A transformer rarely fails all at once. It almost always gives signs — some obvious, some only detectable with instrumentation — for weeks or months before the event. Recognizing them in time is the difference between scheduled maintenance and an unplanned outage with major damage. These are the seven signs that most anticipate a failure.
First, abnormal noise and vibration: a louder hum or new vibration can indicate loosening of the core or winding clamping. Second, overheating: temperatures above normal for the load, or a progressive rise, point to cooling problems, overload or internal degradation. Thermography in service is the fast way to locate the hot spot.
Third, oil leaks and falling level: beyond the environmental risk, they expose the insulation to air and moisture, accelerating its degradation. Fourth, oil appearance and values: dark oil, a burnt smell or out-of-spec physicochemical values indicate ageing or an active thermal problem.
Fifth — and most revealing — the dissolved gases. DGA detects arcing, partial discharge and overheating from the gas signature in the oil, long before any external symptom. A spike in acetylene or an increasing generation rate is a serious alarm even if the transformer 'looks fine'. Sixth, abnormal electrical test results: a power factor rising between measurements, a falling insulation resistance or an out-of-tolerance turns ratio confirm internal degradation.
Seventh: protection operation. Buchholz relay trips, sudden-pressure activations or temperature alarms are not 'false positives' to dismiss lightly — they are the equipment warning you. Every trip should be investigated with diagnostics, not simply reset.
The takeaway: none of these signs should be ignored, but none should be diagnosed by eye either. Confirm with data — DGA, thermography, electrical tests — and decide the intervention on evidence. A transformer that starts giving signs is, almost always, still recoverable; one that has already failed usually costs ten times more.