withstand) rating essential on an ACB?
Icw specifies the fault current the breaker can carry for 1 second without tripping instantaneously, giving downstream MCCBs or MCBs time to clear a feeder fault without blacking out the entire main switchboard.
Which control accessories must be verified when ordering an ACB?
For motorized or automatic transfer panels, verify the control voltage (e.g. 230 V AC or 24 V DC) of the spring-charging motor (M), closing coil (XF), shunt trip release (MX) and undervoltage release (MN), along with auxiliary signaling contacts.
What is the difference between Test and Disconnected positions on a withdrawable ACB?
In the Test position, the main power contacts are isolated from the busbars while the control plug remains connected, allowing safe testing of electrical tripping and closing circuits. In the Disconnected position, both main and auxiliary circuits are physically separated.