Which of the following is NOT classified as a medium-voltage fuse?

Proportion fuses are not classified as medium-voltage fuses because they are typically designed for use in low-voltage applications rather than in medium-voltage systems. Medium-voltage fuses, on the other hand, are specifically engineered to operate within a voltage range generally between 1 kV and 35 kV and are designed to handle the specific current and voltage ratings associated with that classification.

Current-limiting fuses, vented fuses, and dual-element fuses are all types of fuses that can be utilized in medium-voltage applications. Current-limiting fuses are particularly valuable as they limit the amount of fault current that can flow through the system, thereby protecting equipment from damage during fault conditions. Vented fuses are designed to release gases during operation to prevent pressure buildup, and dual-element fuses feature two elements that provide additional overload protection. These characteristics make the other options suitable for medium-voltage protection.