--- title: "Breakdown by treeing and tracking" subject: "High Voltage Engineering" chapter: 6 key: "semester_8/high_voltage_engineering:treeing-tracking-breakdown" source: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/treeing-tracking-breakdown site: BCT Engineering --- # Breakdown by treeing and tracking High Voltage Engineering · Chapter 6: Dielectric Breakdown - Asked in: 2079 Chaitra, 8 marks - Video: [Treeing Tracking mechanism of solid dielectrics |High Voltage Engineering #treeing #tracking #viral](https://www.youtube.com/watch?v=EpxYLO87PQA) — Prasant Decodes - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/treeing-tracking-breakdown - Whole chapter: https://www.bctengineer.com/md/semester_8/high_voltage_engineering/chapter-6 ## Question Explain the breakdown of solid dielectric due to Treeing and Tracking. ## Short answer Treeing forms branched hollow channels inside the solid insulation due to partial discharges at high-stress points, eventually causing failure. Tracking creates a permanent conducting carbon path on the surface from leakage current and dry-band arcing across contamination, leading to flashover. ## Answer **Treeing** and **tracking** are long-term breakdown mechanisms of solid insulation in service. Both progress slowly, step by step, by small discharges at moderate stress, until a conducting or hollow path bridges the electrodes. #### Breakdown due to treeing Treeing is the formation of fine, branched hollow channels inside the solid, which look like a tree or bush. **Cause and process:** 1. At a sharp point, protrusion, metal particle, void or contaminant, the local field is several times the average field. 2. Partial discharges start there. Each discharge erodes the material by electron/ion bombardment, heat and chemical attack (ozone, nitric acid). 3. A thin channel forms; its tip again has high stress, so discharges continue and branch out. 4. The channels grow over months or years and form a tree. When a branch reaches the other electrode, the insulation fails. ```text needle / protrusion (HV) | /|\ / | \ branched channels /\ | /\ (electrical tree) / \|/ \ ----------------------- earthed electrode ``` **Types of trees:** | Type | Cause | Where | |---|---|---| | Electrical tree | Repeated partial discharges | Epoxy, polythene, at defects | | Water tree | Moisture + AC field, no PD | XLPE / PE cables | | Electrochemical tree | Moisture + ionic contamination | Cables in wet soil | Water trees grow at low stress and later turn into electrical trees. **Prevention:** smooth semiconducting screens, clean and dry materials, void-free extrusion, tree-retardant XLPE. #### Breakdown due to tracking Tracking is the formation of a permanent **conducting carbonized path on the surface** of an insulator between electrodes. **Process:** 1. Dust, salt, industrial pollution and moisture form a conducting film on the surface. 2. Leakage current flows through the film and heats it; the film dries unevenly and **dry bands** form. 3. Voltage concentrates across the dry bands, and small arcs (scintillations) strike across them. 4. The arcs decompose organic material, leaving carbon. This carbon track is conducting. 5. The track grows a little with each wetting/drying cycle until it bridges the electrodes; surface flashover and failure follow. ```text HV electrode ====[ surface film ]==== earth ~~~##~~~~##~~~~ dry bands + arcs =====######====== carbon track grows ``` **Features:** - Occurs mostly on organic insulators: resins, epoxies, polymers, bakelite. - Porcelain and glass do not carbonize, so they resist tracking (but may suffer erosion). - Measured by the comparative tracking index (CTI) test. **Prevention:** track-resistant materials (filled silicone rubber, cycloaliphatic epoxy), longer creepage, hydrophobic coatings, regular cleaning. #### Difference | Point | Treeing | Tracking | |---|---|---| | Location | Inside the bulk | On the surface | | Cause | Local high field, PD | Contamination + moisture | | Path | Hollow branched channels | Carbonized conducting track | | Typical in | Cables, cast resin | Outdoor insulators, terminations | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Where does treeing occur and what causes it? A. On the surface, from dry-band arcing B. Inside the bulk, from partial discharges at high local field points C. Inside the bulk, from leakage current heating a moisture film D. On the surface, from electrostatic compression Answer: B. Trees are hollow branched channels grown by discharges at points, voids or contaminants inside the solid. (Key point: Treeing location) 2. What is the correct sequence for tracking? A. Void discharge, channel branching, bridging of electrodes B. Electrostatic compression, collapse to 0.6 d0, breakdown C. Electron avalanche, space charge, streamer D. Wet conducting film, dry bands form, arcs across them, carbon track grows to flashover Answer: D. Leakage current dries the film unevenly; arcs strike across dry bands and carbonize the organic surface. (Key point: Tracking process) 3. Which insulator material resists tracking best? A. Porcelain or glass, which do not carbonize B. Epoxy resin C. Bakelite D. Untreated polymers Answer: A. Tracking needs carbon from organic material; porcelain and glass do not carbonize. (Key point: Tracking-resistant materials) 4. How does a water tree differ from an electrical tree? A. It grows only by repeated partial discharges at high stress B. It forms on the surface of porcelain C. It grows at low stress in moisture-containing polymer cables without partial discharge D. It grows only under DC Answer: C. Water trees need moisture and an AC field, not PD, and can later turn into electrical trees. (Key point: Water trees) ## Related in High Voltage Engineering - [Electromechanical, thermal and treeing breakdown](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Asolid-electromechanical-thermal-treeing) — High Voltage Engineering, chapter 6 - [Solid breakdown due to actual working conditions](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Asolid-breakdown-working-conditions) — High Voltage Engineering, chapter 6 - [Types of breakdown in solid dielectrics](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Asolid-dielectric-breakdown-types) — High Voltage Engineering, chapter 6 - [Dielectric power loss under AC and DC](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Adielectric-power-loss-ac-dc) — High Voltage Engineering, chapter 6 ## Related in other subjects - [Thermal and electromechanical dielectric breakdown](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Athermal-electromechanical-breakdown) — Electrical Engineering Material, chapter 3, asked 4× - [Capacitor plate spacing and dielectric breakdown](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aplate-spacing-breakdown) — Electrical Engineering Material, chapter 3, asked 2× - [Types of dielectric breakdown](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adielectric-breakdown-types) — Electrical Engineering Material, chapter 3, asked 2× - [Dielectric strength and types of breakdown](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adielectric-strength-breakdown) — Electrical Engineering Material, chapter 3