The answer first: a bushing is the insulating sleeve that carries live current through the grounded tank — and it is the most common point of external failure on a distribution transformer
A bushing is the component that lets a high-voltage conductor pass through the earthed metal tank without flashing over to it — a small part that carries a disproportionate share of the failure risk. For a standard oil-immersed distribution transformer you will almost always specify either a porcelain (ceramic) or a composite (silicone rubber / polymer) bushing, rated for the unit's BIL (Basic Insulation Level) and sized so its creepage distance — the shortest surface path along the insulator — matches the pollution level of the site (roughly 16 to 31 mm per kV of phase-to-phase voltage, following IEC 60815). Specify the right rated current, BIL and creepage distance up front, and you eliminate the majority of bushing failures, which typically show up as terminal overheating, moisture ingress, or flashover in coastal and polluted areas. A failed bushing is also one of the few failure modes that can escalate into a tank rupture or fire, because it is a direct path between a live conductor and oil.
Key takeaways
- Bushings are the most common external failure point. They are the physical boundary between a live conductor, oil, and the grounded tank — three things that must never meet.
- Porcelain vs composite is a pollution and mechanical decision. Composite (silicone rubber) is hydrophobic, shatter-resistant and better in heavy pollution; porcelain is cheaper, stiffer and time-proven.
- Specify three numbers, not just a voltage. Rated current (A), BIL (kV), and creepage distance (mm) — the last one is what buyers most often forget.
- Creepage distance follows the site, not the transformer. Clean inland sites need roughly 16–20 mm/kV; coastal, industrial and desert sites need 25–31 mm/kV or more (IEC 60815 pollution classes).
- Bushing health is checkable before failure. Oil level, visual cracks, infrared scans of the terminal, and a tan-delta (power factor) test all catch a bad bushing early.
What exactly is a bushing, and why does a part this small fail so often?
A bushing has three jobs at once: it insulates the live conductor from the grounded tank, it holds the conductor mechanically against wind, cable pull and vibration, and it seals the oil so moisture and air stay out. Most distribution-transformer bushings are of the capacitance-graded (condenser) type at higher voltages, or a simpler solid design at lower voltage classes; in either case the failure modes are the same three: electrical flashover across a polluted or wet surface, thermal ageing at a loose or undersized terminal, and mechanical cracking from shipping or seismic movement.
The reason bushings punch above their weight in the failure statistics is simple: they are the only component that spans three environments at once — live metal, insulating oil, and open air. That is why a transformer you ship to a clean, dry inland site can run for decades on the same bushing, while an identical unit installed 500 m from the coast can flash over in its first humid season if the creepage distance was under-specified.
Porcelain vs composite bushings: which one should you order?
The choice comes down to pollution, mechanical shock and cost.
- Porcelain (ceramic) bushings are the traditional default — rigid, long-proven, and generally lower in first cost. Their weakness is that the smooth, hydrophilic glaze holds a continuous water film in rain and fog, which reduces the effective creepage distance and encourages flashover, and the ceramic can shatter on impact.
- Composite (silicone rubber / polymer) bushings use a fibreglass core wrapped in a silicone shed. The silicone is hydrophobic — it repels water into droplets instead of a film, which keeps the flashover voltage high even in heavy pollution — and the material is shatter-resistant, so a dropped tool or a vandal strike leaves a dent rather than a burst of porcelain fragments.
For most export orders the practical rule is: porcelain for clean, inland, low-cost projects, and composite for coastal, industrial, or high-pollution sites — or anywhere the specification explicitly demands a silicone or polymer shed. The price difference is modest at distribution voltages (industry experience puts a composite bushing at a small premium over porcelain), so the pollution environment, not the budget, should drive the decision.
The three numbers to put on your order (beyond the voltage)
1. Rated current — match it to the winding, not the minimum
The bushing's rated current must carry the transformer's rated secondary current, and the standard sizes are 250 A, 630 A, 1000 A, 1250 A, 2000 A, 2500 A and 3150 A (industry standard series). The most common mistake is sizing the LV bushing to the transformer's kVA rating and forgetting that a heavily loaded or force-cooled unit can run continuously above nameplate — the bushing terminal is exactly where that overload shows up first as localised heating at the contact, which accelerates oil ageing right at the tank wall.
2. BIL — the bushing must survive the same lightning impulse as the winding
BIL, or Basic Insulation Level, is the lightning-impulse withstand the bushing must survive, and it is always specified together with the system voltage (for example, 95 kV BIL for a 12 kV system and 125 kV for a 24 kV system, per IEC 60076-3). A bushing with too low a BIL is a weak point in the whole surge-protection chain — the arrester may do its job, but the bushing is still the first solid insulation a travelling wave meets. Order the bushing BIL at least equal to the transformer's BIL, and do not let a "bargain" bushing silently downgrade it.
3. Creepage distance — the number buyers most often forget
Creepage distance is the shortest distance measured along the insulator surface between the live end and the earthed end, and it is what stops surface flashover in rain, fog, dust and salt spray. The rule of thumb, per IEC 60815 pollution classes, is roughly 16 mm/kV for light pollution, 20 mm/kV for medium, 25 mm/kV for heavy, and 31 mm/kV or more for very heavy (coastal and industrial) pollution. Because it is expressed per kV of phase-to-phase voltage, a 24 kV system in a heavy-pollution zone needs a creepage distance in the region of 600 mm — and if you order by voltage alone, the default bushing may arrive sized for a clean inland site and flash over within a season. This one number is the cheapest insurance you can buy against a first-switch-on failure in a coastal project.
How to spot a failing bushing before it fails
Bushings rarely fail without warning; they leave a trail you can read during routine inspection.
- Oil level and leaks. A porcelain bushing with an oil-impregnated paper core has a small oil reservoir and a sight glass; a falling level or a weeping gasket means moisture is already inside. A composite bushing should show no cracks in the silicone sheds.
- Infrared scan of the terminal. The bushing terminal is a classic hot spot; an infrared scan that shows the LV or HV terminal hotter than the surrounding metal points at a loose or oxidised contact before it turns into a fire.
- Tan-delta (power factor) and capacitance. A tan-delta test of the bushing insulation — standard during commissioning and often repeated in service — catches moisture ingress and ageing long before a flashover. A rising tan-delta value is the single clearest early sign.
- Visual checks after shipping. Bushings are the most damage-prone part in transit; check every bushing for cracks, chips and a shifted porcelain body before you accept delivery, exactly as covered in the pre-shipment inspection.
Sources / 资料来源
- Source: IEC 60137 — Bushings for alternating voltages above 1 000 V
- Source: IEC 60815 — Selection and dimensioning of high-voltage insulators for polluted conditions
- Source: IEC 60076-3 — Insulation levels, dielectric tests and external clearances in air
- Source: IEEE C57.19.01 — Performance characteristics and dimensions for outdoor apparatus bushings
- Source: QDTB Engineering Team — industry experience for composite-vs-porcelain cost premium and the standard bushing current series