Under which condition are secondary circuits for underwater lighting in swimming pools or spas not permitted to be grounded?

The correct answer is that secondary circuits for underwater lighting in swimming pools or spas are not permitted to be grounded when specified as a and either b or c.

This means that if the voltage exceeds 1,000 volts (as specified in option a) or if a delta-connected system (as in option b) is being used, or if the equipment is mobile (as indicated in option c), grounding is not permitted. This provision is primarily focused on safety, as grounding in these particular cases could create a risk of electric shock or malfunction due to specific conditions such as high voltage or the nature of the connections used in delta systems.

By consolidating these specific conditions together, the regulation effectively highlights the importance of assessing multiple factors that could influence safety in electrical installations related to underwater lighting, ensuring that appropriate precautions are taken based on the specific characteristics of the setup. This holistic approach reinforces the commitment to maintaining safety standards within aquatic environments.