Which device must be used to ensure that power is off before working on a transformer?

To ensure that power is off before working on a transformer, a voltage sensing device is essential. This device is specifically designed to detect the presence of electrical voltage in a circuit, allowing a technician to confirm that the power is completely de-energized before beginning work. Using a voltage sensing device helps to enhance safety by ensuring that there are no live voltages present, thereby minimizing the risk of electric shock or other accidents.

In contrast, while a multimeter can measure voltage, it requires a bit more interaction and may not always provide a quick, visual indication of whether the circuit is live in the way that a dedicated voltage sensing device does. An insulation tester evaluates the integrity of insulation but does not confirm the presence of voltage. A current clamp measures the flow of current in a conductor, but this too is not designed for the purpose of indicating whether power is off at electrical equipment. Hence, for the primary goal of ensuring power is off, a voltage sensing device is the most suitable choice.