According to the NEC, under what condition can neutral current flow on metal parts of the electrical system?

Neutral current typically flows in a grounded, closed-loop circuit; however, when there's a fault in the system, such as a short circuit, the neutral can become energized. This can happen due to improper wiring or failure of insulation leading to a dangerous situation where the metal parts of the electrical system, which should normally be grounded, carry current.

The correct answer emphasizes safety measures set by the National Electrical Code (NEC). It highlights that allowing neutral current to flow on metal parts is an unsafe condition that arises not from design intent but as a result of fault conditions. The primary goal of the NEC is to protect people from fire hazards and electrical shock, ensuring that any conditions leading to current flowing on metal parts are treated with a sense of urgency and remedial action.

Addressing scenarios like this—from overloads to open circuits—are covered under different provisions of electrical safety standards, but under no circumstances should there be normal conditions where neutral current flows on these metal parts. Thus, the NEC aims to mitigate such risks to prevent accidents and harm to individuals.