Where on an underground tank is it not allowed to weld a wire from an anode bag?

Welding a wire from an anode bag is prohibited on the shell of an underground tank because the tank's shell is typically designed to withstand environmental stresses and potential corrosive elements. Any welding or alteration to the shell can compromise the integrity of the tank, potentially leading to leaks or structural failures.

Moreover, it's important to ensure that protective measures, such as cathodic protection provided by anodes, are applied in a way that does not interfere with the tank's structural integrity. Anodes are usually connected to other less critical areas where it is safe to do so, minimizing the risk associated with welding on the shell.

In contrast, other locations like the bottom or fittings may be considered more acceptable for such connections because they have different roles and structural factors in the overall design of the tank. For instance, the bottom of the tank may have different protective measures or materials that allow for modifications, and fittings can often accommodate connections without the same risks associated with the tank's shell. The cap also provides a more suitable and manageable area for connections without affecting the tank's overall performance and safety.