It is permissible to use gas piping as an electrical ground if:

Using gas piping as an electrical ground is permissible in certain specific situations, particularly when it is utilized in the flame safeguard circuit. This is because grounding in this context ensures safety by providing a reliable path for electrical faults, preventing the risk of electrical shock or fire in systems where ignition sources can be present. Flame safeguard systems are critical in ensuring that appliances operate safely, and proper grounding can help avoid dangerous electrical conditions that may lead to failures or accidents.

The other options do not provide the necessary context for effective grounding or safety. Constraints such as the size of the piping, the type of joints, or voltage levels do not inherently ensure a safe grounding practice in the context of gas installations. The critical factor for grounding is its application in safety systems such as flame safeguard circuits, which are designed to protect against risks associated with gas-fired appliances.