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Class J fuses are specifically designed to offer overcurrent protection in a way that is tailored to their mounting. The statement indicating that Class J fuses can be installed in Class R fuse mountings is incorrect because these two classes have different physical dimensions and characteristics. Class J fuses are designed to fit into their specific mountings, ensuring compatibility in both size and operational function.

Class J fuses must be installed in Class J fuse mountings to ensure the proper connection and performance. This prevents any risk of incorrect application, which could compromise safety or operational effectiveness.

In contrast, the other statements provide accurate information about the installation and characteristics of various fuse classes. For example, Class R fuses can be used in their designated mountings and have the versatility to adapt to Class H mountings as well, reflecting their design purpose and broader compatibility.

Additionally, the mention of Class R, J, CF, and L fuses being current-limiting devices signifies their shared functionality in protecting circuits against short-circuit conditions, showcasing their design as current-limiting devices intended to enhance safety within electrical systems.