ELEC5205 Chap.4 Solid, Liquid and Surface Breakdown
Solid, Liquid and Surface Breakdown
Location decides the failure label
Real solid insulation contains voids, droplets, particles and electrode irregularities that concentrate field and initiate local heating or discharge. Branching electrical trees leave damaged channels through bulk material. Liquid paths can disperse when voltage is removed.
On contaminated surfaces, leakage heating creates dry bands, local arcs carbonise the path and flashover bridges the remaining gap. Persistence, location and timescale distinguish the mechanisms.
Follow a bulk defect from field concentration to damage
A gas void in a solid has different permittivity from the surrounding dielectric, so voltage division can place a high local field across the void.
Repeated partial discharges erode, heat and chemically change nearby material. Electrical trees then grow as branched channels through the bulk, often from an electrode defect or stressed interface. The remaining structure records cumulative damage even when the supply is removed.
Describe inception, repeated local attack and eventual bridge as distinct stages.
Distinguish liquid evidence by persistence
Particles, bubbles and droplets in a liquid can align with the field, distort stress and support a temporary discharge path. Flow, buoyancy and removal of voltage may disperse that path, unlike a carbonised solid channel. Moisture and contamination still affect both inception and recovery.
A post-event inspection should therefore ask what residue, gas, particle motion or surface mark remains. The absence of a permanent branch does not prove that the event was harmless or that the liquid condition is acceptable.
Trace tracking as a thermal-electrical feedback loop
Contamination and moisture allow leakage current along an insulating surface.
Nonuniform drying produces a high-resistance dry band, concentrating voltage across a short region. Local arcing heats and carbonises the surface, which redistributes current and prepares another arc. The carbonised track is progressive material damage. Flashover is the later event that spans the remaining insulation path.
Using those terms interchangeably hides the sequence and can lead to the wrong maintenance response.
Classify by path, residue and timescale
Start every scenario by locating the activity: within the bulk, in a liquid volume or along a surface. Next ask whether the path is permanent, dispersible or dependent on continuing contamination.
Then identify the timescale, from short discharge pulses to long ageing and final high current. Conclude with a control linked to the mechanism: remove voids, filter or dry liquid, increase creepage, improve surface profile, clean pollution, or reduce local field. A label without that causal chain earns little confidence.
What this chapter covers
- 01
Electrical Treeing
- 02
Water Trees
- 03
Liquid Recovery
- 04
Surface Tracking
- 05
Dry-Band Arcing
- 06
Flashover
Worked example · free
Diagnose a contaminated surface
- 2Identify the conductive film and leakage heating.
- 2Sequence dry-band arcing and carbonisation.
- 2Name the final event as flashover and justify the location.
Key terms
- Electrical Tree
- Electrical Tree is a persistent branching channel created through bulk solid insulation by concentrated electrical stress.
- Dry-Band Arc
- Dry-Band Arc is a local discharge across a dried section of an otherwise conducting contaminated surface.
- Flashover
- Flashover is the rapid discharge that bridges the remaining path along an insulation surface and surrounding gas.
Solid, Liquid and Surface Breakdown FAQ
Where do electrical trees begin?
They can begin at gas voids, conductive particles, water droplets or microscopic electrode protuberances where the local field is enhanced. Heating, bond breaking and electrostatic forces extend the initial damage into branching hollow channels.
How do liquid and solid trees differ after voltage removal?
A damaged solid retains its channels because material has been decomposed or cracked. In a liquid, bubbles and aligned particles can disperse or rise after the field is removed, so the temporary path may not remain visible.
What is the difference between tracking and flashover?
Tracking is the slow accumulation of carbonised surface damage through leakage heating and repeated dry-band arcs. Flashover is the fast terminal discharge across the remaining surface-air path once insulation between the electrodes is effectively lost.
Exam move
Sort examples by bulk, liquid and surface location before memorising names. Reproduce the tracking sequence from moisture to flashover, then compare what remains after voltage removal in solid and liquid cases. Use the old solved-paper themes only as a checklist; answer fresh scenarios in your own words. Build a three-column inspection ledger for path location, persistent residue and event timescale.
Use it to classify unfamiliar evidence before choosing a term. For each mechanism, connect one intervention to the causal step it interrupts: remove voids, dry or filter liquid, clean contamination, increase creepage, smooth a field concentration or limit thermal stress. When evidence mixes locations, describe the sequence explicitly rather than choosing the most dramatic label and forcing every observation into it.
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