What must be used to line an excavation deeper than 12 inches when installing a mechanical appliance?

In the context of installing a mechanical appliance, ensuring the proper support and safety measures in an excavation deeper than 12 inches is crucial. A copper pan is used in some specific applications because it can act as a barrier, protecting both the appliance and the surrounding environment from potential contaminants or water intrusion. Copper is resistant to corrosion, which enhances its durability and effectiveness in such scenarios.

Moreover, copper pans can be integral in managing the area around the appliance, especially if a watertight seal is necessary. This characteristic makes copper a suitable material for certain mechanical installations where moisture or other environmental factors may present a risk.

Other materials like concrete, wood planks, and steel plates might serve various purposes in construction and excavation but may not provide the same level of protection or suitability for lining an excavation specifically in the context of propane and natural gas appliance installations. The specific requirement for copper indicates its importance in maintaining safety and integrity during the installation process.