Contents
Standards:
- IEC 60947-2
- SBC 401 (Electrical Code – KSA)
- IEEE 1015
- NEC 240 (contextual comparison)
📘 1.0 What is an ACB?
Air Circuit Breakers (ACBs) are heavy-duty circuit protection devices used for main LV incomer, generator output, and bus-coupler protection in installations with current ratings from 800 A to 6300 A.
They interrupt fault current in air, using arc chutes and blowout devices. Modern ACBs feature electronic LSIG protection, remote control, communication, and zone-selective interlocking.
🧱 2.0 ACB Construction
Major Components:
- Fixed & moving contacts
- Arc chute with splitter plates
- Blowout coil for arc displacement
- Trip mechanism
- Spring or motor charging system
- Electronic trip unit (LSIG or LSIGM)
- Plug-in/drawout chassis with mechanical interlock
📌 Construction resembles MCCBs but on a much larger scale with modular withdrawable design.
⚡ 3.0 Working Principle
| Operation | Description |
|---|---|
| Normal | Current flows through main contacts |
| Fault | Arc forms → arc chute splits & cools it → blowout coil deflects arc |
| Trip | Electronic unit sends signal → release mechanism opens contacts |
| Reset | Manual or motorized spring recharge resets breaker |
Modern ACBs are remote-controlled, monitored, and fully programmable.
📚 4.0 IEC 60947-2 Clauses for ACBs
| Clause | Description |
|---|---|
| 2.1.3 | Icu (Ultimate breaking capacity) |
| 2.1.4 | Ics (Service breaking capacity) |
| 4.3 | Dielectric strength |
| 4.4.1 | Time-current coordination (LSIG trip unit) |
| 4.5.2 | Selectivity and backup protection |
ACBs must meet Type B or Type A coordination depending on backup breaker.
🔌 5.0 Ratings & Settings
| Term | Description |
|---|---|
| Inm | Frame rating (e.g., 1600A, 3200A, 6300A) |
| Ir | Long-time adjustable protection (40–100% of Inm) |
| Isd | Short-time delay protection (2–10× Ir) |
| Ii | Instantaneous trip (5–20× Ir or disabled) |
| Ig | Ground fault trip (20–100% of Ir) |
| Icw | Withstand current for 1s or 3s |
📌 Example:
A 3200A ACB may be set at Ir = 2800A, Ii = 14,000A, Ig = 640A
🔧 6.0 LSIG Trip Functions
Trip Unit: L = Long | S = Short | I = Instant | G = Ground
| Protection | Adjustable | Range |
|---|---|---|
| Long-time | ✅ | 0.4–1 × Inm |
| Short-time | ✅ | 2–10 × Ir |
| Instantaneous | ✅ or Disabled | 5–20 × Ir |
| Ground Fault | ✅ | 20–100% Ir |
| Delay (sec) | ✅ | 0.05 – 1.0 s |
Most ACBs include Zone-Selective Interlocking (ZSI) to improve selectivity.
🧠 7.0 Application Chart
| Application | ACB Size | Notes |
|---|---|---|
| LV Main Incomer | 2500–6300 A | Feeds MSB |
| Generator Output | 1600–3200 A | Sync-enabled |
| Transformer Secondary | 800–2500 A | With ZSI |
| Tie Breaker | 1600–3200 A | With interlocking |
| Bus-Coupler | 2500–4000 A | Auto transfer logic |
✅ ACBs are preferred at critical points, with high fault levels (>50kA) and advanced control needs.
🏆 8.0 Top ACB Brands & Models
| Brand | ACB Model | Range |
|---|---|---|
| Schneider Electric | MasterPact MTZ | Up to 6300 A |
| ABB | Emax 2 | Up to 6300 A |
| Siemens | 3WL | Up to 6300 A |
| Terasaki | TemPower 2 | 800–6300 A |
| Eaton | NRX Series | Up to 4000 A |
All are IEC 60947-2 compliant and available with advanced trip units (Micrologic, PR123, ETU800, etc.)
📊 9.0 ACB vs MCCB
| Feature | ACB | MCCB |
|---|---|---|
| Max Current | Up to 6300 A | Up to 2500 A |
| Trip Unit | Electronic (LSIG) | Thermal-Mag or Electronic |
| Install | Withdrawable | Fixed/Mounted |
| Arc Quenching | Air | Magnetic blow-out |
| Monitoring | Fully programmable | Limited (in thermal-magnetic) |
| Coordination | ZSI, GFL | Basic selectivity |
📎 10.0 Final Notes
✅ Always ensure:
- ACB matches the short-circuit level at its location (Icu ≥ fault level)
- Set LSIG based on coordination study
- ZSI enabled for backup protection
- Breakers are tested under IEC 60947-2 and SBC 401-compliant
📚 References
- IEC 60947-2: Low-voltage switchgear – Circuit Breakers
- SBC 401 – Saudi Building Code (Electrical)
- ABB Emax E2/E4/E6 catalogs
- Schneider Electric MasterPact MTZ handbook
- Siemens 3WL selection guide
- IEEE 1015 – Application of Low-Voltage CBs