Which of the following statements about dry motoring is true?

The correct statement about dry motoring is that it is limited to a maximum of 30 seconds. This limitation is crucial because dry motoring involves running the engine's starter without igniting fuel, primarily to check the mechanical condition and movement of the engine components. Continuous operation beyond this time can lead to overheating of the starter system and potential damage.

In contrast, the other options suggest either an unreasonably long operation time or incorrect requirements for rest or cool-down periods. While some procedures may have specific cool-down requirements, the standard practice in dry motoring emphasizes keeping attempts limited to avoid complications. Hence, understanding this time constraint is essential for ensuring the safe and effective use of the dry motoring technique.