Is Your Transformer Safe to Switch On After a Fault? A Return-to-Service Checklist

When a transformer trips on a fault, gets flooded, or sits out of service for months, the pressure to switch it back on is enormous. Production is stopped, tenants are complaining and the generator is burning diesel. But energising a damaged transformer can turn a repairable fault into a total loss, and in the worst case cause a fire or explosion.

A return-to-service assessment answers one question with evidence: is this transformer safe to energise? Here is the checklist we follow.

When you need a return-to-service assessment

• After the transformer or its upstream breaker tripped on a fault
• After the Buchholz relay, pressure relief device or temperature protection operated
• After flooding, fire nearby, vandalism or oil theft
• After the transformer has been out of service for a long period
• Before putting a relocated or second-hand transformer into service

Step 1: Make it safe and gather the facts

Keep the transformer isolated, locked off and earthed on both the HV and LV sides. Then collect the history: what tripped, which protection operated, what the load was, and whether anyone saw smoke, noise or oil spray. This history often points straight to the cause.

Step 2: External inspection

• Oil leaks, a low oil level or oil sprayed around the tank
• Cracked or tracked bushings
• Burnt cable terminations and damaged cable boxes
• Bulging tank walls or an operated pressure relief device
• Gas collected in the Buchholz relay
• Condition of the silica gel breather

Step 3: Electrical tests

• Insulation resistance (IR) and polarisation index (PI): HV to earth, LV to earth and HV to LV. Low or falling values point to damaged or wet insulation.
• Turns ratio (TTR): on every tap. A wrong ratio on one phase suggests shorted turns.
• Winding resistance: on every phase and tap. Imbalance between phases points to a loose internal connection or damaged tap changer contacts.
• Vector group check: confirms the connections are correct, especially after any internal work.
• Magnetising current: useful for finding core or winding problems.

Step 4: Oil tests

• Breakdown voltage (dielectric strength) and moisture content show whether the oil can still insulate.
• Dissolved gas analysis (DGA) shows what kind of fault happened inside: overheating, arcing or partial discharge.

Step 5: Check the protection

Before energising, confirm that the protection that should clear the next fault actually works: Buchholz and temperature trips, the upstream breaker or fuses, and any relay settings. A transformer that tripped once can trip again, and the protection must be ready.

Step 6: Decide and document

The engineer compares results with the transformer’s previous records, the manufacturer’s data and accepted practice, and then recommends one of three outcomes:

1. Safe to energise, with any minor remedial work listed
2. Energise after remedial work, such as oil filtration, termination repair or a new breather
3. Do not energise: the transformer needs workshop repair or replacement

Everything should be written up in a report you can show to management, insurers and your DisCo if needed.

Energising safely

When the transformer passes, energise it with no load first, listen and watch for anything unusual, then add load in stages while monitoring temperature. Keep the report with your asset records. It becomes the baseline for future tests.

For routine care between faults, read our transformer maintenance and testing guide.

Talk to Corustar

Corustar Technological Services is an electrical engineering contractor based in Ikeja, Lagos, working across Nigeria. See our Transformer Services service, request a quote, or call +234 816 976 9672. For outages and faults, call our 24/7 emergency line on 0705 739 9297.