On-site core inspection—often called inspection for core lifting or inspection for bell/cover lifting—is one of the highest-risk activities in transformer commissioning and overhaul. Opening the main tank exposes the core, windings, tap changer, and solid insulation to moisture, dust, mechanical shock, and confined-space hazards. For oil-immersed distribution and power transformers, the work must follow a controlled procedure based on the manufacturer’s technical file, applicable installation standard, and site environmental limits.

The framework below is written for EPC supervisors, substation commissioning engineers, and utility maintenance teams. It can be applied to 10 kV/35 kV oil-immersed units as well as 110 kV/220 kV power transformers, with the exact exposure time, heating method, and gas-handling steps adjusted to the manufacturer’s instructions.

When On-Site Core/Bell Inspection Is Required

Not every delivered transformer must be opened. Many units arrive with acceptable impact records, tightness checks, and factory test reports, and can be installed without body inspection. Site bell removal is normally considered when:

• The transport impact recorder exceeds the agreed threshold or data is unavailable for a large/high-voltage unit.
• There is evidence of oil leakage, pressure loss for gas-filled units, or suspicion of internal displacement after handling.
• Short-circuit, lightning, or internal-fault indicators require visual confirmation.
• The contract or local installation code requires body inspection for the voltage/class/transport mode.
• The manufacturer’s technical documentation requires opening after prolonged storage, suspected moisture ingress, or special site conditions.

For Xinda/Xinda-style portfolios covering 10 kV S11-M/S13-M.RL, 35 kV S11/SZ11, 110 kV/220 kV oil power units, and S(B)H15-M amorphous oil units, the decision should come from the project technical file rather than a generic rule.

Pre-Inspection Isolation and Safety Control

Before any cover bolt is loosened, treat the transformer as a multi-hazard workplace.

• Electrical isolation and LOTO: De-energize all HV/LV bushings, neutral, auxiliary power, cooling fans, motor tap-drive, heating, and SCADA circuits. Apply lockout/tagout on every source, including temporary site power. Verify zero voltage on all terminals before touching fittings.
• Grounding: Bond the tank, conservator supports, Radiators, and control cabinet to the station earth. Keep bushings grounded during lead removal to avoid induced voltage; only disconnect for specified testing.
• Oil system isolation: Close and lock valves to radiators, coolers, conservator, OLTC diverter/oil compartment, and purifier where the inspection scope does not require them open. If any isolated compartment still contains oil, drain or lock-isolate to prevent engulfment.
• Fire safety: Stage suitable extinguishers for oil work; prohibit open flame near open tank; control hot-work permits for any welding on brackets or fittings.
• Permits and roles: Use confined-space entry permit for tank entry, appointed lifting supervisor, safety watcher, tool registrar, and manufacturer technical representative where required.

Environmental and Atmospheric Limits

Moisture control is the single most important condition for bell removal.

Reference rules commonly used in Chinese installation practice and aligned with international liquid-immersed guidance:

• Do not perform body inspection in rain, snow, dense fog, or when wind reaches force 4 and above.
• Do not open the tank when relative humidity is above 75%.
• Prefer indoor or enclosed tent work. Outdoor bell removal requires rainproof, dustproof, and temperature-controlled shelter.
• Ambient air temperature should not be below 0°C where practicable; transformer body temperature should not be lower than ambient. If the body is colder, preheat so the core/body is approximately 10°C above ambient, or follow the manufacturer’s alternative heating instruction to avoid condensation.
• Air-exposure clock: for conventional oil-filled body inspection, when relative humidity is below 75%, total exposure from start of oil draining—or from removing the cover/openings for un-oiled units—to start of vacuum treatment or oil filling should not exceed 16 hours; tighter limits apply at higher humidity bands per the governing standard and product file.
• For direct tank entry of gas-filled units, supplement with dry-air purging at dew point ≤ -40°C, keep oxygen ≥ 18%, and maintain internal relative humidity ≤ 20% during internal inspection; internal inspectors should be limited and tool-controlled.

IEEE C57.93 gives a complementary international rule: avoid opening at high humidity without moisture-control measures; for nitrogen/dry-air shipped units, replace inert gas with breathable dry air, monitor oxygen and combustible gas, and never allow entry before acceptable atmosphere is confirmed.

Gas-Filled and Nitrogen-Transport Units: Pre-Bell Safety

Many 110 kV/220 kV and some 35 kV units are shipped with dry air or nitrogen. Mishandling the gas blanket causes asphyxiation and trapped-gas risk.

• Before cover removal, confirm tank pressure trend from storage records; check for abnormal drop that may indicate leakage.
• For nitrogen-filled transport, do not allow personnel entry before nitrogen is removed and replaced with breathable air. For bell lifting inspection, allow the body to stand in air for at least 15 minutes after opening so residual nitrogen diffuses before detailed internal checks.
• For tank entry, purge with dry air or use vacuum/purge per the manufacturer; verify oxygen, combustible gas, and—where fault suspicion exists—CO/hydrocarbon levels before entry.
• Maintain continuous atmosphere monitoring during internal work; keep one attendant outside the tank in visual/voice contact.
• If the unit uses dry-air positive pressure during transport, follow the declared pressure band in the technical file; Xinda oil units using gas custody should follow their packing/transport instruction rather than a generic pressure assumption.

Oil Drainage, Accessory Removal, and Lifting Preparation

A structured dismantling sequence reduces collision and contamination risk.

1. Collect baseline data: nameplate, vector group, tap position, oil type, factory test report, impact record, and storage pressure log.
2. Take oil samples before full drainage if the tank still contains oil—test breakdown voltage, moisture, gas, and acidity according to the applicable maintenance standard and contract.
3. Drain oil to a clean, dry, bonded storage tank through bottom valve/filter set; lower oil below the bell/leading level required for the inspection scope. Keep oil temperature and exposure records.
4. Remove specified externals in controlled order: bushings, surge arresters/terminals as required, conservator piping, gas relay, pressure-relief/buchholz accessories, radiators/coolers if specified, fans, thermometers, tap mechanism, silica-gel breather, and all bell bolts. Tag and photograph every connection.
5. Disconnect internal links to the bell: neutral leads, tap selector links, grounding straps, CT secondaries shorted and grounded per safety rules, sensor cables. Confirm no tension remains on the cover.
6. Verify lifting points, hook center, sling rating, and spreader beam. For bell lifting, keep sling angle to the vertical within the manufacturer’s limit; general good practice often uses not greater than 30° when no special beam is used, but the approved rigging calculation and lifting drawing govern.
7. Appoint a single signaling person; inspect tank corners, core clamps, and winding tops for clearance before first lift.

Bell Lifting and Core/Body Exposure

• Lift slowly in stages. Stop after a few centimeters to check for sticking, forgotten links, distorted gaskets, or contact between core/clamps and tank wall.
• For bell-type units, lift the cover and place it on clean supports; for top-mounted active-part units, lift the complete core-winding assembly only when the design and rigging permit.
• Never use bushing holes, fan frames, radiator nozzles, or unidentified brackets as lifting points.
• Prevent tools, bolts, washers, and cleaning materials from falling into the tank; use tool belts, numbered tool lists, and drop-in prevention trays.
• Begin the inspection clock at first oil drain or first cover/opening, and continuously track elapsed exposure against the humidity-based limit.

Core and Winding Inspection Checklist

The objective is to confirm the active part has no transport damage, moisture ingress, loosening, or contamination.

Core

• Silicon-steel/laminated or wound-core clamping bolts tight; no loose punchings.
• Core ground strap single-point connection verified where design requires; no unintended multiple grounds.
• For S13-M.RL three-dimensional wound core or S11-M·R wound core, check core symmetry, binding, and winding support without disturbing factory bonding unless instructed.
• Amorphous S(B)H15-M cores handled per special instruction—amorphous ribbon edges and wound structure are sensitive to mechanical shock; do not hammer, pry, or re-torque beyond the technical file.
• Cooling ducts/channels free of debris.

Windings

• Axial/radial clamping, spacers, and pressure pads in position; no displacement, bulging, or indentation.
• Paper/insulation surfaces dry, no sludge, carbon tracking, discoloration from overheating, or moisture staining.
• LV foil/ribbon joints, HV layer/disc connections, and lead exits intact; tap windings and transition joints secure.
• For dry-type comparison only, epoxy cast SC(B)10/SCB18 or VPI SG(B)10 would be judged on casting cracks, delamination, and PD signs—but this article’s bell procedure applies to oil-immersed tanks.

Leads and supports

• Lead clearances to tank/ground per design; no scorching, loose tying, or contact with sharp edges.
• Bushings draw leads centered; CTs mounted, secondary terminals secured and shorted/tagged as required.
• Insulating cylinders/barriers undamaged.

Tap changer

• Off-circuit tap: fixed/contacts clean, position matches record, insulating shaft not cracked.
• On-load tap changer: selector/diverter contacts per manufacturer; motor drive neutral; oil compartment isolated/clean; transition resistors unchecked unless fault suspected and test plan exists.
• Regulating switch air-exposure time follows the stricter tap-device table in the governing standard when separate from main body.

Tank and bottom

• Bottom free of metal filings, moisture pools, sludge, and packing remnants.
• Grounding terminals, magnetic shield bolts, and structural supports tight.
• Gasket grooves clean; reused gaskets only if approved by the manufacturer and condition-inspected.

Key Risks and Corresponding Controls

Moisture absorption

Risk: high humidity, condensation, long exposure, rain ingress → insulation resistance drop, PD, future failure.

Controls: tent/enclosure, humidity logging, body preheating, dew-point-controlled dry air for entry, minimized exposure, vacuum drying/oil filling per technical file.

Asphyxiation and toxic gas

Risk: nitrogen/dry-air blanket, fault-generated gases, solvent cleaning vapors.

Controls: pre-entry gas replacement, continuous O₂/combustible/CO monitoring, confined-space permit, outside attendant, no entry before acceptable readings.

Mechanical lifting damage

Risk: tilted bell, struck windings, bent lifting lug, sling failure.

Controls: approved rigging plan, spreader beam, limited sling angle, staged lift, four-corner monitoring, verified lifting points only.

Foreign objects left inside

Risk: tools, bolts, wipes, metal filings cause turn-to-turn fault or blocked flow.

Controls: numbered tools, sign-in/sign-out, non-shedding cloths, magnetic sweeps, final borescope/visual check, close-out signature by manufacturer representative.

Oil contamination

Risk: dirty storage tank, wrong oil grade, water in oil, mixing fluids.

Controls: dedicated filtered oil system, sampled and tested oil, sealed transfer lines, documented oil type per specification, no mixing of mineral/ester/silicone without approval.

Electrical restart hazards

Risk: incomplete LOTO, CT open-circuit, electrostatic induction, test voltage contact.

Controls: full LOTO, CT secondaries shorted/grounded until testing, bushing grounding during lead work, approved test permits for insulation/resistance/ratio/impulse.

Reassembly, Vacuum, and Oil Filling

Reassembly quality determines whether the inspection improves reliability or introduces new defects.

• Clean contact surfaces, replace gaskets as specified, and tighten bell bolts in the manufacturer’s cross/torque sequence.
• For internal entry jobs, refill/continue dry-air supply until the tank is closed; then perform leak check on manholes, flanges, and bushings.
• Vacuum treatment: apply vacuum level, holding time, and maximum allowable tank stress per the product technical file; never exceed tank design vacuum.
• Oil filling: use filtered, tested oil of the specified type; fill through the approved bottom/valve path where required, avoid entraining air, maintain positive dryness control, and record oil temperature, vacuum, and filling volume.
• For nitrogen/dry-air shipped units, complete gas replacement and atmosphere verification before any person re-enters after partial assembly.
• Refit radiators, conservator, buchholz/pressure relays, thermometers, tap drive, breather with correct silica gel, and all electrical terminals.

Post-Inspection Testing and Acceptance

After bell closure and oil conditioning, perform the tests defined by the project and applicable standard family—typically IEC 60076 or the contract’s GB/IEEE equivalent:

• Winding resistance, ratio, vector group verification
• Insulation resistance and, where specified, polarization index
• No-load loss/no-load current and load loss/impedance at agreed tap
• Power-frequency withstand and, for applicable HV units, induced overvoltage/impulse
• Oil dielectric, moisture, DGA where the unit size or fault suspicion justifies it
• Accessory functional tests: buchholz alarm/trip, WTI/OTI, tap motor limits, cooling-group interlocks, SCADA signals

For Xinda 110 kV/220 kV units, factory routine/type test infrastructure includes integrated test benches, impulse generators, regulators, intermediate transformers, frequency-doubling sets, and HV withstand systems; site retests should be scoped to confirm site-handling impacts, not to replace factory type tests.

Application to Xinda Product Lines

The procedure scales differently by product type:

• 10 kV oil: S11-M sealed, S11-M.ZT/S13-M.ZT on-load capacity-regulating, S13-M.RL 3D wound core, D11 single-phase—usually only open on impact anomaly, leakage, or contract requirement; keep exposure tight because distribution schedules are sensitive to moisture.
• Amorphous oil S(B)H15-M: core handling instructions must be requested from the manufacturer; avoid mechanical shock and unauthorized core retorque.
• 35 kV S11/SZ11: inspect tap compartment separately; SZ11 on-load regulation requires OLTC drive/contact checks per the switching device manual.
• 110 kV/220 kV power: gas-filled transport, bell lift, direct internal entry, DGA, and full post-open test plan are common on major projects; use manufacturer technical representative and stricter atmospheric control.
• Special oil units: rectifier, furnace, mining-general, high-overload—bell inspection must include application-specific leads, cooling, and protection interfaces; do not use a standard distribution checklist unchanged.

Site Procedure Summary Checklist

1. Confirm need for body/bell inspection from impact data, leakage, tests, and contract.
2. LOTO, grounding, permits, fire cover, lifting plan, tool register.
3. Record weather: temperature, RH, wind, rain/snow; only proceed within approved limits.
4. Gas units: pressure check, nitrogen/dry-air purge, O₂/combustible verification, 15-minute diffusion for nitrogen bell lift where specified.
5. Oil sampling, filtered drainage, accessory removal with tags.
6. Rigging check, staged bell lift, no unauthorized core disturbance.
7. Inspect core, windings, leads, tap, CTs, tank bottom; photograph and record.
8. Track exposure time; close, vacuum, fill with tested oil, leak-check.
9. Routine + specified type/special tests; review DGA for large/HV units.
10. Issue inspection report with before/after photos, oil certificates, test data, and deviation list.

Conclusion

On-site transformer core/bell inspection is not a routine “open and look” task. It is a controlled operation where atmospheric limits, gas safety, lifting geometry, oil cleanliness, and post-open testing decide whether the unit returns to service more reliable or more vulnerable. For oil-immersed 10–220 kV projects, the practical rule is simple: open only when justified, prepare the environment first, control exposure by the clock and humidity, inspect against a written checklist, and close with vacuum/oil treatment plus verified tests.

Xinda Transformer supports this workflow with 220 kV-and-below oil and dry manufacturing in Liaocheng—registered capital RMB 150 million, 150 mu site, 53,000 m² building area, 470+ production equipment units, 47-set-class test infrastructure where applicable, and product coverage from S11/S13/S20/S22 oil, SC(B)/SG(B) dry, S(B)H15/SCBH15 amorphous, to 35–220 kV power and special transformers. For any site bell-removal job, request the model-specific rigging drawing, gas-handling instruction, exposure limits, vacuum/oil-filling parameters, and recommended post-inspection test schedule before the cover is removed.

Planning commissioning or post-transport body inspection for an oil-immersed transformer?​ Send Xinda technical team the nameplate, transport mode, impact record, site temperature/humidity range, and intended inspection scope. We will return a model-specific bell-removal procedure, safety permit list, exposure-time plan, and factory-aligned retest schedule—without assuming generic limits that your product file does not support.