What actually causes failure
The dominant cause is insulation degradation, accelerated by heat and moisture. Every 6 to 8 degrees Celsius of sustained overtemperature roughly halves insulation life, so chronic overloading kills a transformer years before its time without ever tripping anything. After that come lightning and switching surges, moisture ingress through a failed gasket, and mechanical damage from through faults that loosen windings. Manufacturing defects exist but are a small share.
Dissolved gas analysis is the real test
On a sealed unit you cannot see inside, so the oil is the diagnostic. Dissolved gas analysis identifies what is happening by which gases are present. Acetylene points to arcing, ethylene to serious overheating, methane and ethane to lower temperature faults, and hydrogen to partial discharge. Taken over time rather than once, it shows a trend, and the trend is worth more than any single reading.
The other tests worth doing
Insulation resistance and polarisation index for the general condition of the insulation. Turns ratio to confirm windings and tap positions are intact. Winding resistance to find loose connections and broken strands. Power factor or dissipation factor for insulation quality. Oil dielectric breakdown voltage and moisture content for the fluid itself. On a new unit, an acceptance test before energising is far cheaper than finding the problem after.
What a multimeter can and cannot tell you
A multimeter will confirm continuity on a winding and give a rough turns ratio on a small de energised transformer, which is enough to find an open circuit. It will not find degraded insulation, moisture, partial discharge or an incipient fault, and those are what actually fail. For a medium voltage pad mounted unit, never open anything to test it. Have it de energised, grounded and tested by someone qualified with the right instruments.
Common questions
What is the most common cause of transformer failure
Insulation degradation driven by heat and moisture. Sustained overloading raises winding temperature, and every 6 to 8 degrees Celsius of overtemperature roughly halves insulation life, so a chronically overloaded unit fails years early.
What tests are performed on transformers
Dissolved gas analysis on the oil, insulation resistance and polarisation index, turns ratio, winding resistance, power factor, and oil dielectric breakdown and moisture. Acceptance testing before energising is standard on new units.
How can I test a transformer with a multimeter
On a small de energised unit you can check winding continuity and take a rough turns ratio by measuring input and output voltage, which finds an open winding. It will not detect degraded insulation, moisture or partial discharge. Never open a medium voltage pad mounted transformer to test it.
What does dissolved gas analysis tell you
Which fault is developing inside a sealed tank. Acetylene indicates arcing, ethylene serious overheating, methane and ethane lower temperature faults, and hydrogen partial discharge. Trended over time it gives warning long before failure.
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