Existing metallic water service piping used for electrical grounding cannot be replaced with what until other means of grounding are provided?

The reason why plastic piping cannot replace existing metallic water service piping used for electrical grounding lies in its inability to conduct electricity. Metal pipes, such as those made of steel or copper, have conductive properties that allow them to function effectively as grounding systems. Grounding is crucial for protecting electrical systems and ensuring safety by providing a path for electrical faults to dissipate into the earth.

When a grounding mechanism is installed, it must effectively handle any fault current, which plastic piping cannot do as it is non-conductive. This absence of conductivity means that if the metallic pipe were to be replaced with plastic without establishing an alternative grounding system, it would create a hazardous situation where there is no reliable path for electrical current, increasing the risk of electrical shock, equipment damage, or fire.

In the case of replacing metallic plumbing with other metallic options such as steel or copper, these materials maintain the grounding capability that is critical for safety. Thus, it is clear why nonmetallic options like plastic are excluded from consideration in this context.