Download the PDF formA4, 2 pages
Print it at 100 % on A4. Page 1 is the form you fill in. The next page is the worksheet and reference.
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What is inside
- Transformer details: make, rating, vector group, HV and LV voltage, oil and winding temperature, ambient and weather
- Readings at 15 s, 60 s and 10 min for HV to LV and earth, LV to HV and earth, HV and LV to earth, HV to LV, tertiary and core to earth
- Columns for the temperature factor, IR60 at 20 °C, DAR and PI, plus an evaluation table against your acceptance value
- A temperature factor table from 10 to 80 °C, the formulas and a worked example
- Guide values clearly marked as rules of thumb, and method notes on isolation, guard use and discharge
How it was checked
Temperature factors are calculated from K20 = 2^((T − 20) / 10), the usual halving per 10 °C rule referred to 20 °C. For oil-filled units the real slope can differ, so use the manufacturer or IEEE C57.152 table where you have one. The 1 MΩ per kV rule is a guide, not a standard limit.
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