When jump-starting a heavy truck with a 24V electrical system using booster cables, which of the following is a critical safety precaution?

1. Ensure the booster power source is 12V to prevent alternator overspeed
2. Verify the booster vehicle/source matches the 24V configuration, and connect positive to positive and negative to a clean chassis ground
3. Connect the negative clamp to the positive terminal of the flat battery to complete a reverse-charge loop
4. Keep the ignition key in the 'START' position in both vehicles while connecting the clamps