Transformer Winding Fault Diagnosis in a Workshop
A trainee in a vocational workshop connects a 230 V / 12 V step-down transformer to a 230 V supply. The secondary is left open-circuited. On switching on, the transformer hums loudly but no voltage appears across the secondary terminals. The trainee measures the primary winding resistance as 0.8 Ω and the secondary as 0.05 Ω. A second identical transformer measures 4.5 Ω and 0.3 Ω respectively. The supply voltage is confirmed at 230 V with a multimeter, and the fuse is intact. The workshop supervisor suspects a shorted primary turn rather than an open secondary.
1. Using the resistance readings and the observed symptoms, decide which fault is more likely and justify your reasoning with numbers.
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Compare the two transformers. The suspect unit shows a primary resistance of 0.8 Ω against 4.5 Ω for a healthy unit of the same design - a reduction to about 18% of the expected value. A shorted turn effectively removes turns from the primary, so both its inductance and its copper resistance fall. An open secondary would not change the primary resistance at all, so the low primary resistance points to a shorted primary turn.
The loud hum is consistent with this: with fewer effective turns the magnetising current rises sharply, saturating the core and producing mechanical vibration. An open secondary would leave the transformer quiet and drawing only normal no-load current. The intact fuse and confirmed 230 V supply rule out supply-side problems. Conclusion: the evidence supports a shorted primary turn, and the transformer should be isolated and rewound or replaced rather than tested further under power.
2. State two safety actions the trainee should take before any further measurement, and explain why each matters.
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First, switch off and isolate the transformer from the 230 V supply, then verify dead with the multimeter before touching terminals. A shorted primary turn can overheat rapidly and create a fire or shock hazard, and a transformer that is still energised cannot be safely handled.
Second, discharge and lock off the supply point, and remove the fuse or tag the breaker so no one can re-energise the circuit while the trainee works. Because the fault causes abnormally high primary current, an unnoticed re-energisation could damage the winding further or start a fire. Only after isolation should resistance or insulation testing proceed, and the faulty unit should be labelled as defective.