The transformer has been operating normally for years. There is no trip, no alarm and no complaint from the building.
During a planned electrical inspection, however, the engineer notices something unusual: one LV termination is significantly hotter than the other phases. The transformer is still carrying the building load, but the connection has started deteriorating.
If that condition is ignored, the next indication may not be another maintenance observation. It may be an overheated termination, damaged insulation or an unplanned electrical shutdown.
This is why transformer maintenance should not be reduced to “check transformer condition — OK.”
A proper transformer maintenance checklist looks for developing problems while the transformer is still operating normally: abnormal temperature, contamination, loose connections, deteriorating insulation, ventilation problems, oil leakage, moisture and changes in electrical-test results.
For FM engineers, the objective is simple: identify deterioration early enough to plan the repair instead of waiting for the transformer to decide the shutdown date.
Why Transformer Maintenance Matters
A transformer is one of the most critical assets in a building’s electrical distribution system.
A typical arrangement may be:
Utility MV Supply → Transformer → LV Main Switchboard → MDB → SMDB → DB → Loads
If an FCU fails, one room may be affected.
If an MDB feeder trips, one section of the building may lose power.
If the main transformer fails, a much larger part of the facility can be affected.
Your recently published Single Line Diagram Explained guide is useful for understanding exactly where the transformer sits in this distribution hierarchy:
Transformer PPM therefore focuses on three objectives:
- Maintaining safe operation
- Detecting deterioration before failure
- Building condition history through repeatable measurements
A transformer that has not tripped is not necessarily a transformer in good condition.
Know Which Transformer You Are Maintaining
The maintenance programme depends heavily on transformer type.
Two common building applications are:
Dry-Type Transformer
Dry-type transformers do not use insulating oil as the main cooling and insulation medium.
You commonly find them in:
- Commercial buildings
- Hospitals
- Malls
- Data centres
- Indoor substations
- High-rise developments
Their maintenance concentrates heavily on:
- Cleanliness
- Ventilation
- Winding condition
- Connections
- Temperature
- Moisture
- Insulation
- Fans and temperature-control equipment
Oil-Immersed Transformer
These transformers use insulating liquid for insulation and heat removal.
Maintenance may therefore include:
- Oil level
- Oil leakage
- Bushings
- Breather condition
- Oil temperature
- Dissolved gas analysis
- Oil quality
- Pressure-relief devices
- Cooling equipment
The manufacturer’s O&M manual should always be your primary maintenance reference.
Transformer Nameplate: Check This Before PPM
Before maintaining a transformer, understand what you are looking at.
A nameplate may show:
| Parameter | Example | What It Tells You |
| Rated power | 1500 kVA | Transformer capacity |
| Voltage | 11 kV / 415 V | Primary and secondary voltage |
| Frequency | 50 Hz | Designed supply frequency |
| Vector group | Dyn11 | Winding connection/phase relationship |
| Impedance | 6% | Important for fault and system studies |
| Cooling | AN / AF | Cooling arrangement |
You may also see:
- Manufacturer
- Serial number
- Year
- Tap positions
- Insulation class
- Temperature rise
- Total mass
Do not maintain equipment without first confirming its identity.
On sites with multiple transformers, a PPM report saying only “Transformer checked” is poor documentation.
Record the actual asset number, for example:
TR-01 — 1500 kVA — 11/0.415 kV
Transformer Maintenance Frequency
There is no universal maintenance interval that fits every transformer.
The frequency depends on:
- Manufacturer recommendation
- Transformer type
- Loading
- Environment
- Dust
- Humidity
- Criticality
- Previous condition
- Applicable electrical-maintenance programme
For dry-type transformers, Schneider Electric states that inspection frequency depends on operating conditions. It notes that annual inspection may be sufficient in clean, dry locations, while locations contaminated with dust or chemical fumes may require inspection at three- or six-month intervals. Ckm Content
This is a much better approach than blindly assigning the same annual PPM to every transformer.
A dusty electrical room with poor ventilation deserves more attention than a clean, controlled substation.
Routine Transformer Inspection Checklist
Routine inspections can identify problems without dismantling equipment.
| ☐ | Task | What to Check / Accept Standard |
| ☐ | Check operating status | No unexplained alarm or abnormal indication |
| ☐ | Check temperature | Compare phases/areas and previous readings |
| ☐ | Listen for noise | No new buzzing, rattling or abnormal sound |
| ☐ | Check ventilation | Louvres and airflow paths unobstructed |
| ☐ | Inspect room | Clean, dry and free from water ingress |
| ☐ | Check smell | No burning or overheated-insulation smell |
| ☐ | Check loading | Compare current/load with normal trend |
| ☐ | Inspect enclosure | No damage, corrosion or loose panels |
| ☐ | Review alarms | Investigate temperature or fan alarms |
| ☐ | Check housekeeping | Required access and clearances maintained |
The important word here is compare.
A temperature of 70°C recorded once tells you less than:
- Month 1: 52°C
- Month 2: 55°C
- Month 3: 63°C
- Month 4: 71°C
Trend data shows deterioration.
Transformer Room Inspection
Do not inspect only the transformer.
Inspect its environment.
For a dry-type transformer especially, cooling depends on adequate airflow.
Check:
- Supply and extract ventilation
- Air-conditioning where provided
- Blocked ventilation openings
- Dust accumulation
- Water leakage
- Condensation
- Storage around equipment
- Pest ingress
- Lighting
- Access
- Fire stopping
- Door condition
IEC 60076 normal-service conditions include environmental assumptions such as ambient-air limits, and Schneider’s summary of IEC 60076-1 gives a normal maximum ambient-air temperature of 40°C, unless the equipment is designed for other conditions. Schneider Electric
That does not mean 40°C should be your target transformer-room temperature.
It means you must know the transformer design conditions and ensure the actual environment remains within them.
Dry-Type Transformer Maintenance Checklist
During an authorised shutdown, a more detailed inspection becomes possible.
| ☐ | Task | What to Check / Accept Standard |
| ☐ | Inspect windings | Clean, dry and without visible damage |
| ☐ | Remove contamination | Dust/debris removed using approved method |
| ☐ | Inspect insulation | No cracking, tracking or discoloration |
| ☐ | Check connections | Inspect/tighten to manufacturer requirement |
| ☐ | Inspect bus connections | No overheating or physical deterioration |
| ☐ | Check cooling fans | Correct operation where fitted |
| ☐ | Inspect temperature sensors | Secure and functioning |
| ☐ | Check enclosure ventilation | No obstruction |
| ☐ | Perform required tests | Compare results with baseline/history |
| ☐ | Inspect earthing | Secure and in good condition |
Schneider’s dry-type transformer instructions specifically call for removing excessive dirt from windings and insulators so that air circulation is not restricted and insulation problems are reduced. They also require checking bolted electrical connections before energisation. Product Info
Do not clean blindly
Transformer windings and insulation are not ordinary plant-room surfaces.
Use the manufacturer’s approved cleaning method.
Avoid introducing:
- Moisture
- Conductive contamination
- Damage to insulation
- Foreign material inside the enclosure
The transformer must be isolated, proved dead and maintained by authorised competent electrical personnel under the site’s electrical-safety procedure.
Thermographic Inspection
Thermal imaging is one of the most useful condition-monitoring tools for electrical systems.
It can reveal abnormal heating at:
- Cable terminations
- Busbar connections
- Transformer terminals
- Tap connections
- LV connections
Eaton notes that transformer connection points are locations where developing thermal faults can occur and promotes thermal monitoring specifically to identify these problems before they result in outages or equipment damage. Eaton
Compare phases, not just temperatures
Suppose you record:
Phase L1 termination: 54°C
Phase L2 termination: 55°C
Phase L3 termination: 83°C
The 83°C reading deserves investigation because one comparable phase is substantially hotter.
Possible reasons include:
- Loose connection
- High contact resistance
- Unequal loading
- Cable termination deterioration
- Connection damage
Do not create a universal rule such as “anything above 70°C fails.”
Thermographic interpretation should consider:
- Load at inspection time
- Ambient temperature
- Similar components
- Manufacturer limits
- Historical readings
Record the electrical load when taking thermal images whenever possible.
A thermogram without loading information loses much of its diagnostic value.
Transformer Loading
A transformer rated 1500 kVA should not automatically be described as “50% loaded” because the building load is 750 kW.
kW and kVA are not the same.
For a three-phase system:
kVA = √3 × Voltage × Current / 1000
And approximately:
kW = kVA × Power Factor
If a building is consuming:
900 kW at PF 0.90
Apparent power is:
900 ÷ 0.90 = 1000 kVA
For a 1500 kVA transformer:
Loading = 1000 ÷ 1500 × 100 = 66.7%
Monitor loading particularly during peak building operation.
Insulation Resistance Testing
Insulation resistance testing is commonly included in transformer commissioning and condition assessment.
Eaton lists insulation resistance among the maintenance and commissioning tests used for dry-type and liquid-filled transformers, alongside tests such as transformer turns ratio and winding resistance. Eaton
The test should be carried out only under the correct isolation and testing procedure.
Do not publish one generic “acceptable megohm value” for every transformer.
Results depend on factors including:
- Transformer type
- Voltage class
- Insulation system
- Temperature
- Manufacturer criteria
- Test voltage
- Previous readings
Trend the readings.
If previous results were consistently high and the latest test drops sharply under comparable conditions, investigate the reason even if somebody says the value still “passes.”
Possible causes include:
- Moisture
- Dirt
- Insulation deterioration
- Test-condition differences
Schneider’s specific dry-type instructions, for example, call for moisture investigation and drying where required and give product-specific insulation-resistance guidance. That illustrates why the manufacturer’s manual matters more than a generic internet limit. Product Info
Winding Resistance Test
Winding resistance testing helps assess the condition and consistency of transformer windings and connections.
It can help identify issues such as:
- Poor connections
- Tap-changer problems
- Abnormal winding resistance
- Phase-to-phase inconsistencies
Do not judge the result by looking at one resistance number alone.
Compare:
- Phase readings
- Manufacturer data
- Previous test results
- Temperature-corrected results where applicable
A significant unexplained change is often more useful diagnostically than an isolated number.
Transformer Turns Ratio Test
The transformer turns ratio test verifies the relationship between primary and secondary windings.
It is useful during:
- Commissioning
- Major maintenance
- Fault investigation
- Tap-related investigation
Eaton includes transformer ratio testing among the standard transformer maintenance/commissioning tests covered in its service training. Eaton
If results do not correspond with expected ratios or tap positions, specialist investigation is required.
Oil-Immersed Transformer Maintenance
Oil-filled transformers need additional checks because the insulating liquid itself provides valuable information about transformer condition.
Typical inspection points include:
- Oil level
- Oil leakage
- Oil temperature
- Bushings
- Gaskets
- Radiators
- Cooling fans
- Conservator
- Breather
- Pressure-relief device
- Protection relays
Schneider’s current oil-immersed transformer manual includes periodic inspection, oil sampling, dissolved-gas analysis and maintenance of accessories such as bushings, breathers, oil-level gauges and pressure-relief devices. Schneider Electric
Oil leakage should not become “normal”
A small leak that has existed for six months is still a leak.
Record:
- Exact location
- Severity
- Oil level
- Whether leakage is increasing
- Required repair
Repeatedly wiping oil from the transformer is housekeeping, not corrective maintenance.
Dissolved Gas Analysis
For applicable oil-filled transformers, Dissolved Gas Analysis (DGA) is a valuable condition-assessment tool.
Transformer faults can generate gases in the insulating oil.
The gas pattern and trend can help specialists investigate conditions associated with:
- Thermal faults
- Electrical discharges
- Arcing
- Insulation deterioration
Do not interpret one DGA result in isolation unless there is a clear critical condition.
Trending is important.
A changing gas-generation rate can be more informative than a single sample.
Oil sampling must also be performed correctly; a poor sample can compromise the analysis.
Transformer Earthing
Inspect transformer earthing connections during planned maintenance.
Check:
- Physical connection
- Corrosion
- Damage
- Loose hardware
- Continuity/testing as required by the maintenance programme
Earthing provides a controlled path for fault current and supports the operation of protective devices.
For the wider building earthing arrangement, use: Earthing System in Buildings
Common Transformer Problems
| Symptom / Fault | Diagnosis & Action |
| Abnormally high temperature | Check loading, ventilation, fans, ambient condition and connections |
| One termination hotter than others | Check phase load, contact resistance and termination condition |
| Excessive humming/vibration | Compare with normal condition; inspect mounting and electrical/mechanical condition |
| Dust on dry windings | Plan safe cleaning using manufacturer-approved procedure |
| Low insulation-resistance trend | Investigate moisture, contamination and insulation condition |
| Oil leakage | Locate source, check oil level and arrange corrective repair |
| Repeated high-temperature alarm | Verify sensor, loading, cooling and actual transformer temperature |
| Cooling fan not operating | Check fan supply, controls, motor and temperature-control logic |
| Unusual burning smell | Treat as abnormal; investigate immediately under safe operating procedure |
A Practical FM Example
Your building has two 1500 kVA transformers.
During monthly inspection, both appear normal.
During annual thermography, however, the engineer records one LV termination on Transformer 2 significantly hotter than the comparable phases.
The transformer has not tripped.
There is no BMS alarm.
Occupants have noticed nothing.
This is exactly when preventive maintenance has succeeded.
The team can now:
- Record the thermal image.
- Record transformer loading.
- Compare phase currents.
- Review previous thermography.
- Plan an authorised shutdown.
- Inspect the termination.
- Correct the defect.
- Re-scan after restoration.
Waiting until the connection fails would turn a planned maintenance issue into an operational emergency.
Transformer PPM Records to Keep
Every transformer should have an asset history containing:
- Asset identification
- Nameplate details
- Load readings
- Temperature readings
- Thermography reports
- Insulation-resistance results
- Winding-resistance results where applicable
- Ratio-test results where applicable
- Oil-analysis/DGA results where applicable
- Alarm history
- Corrective maintenance
- Shutdown history
- Photographs
- Test certificates
The value is not the individual report.
The value is being able to compare this year against last year.
Your wider electrical programme can be structured using: Electrical Preventive Maintenance Checklist
Frequently Asked Questions
1. How often should a transformer be maintained?
There is no single interval suitable for every transformer. The manufacturer, operating environment, loading, criticality and condition history should determine the programme. For some dry-type transformers in clean, dry environments, annual inspection may be sufficient, while dusty or chemically contaminated environments may justify three- or six-month inspections. Ckm Content
2. What is the most important transformer maintenance test?
No single test covers every failure mode. Visual inspection, loading and temperature trending, thermography, insulation testing and other electrical tests each identify different conditions. For oil-filled transformers, oil testing and DGA can add important internal-condition information.
3. Is transformer humming normal?
A transformer normally produces some audible hum during operation. A new, significantly louder or mechanically different sound deserves investigation, particularly if accompanied by vibration, overheating or load changes. The useful comparison is with the transformer’s established normal condition.
4. Why does a transformer overheat?
Common causes include excessive loading, high ambient temperature, restricted ventilation, cooling-fan failure, abnormal connections and internal defects. Do not respond only by adding cooling to the room. Check the transformer load and determine why the temperature has increased.
5. Can transformer maintenance be performed while it is energised?
Routine external observations and properly planned condition monitoring may be possible while equipment is operating, depending on the installation and electrical-safety programme. Internal inspection, cleaning, tightening and many electrical tests require safe isolation. Work must follow the site’s electrical safety, permit, LOTO and test-for-dead procedures and be performed by authorised competent personnel.
Conclusion
Good transformer maintenance is mostly about detecting change.
A hotter connection, increasing load, falling insulation-resistance trend, new vibration, oil leakage or repeated temperature alarm may appear minor while the transformer is still operating. Those are exactly the conditions a good PPM programme should catch before they develop into failures.
For the wider electrical distribution system, continue with the Single Line Diagram Explained guide and Electrical Preventive Maintenance Checklist.
