What device must be used to confirm that the power is off before starting work on a transformer?

A voltage sensing device is essential for confirming that power is off before starting work on a transformer because it is specifically designed to detect the presence of voltage in electrical systems. This tool typically provides immediate feedback, informing the user whether a circuit is energized or de-energized, which is crucial for ensuring safety before performing any maintenance or repairs.

While a multimeter can measure voltage, it usually involves making direct electrical contact and can pose a risk if not used correctly. A current clamp is useful for measuring current flowing in a conductor without direct contact but does not indicate whether a circuit is energized. A power analyzer provides comprehensive data about electrical consumption and may not directly indicate the presence or absence of voltage. Therefore, a voltage sensing device is the most efficient and direct method to ensure safety prior to working on a transformer.