How can the oil level in a transformer be checked safely?

Check oil level only with the transformer de energized, locked out, and cooled to avoid hot oil spray and pressure effects. Use the installed level gauge or sight glass on the conservator or main tank, and confirm against the nameplate fill reference temperature if provided. Inspect for leaks around gaskets and valves. If topping up is needed, use clean, dry, approved oil and a filtered transfer setup to prevent moisture ingress.