What tool should a Qualified Electrical Worker use to check that isolation is complete before installing protective grounds?

The tool that a Qualified Electrical Worker should use to check that isolation is complete before installing protective grounds is the potential tester. This device allows the user to verify that there is no voltage present on the equipment being worked on. It accomplishes this by measuring the voltage between the conductor and ground as well as between different conductors. Ensuring that the equipment is de-energized is a critical safety step before proceeding with grounding or any other work.

While other tools mentioned have important functions, a multimeter measures various electrical properties like voltage, current, and resistance, but it may not clearly indicate the absence of voltage in a safe manner when working with live circuits. An insulation resistance tester is primarily used to check the insulation integrity of electrical systems, which is important, but does not serve the specific purpose of confirming that isolation from live sources has occurred. A continuity tester checks for a complete path in a circuit but does not confirm the absence of voltage, thereby posing a risk if the equipment is still energized. Thus, the use of a potential tester is the most appropriate choice for ensuring that the system is isolated before additional safety measures are applied.