A transformer combines some of the most dangerous aspects of electricity: high voltages, stored energy, flammable oil, and heavy equipment. No maintenance task justifies skipping a single safety step — a professional is known by their adherence to procedure, not their speed.
The Golden Steps Before Any Contact
- 1. Work permit: No work without an approved permit specifying the task, hazards, procedures, and the person responsible for isolation.
- 2. Disconnection from all sources: Open the breakers on both sides — a transformer is fed from two directions — and don't forget backfeed sources and auxiliary service circuits.
- 3. Lockout-Tagout (LOTO): A personal lock and warning tag on every isolation point; your key stays with you, not anyone else.
- 4. Verify absence of voltage: Using a device suitable for the voltage level; the device is tested before and after the measurement on a known source.
- 5. Visible grounding: Ground all terminals on both sides and assume that any ungrounded terminal is live. Windings retain charge, even after megger tests.
- 6. Secure the surroundings: Barriers, signage, and safe distances from adjacent live parts in the substation.
Hazards Specific to Transformers
| Hazard | Precaution |
|---|---|
| Charge stored in the windings and insulation | Discharge and ground before and after tests |
| Hot, flammable oil | Allow the transformer to cool, have fire-fighting equipment ready, no open flames |
| Current transformers (CTs) with open secondary | Never open a CT secondary under load — lethal voltage (see CT and VT) |
| Working at height and on top of the tank | Approved fall-protection equipment and anchor points |
| Heavy weights (bushings, radiators) | Approved lifting equipment and a lifting plan |
Sample answer: After obtaining the work permit: disconnect from all sources (the high-voltage and low-voltage sides and any backfeed), then apply lockout-tagout (LOTO) at every isolation point, then verify the absence of voltage with a suitable device tested before and after, then visibly ground all terminals on both sides while discharging any stored charges, and finally secure the work area with barriers and signage from adjacent live parts.
Disconnecting only one side and forgetting that the transformer may remain live from the other side or via backfeed through the network. The rule: a transformer has two sides, and complete isolation must cover every possible source without exception.
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