What must be done before working on an electrical circuit?

Before working on an electrical circuit, it is essential to disconnect the circuit to ensure safety. This involves turning off the power supply and isolating the circuit, preventing any accidental electrical shocks or injuries while working. Disconnecting the circuit ensures that there is no live current flowing, which is crucial for maintaining a safe working environment.

While testing the circuit with a voltmeter or installing additional switches are important tasks in electrical work, they should only be performed after ensuring that the circuit is disconnected. Ensuring the power is on is counterproductive when you're trying to safely work on a circuit, as it could lead to significant risks. Disconnecting the circuit is the fundamental first step that establishes a safe condition for any further actions.