Drop-Out Fuse Unit: A Critical Overcurrent Protection Device
The Drop-Out Fuse Unit (DOFU) is a specialized expulsion fuse. We widely apply it in overhead medium-voltage distribution networks, generally up to 33kV. It functions as a reliable overcurrent protection device as well as an isolating switch. Such dual functionality makes it a core component for ensuring safety and reliability in the protection of pole-mounted transformers and feeders.
Structurally, the DOFU comprises three main components. These include a reinforced porcelain or polymer insulator housing, a fusible element (link) enclosed in a fiberglass tube, and a pivotal spring-loaded contact mechanism. Under normal load conditions, the fuse link maintains tension. This tension keeps the upper contact arm tightly secured in the closed position within the insulator’s contact jaws, thereby completing the circuit through the unit.
Its key operating characteristic is the “drop-out” action, which is where the device gets its name. When a sustained overcurrent or short-circuit fault happens, the fusible element melts. This melting releases the tension and activates the pivotal mechanism. The upper contact arm then rotates freely downward, opening the circuit in both a physical and visible manner. This action achieves clear isolation of the faulty section—a critical safety feature for line crews. Meanwhile, the sand filler surrounding the fusible element or the deionizing gas generated by the fiberglass tube liner produces an expulsion effect. This effect efficiently extinguishes the resulting electric arc.
The DOFU boasts several notable advantages. It is cost-effective, features a simple design, and allows for easy maintenance. The drop-out action not only provides a safe isolation point but also offers a visual indication of the device’s operation. After the fault is cleared, we can readily replace the fusible link to resume service. What’s more, its design inherently enables effective arc interruption for fault currents within the specified range.
The drop-out fuse unit remains an indispensable component in electrical distribution systems. It delivers a robust, economical, and visually verifiable solution for overcurrent protection and isolation, thus safeguarding both system reliability and personnel safety.
