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The proper installation of dielectric bushings is crucial for ensuring effective electrical insulation and minimizing the risk of short circuits or electrical faults. One necessary condition is that no electric current should pass through them during installation. This is vital because allowing electric current to flow through the bushings can create excessive heat, leading to potential damage to the bushing material and reducing its insulating properties. It also poses a safety risk to the installers and can compromise the integrity of the installation.
In contrast to the correct answer, other conditions listed can be important for various aspects of UST operations but are not as critical to the specific installation of dielectric bushings. For instance, while ensuring that no air is trapped can contribute to optimal installation, it is not as fundamental as preventing current flow. Likewise, having a dry excavation is important for general safety and structural integrity, and pre-heating may apply under specific circumstances, but these conditions do not directly interact with the needed electrical isolation essential for dielectric bushings to function correctly.