When working on an existing electrical system what should the first test to be carried out be?

When working on an existing electrical system, the first test to be carried out should be a visual inspection, followed by verifying the system's de-energization using a suitable voltage tester. This ensures that the system is safe to work on and identifies any obvious signs of damage, wear, or incorrect installations. After confirming de-energization, further tests such as insulation resistance and continuity testing can be conducted to assess the system's integrity.