Cross jet explosion pot in bulk oil CB

A cross jet explosion pot is an arc control device used in bulk oil circuit breakers to quench the arc with jets of oil blown across it, using the pressure created by the arc itself. It is suited to high currents.

Construction

• A strong insulating pot (fibreglass or similar) mounted around the fixed contact, inside the oil tank.
• One side of the pot has a set of arc splitter / side vents (exit ports) arranged one above the other.
• The moving contact is a rod that slides down through the bottom throat of the pot.
• The upper part of the pot forms a closed space where gas pressure builds up.

        ┌────────────────────┐
        │   Fixed contact    │
        │  ┌──────────────┐  │
 gas    │  │ pressure     │  │
 pocket │  │ space        │  │
        │  └──────┬───────┘  │
        │  ~arc~  │   ═══►   │ side vents
        │  ~arc~  │   ═══►   │ (oil + gas
        │  ~arc~  │   ═══►   │  jets out)
        │         │          │
        └─────────┼──────────┘
              moving contact
                  ▼ moves down

Working

1. In the closed position the moving contact sits in the fixed contact inside the pot.
2. When the breaker trips, the moving contact moves downward and an arc is drawn inside the pot.
3. The arc decomposes oil into gas (mostly hydrogen). Because the pot is closed, a high-pressure gas bubble forms in its upper part.
4. As the moving contact travels down, it uncovers the side vents one after another.
5. The high-pressure gas forces oil out through these vents, producing jets of oil and gas across the arc (cross jet).
6. The cross jets push the arc sideways into the vents, lengthen it, cool it and remove ionised products.
7. The arc is extinguished at a current zero, usually before the moving contact leaves the pot. Fresh oil then fills the gap and builds up dielectric strength.

Features

• Self-generated pressure, so the higher the fault current, the stronger the blast; good for high fault currents.
• At low currents pressure is small and arcing time is long; for this the self-compensated explosion pot (cross jet plus plain pot) is used.
• Short arc length and arcing time compared with a plain break oil CB.