What must be done before working on or inside a transformer?

Before working on or inside a transformer, it is essential to turn off all power to ensure safety. Transformers can carry high voltages that pose serious risks of electrical shock or arc flash. De-energizing the transformer eliminates the risk of accidental contact with live electrical components, thus safeguarding the person performing the work.

While informing a supervisor, wearing protective gear, and checking voltage levels are all important safety practices in electrical work, they do not directly prevent the immediate danger associated with high voltage when working on a transformer. Ensuring that the power is turned off serves as the first and most critical step in any electrical maintenance or repair process, making it the most vital action to take before approaching the equipment.