--- title: "Cross jet explosion pot in bulk oil CB" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:cross-jet-explosion-pot" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/cross-jet-explosion-pot site: BCT Engineering --- # Cross jet explosion pot in bulk oil CB Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2078 Chaitra, 6 marks - Video: [58 Self Pressure Generated Oil Circuit Breaker](https://www.youtube.com/watch?v=esZ2EpTw6cs) — Dr Padmini S Panneer Selvam - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/cross-jet-explosion-pot - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Describe the working of cross jet explosion pot in bulk oil circuit breaker with a neat sketch. ## Short answer In a cross jet explosion pot, the arc decomposes oil into high-pressure gas. As the moving contact uncovers side vents, oil jets are forced across the arc, cooling, lengthening, and deionizing it to extinguish at current zero. It uses self-generated pressure, effective for high fault currents. ## Answer 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. ```text ┌────────────────────┐ │ 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Where does the pressure that drives the oil jets in a cross jet explosion pot come from? A. An external pump B. A compressed air reservoir C. The arc itself, which decomposes oil into gas in the closed pot D. A spring Answer: C. The pressure is self-generated, so a larger fault current produces a stronger blast. (Key point: Source of pressure) 2. How do the cross jets form as the moving contact moves down? A. The contact uncovers the side vents one after another, letting oil and gas jet across the arc B. The vents close one by one C. Oil is sucked in from the bottom D. The pot opens completely Answer: A. Uncovered vents release high-pressure oil/gas sideways across the arc, lengthening and cooling it. (Key point: Side vents) 3. For which fault conditions is a plain cross jet pot best suited? A. Low currents B. No-load current C. High fault currents D. Only DC Answer: C. Pressure rises with current; at low currents the pressure is small and arcing time long. (Key point: Suitable currents) 4. Why is a self-compensated explosion pot used? A. To reduce oil use B. To handle low currents where cross jet pressure is weak, by adding a plain pot action C. To increase the voltage rating D. To avoid arcing entirely Answer: B. It combines cross jet and plain pot features so both high and low currents are cleared efficiently. 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