Schedule the frequency of routine testing based on the criticality of the transformer. Despite the name, it isn't actually testing. You want to collect certain data and compare to the baseline data from acceptance testing and trend data points over time, watching for the trendline to change.
Collect data in engineering units. Rather than have a checkbox for "OK" next to temperature, record the actual temperature. This permits trending and prevents several problems.
Collect the following data.
All types of transformer units:
Liquid-filled type transformer units:
Ensure the test form has a free text area for noting observations (e.g., leaks, smells, audible hum).