What type of equipment must not have conditions that require grounding for 50–1000 volts?

The focus is on control circuits that are specifically designed for use by qualified personnel. These are circuits that function at voltages between 50 and 1000 volts but are often considered in safe environments where qualified individuals have specialized training to handle them. Grounding requirements are typically meant to protect people and sensitive equipment from fault conditions, and in the case of properly designed and maintained control circuits, the need for grounding may not apply due to their controlled environment and specific usage.

When dealing with control circuits, the intention is to ensure that personnel are knowledgeable and capable of safely managing the risks associated with the circuit's operation. The design and function of these circuits are often such that they minimize the hazards that grounding would typically address.

In contrast, other choices involve scenarios where grounding is a critical safety requirement, either because of the risk of electric shock or to maintain system integrity in general power systems or other voltage conditions.