Contents
- 1 📘 1.0 What is a VCB?
- 2 🧱 2.0 VCB Construction
- 3 ⚡ 3.0 Working Principle of VCB
- 4 📚 4.0 IEC 62271-100 Key Clauses
- 5 🔧 5.0 VCB Ratings (Typical)
- 6 🔌 6.0 Applications of VCBs
- 7 🔁 7.0 VCB vs Other MV Breakers
- 8 🏆 8.0 Top Brands & Models
- 9 🧠 9.0 VCB with Protection Relays
- 10 📎 10.0 Key Selection Tips
- 11 📚 References
Standards:
- IEC 62271-100 (High-voltage switchgear and controlgear)
- SBC 401 (Electrical Installations – Medium Voltage)
- IEEE C37.06
- NEMA SG-4
📘 1.0 What is a VCB?
Vacuum Circuit Breakers (VCBs) are the most common switching and protection devices used in medium-voltage (MV) systems (3.3 to 36 kV). They extinguish arcs by separating contacts inside a sealed vacuum interrupter, preventing arc re-striking due to the high dielectric strength of vacuum.
They’re used in:
- MV Substations
- Ring Main Units (RMUs)
- Generator & Transformer protection
- Distribution feeders
🧱 2.0 VCB Construction
| Component | Function |
|---|---|
| Vacuum Interrupter Bottle | Main arc-extinguishing chamber |
| Moving & Fixed Contacts | Conduct current; separate during fault |
| Insulated Nozzle | Directs arc into vacuum |
| Operating Mechanism | Spring/motor-charged for contact movement |
| Housing | Air-insulated or cast resin insulated unit |
| Drive Linkage | Transfers energy to the vacuum bottle |
📌 Modern VCBs are often mounted inside withdrawable metal-clad switchgear.
⚡ 3.0 Working Principle of VCB
| Phase | Action |
|---|---|
| Normal | Contacts closed → current flows |
| Fault | Relay trip → contacts separate |
| Arc Formation | Ionized path forms briefly |
| Arc Extinction | Vacuum causes rapid de-ionization (0.1–0.5 ms) |
| Recovery | High dielectric strength restores insulation instantly |
Vacuum’s dielectric strength is ~8× higher than SF₆ or air → no re-strike occurs.
📚 4.0 IEC 62271-100 Key Clauses
| Clause | Description |
|---|---|
| 4.101 | Rated short-circuit breaking current (Isc) |
| 5.105 | Making capacity (Ik) |
| 5.106 | Electrical & mechanical endurance |
| Annex G | Synthetic test conditions for MV CBs |
| BIL (Basic Insulation Level) | 75–170 kV (impulse rating) |
📌 SBC 401 (KSA) adopts IEC 62271-100 for all MV switchgear design & testing.
🔧 5.0 VCB Ratings (Typical)
| Parameter | Range |
|---|---|
| Rated Voltage (Ur) | 3.3 / 6.6 / 11 / 22 / 33 kV |
| Rated Current (Ir) | 630 A to 4000 A |
| Breaking Capacity (Isc) | 16 / 25 / 31.5 / 40 / 50 kA |
| Short-Time Withstand (Icw) | 16–50 kA for 1–3 s |
| Impulse Withstand (BIL) | 75–170 kV |
| Making Capacity (Ik) | 2.5 × Isc (peak) |
🔌 6.0 Applications of VCBs
| Location | Voltage | Use Case |
|---|---|---|
| Primary Substation | 11 / 22 / 33 kV | Feeder & transformer protection |
| RMU | 11 kV | Load-side breaker |
| Motor Starters | 6.6 / 11 kV | HV motors with fast reclosing |
| Capacitor Banks | 11 kV | With pre-insertion resistors |
| Industrial Plants | 6.6 / 11 kV | Process critical equipment |
🔁 7.0 VCB vs Other MV Breakers
| Feature | VCB | SF₆ CB | Oil CB |
|---|---|---|---|
| Arc Medium | Vacuum | SF₆ Gas | Mineral Oil |
| Insulation | Air or Resin | Gas | Oil |
| Size | Compact | Large | Bulky |
| Maintenance | Minimal | Medium | High |
| Environment | Eco-friendly | Greenhouse gas | Fire risk |
| Lifespan | 20,000+ operations | 5000–10,000 | <5000 |
✅ VCBs are now the global standard for indoor MV breakers due to safety, size, and eco-compatibility.
🏆 8.0 Top Brands & Models
| Brand | Model | Range |
|---|---|---|
| ABB | VD4 | 630 A to 4000 A |
| Siemens | 3AH | 630 A to 3150 A |
| Schneider Electric | Evolis | Up to 4000 A |
| Eaton | VCP-W | Up to 3000 A |
| Terasaki | HVX | 12 to 36 kV |
All are fully IEC 62271-100 tested and used in SEC (Saudi Electric Co.) substations.
🧠 9.0 VCB with Protection Relays
VCBs must work in conjunction with numerical protection relays, such as:
- Micom P123 / P142 / P443 (Schneider)
- Siemens Siprotec 5 / 7SJ series
- ABB Relion 611 / 630
- SEL relays (for arc protection)
✅ Protection coordination is essential for fast, selective fault clearing and must include:
- CT ratio validation
- Time-current coordination (IEC curves)
- Zone selectivity or fast bus transfer logic
📎 10.0 Key Selection Tips
✅ Choose VCB when:
- You have frequent operations (>10/day)
- Fire/explosion risk must be avoided
- Space is limited (compact switchgear)
- Clean indoor installation is preferred
- High dielectric withstand is required
📚 References
- IEC 62271-100: High-voltage alternating-current circuit-breakers
- SBC 401 – Medium Voltage Installation Guide (KSA)
- IEEE C37.06: Standard ratings for HV CBs
- ABB VD4 Technical Manual
- Schneider Evolis Application Guide
- Siemens Medium Voltage Switchgear Handbook