What type of fuse has a grooved ring in one ferrule?

The type of fuse that features a grooved ring in one ferrule is indeed recognized as Type R. This distinctive design allows Type R fuses to be utilized with special fuse holders that accept this specific configuration, preventing the incorrect insertion of other fuse types that may not have the same ring structure. The grooved ferrule serves as a safeguard, enhancing both the safety and reliability of the electrical system by ensuring that only compatible fuses are used, which can significantly reduce the risk of circuit failure or damage. The unique design element of the grooved ring is crucial for its intended application and adherence to specific electrical codes and practices.