What is the requirement for grounding secondary circuits of transformers in relation to primary winding voltage?

The requirement for grounding secondary circuits of transformers is stipulated to ensure safety and proper function under the National Electrical Code (NEC). For transformers, particularly those serving larger voltages, the NEC establishes guidelines for grounding to reduce the risk of electrical shock and to help protect equipment.

In the context of this question, the requirement states that when the primary winding voltage of a transformer exceeds a certain level, specifically 300 volts, the secondary circuits must be grounded to maintain safety standards. Grounding provides a reference point for electrical systems, helping to prevent stray voltages and ensuring that excess current has a safe path to ground.

Grounding secondary circuits at or above 300 volts is critical because it enhances system stability and reduces the potential for electrical hazards. If the secondary circuits are not grounded, there could be an increased risk of electrical fault conditions that may lead to severe consequences, such as equipment damage or electric shock.

In summary, the grounding requirement for secondary circuits related to transformers is crucial for safety, especially when the primary voltage exceeds 300 volts, making the third option the most appropriate answer.