In-depth Analysis of the Working Principle and Structural Composition of Drop-out Fuses

Drop-out fuses are the most widely used overcurrent protection devices in 10kV distribution lines and distribution transformers, known for their simple structure, reliable operation and low cost. Although not large in size, their internal working mechanism is intricately designed and interlinked.

I. Structural Composition

Drop-out fuses consist of the following main components:

1. Insulation Support (Insulator)

Bears the insulation between live parts and ground, while also serving as the structural support for the entire fuse. Usually made of porcelain or composite insulating materials, it must meet the insulation requirements of the corresponding voltage level.

2. Fixed Contact Assembly

Divided into upper and lower fixed contacts, installed at both ends of the insulation support. The upper fixed contact ("duck bill") contains phosphor bronze elastic pieces providing reliable contact pressure; the lower fixed contact provides support and current conduction.

3. Fuse Tube

The fuse tube is the core component of the drop-out fuse, consisting of two layers of structure:

• Outer Layer: Phenolic paper tube or epoxy fiberglass cloth tube, providing mechanical strength and protection
• Inner Layer (Arc Extinguishing Tube): Gas-generating material such as steel paper tube, which decomposes to produce large amounts of gas under arc action for arc extinction
• Inside Tube: Fuse element (copper-silver alloy) passes through the tube, with both ends fixed to the upper and lower moving contacts respectively
• Upper End Cap: Contains a thin melting piece that melts under high current to form dual-end exhaust

4. Moving Contacts

The upper and lower moving contacts are respectively located at both ends of the fuse tube, maintaining close contact with the fixed contacts through the tension of the fuse element.

II. Detailed Working Principle

Stage 1: Normal Operation

The fuse element passes through the fuse tube and is tightened at both ends, generating certain tension. The tension of the fuse element pulls the upper moving contact upward, keeping it in close contact with the elastic piece inside the upper fixed contact ("duck bill"). The lower moving contact is simultaneously pulled tight to contact the lower fixed contact. At this time, current flows normally through the path: upper fixed contact → fuse element → lower fixed contact.

Stage 2: Fault Melting (Small Current)

When overload or short circuit faults occur, the fault current passes through the fuse element, rapidly raising its temperature. The fuse element material (copper-silver alloy) has a high melting point and breaks after reaching the melting temperature. The moment the fuse element melts, an arc is generated, and the arc extinguishing tube decomposes under the high temperature of the arc to produce large amounts of gas. The gas forms high pressure inside the tube and is ejected from the upper end (single-end exhaust), strongly blowing the arc longitudinally, causing it to rapidly elongate and extinguish at current zero crossing.

Stage 3: Dual-end Exhaust for High Current

When the interrupting current is large, the gas pressure inside the fuse tube rises sharply, and the thin melting piece in the upper end cap also melts, forming simultaneous exhaust from both upper and lower ends (dual-end exhaust). Dual-end exhaust reduces the peak pressure inside the tube, prevents tube bursting, and accelerates arc extinction.

Stage 4: Fuse Tube Drop

After the fuse element melts, the upper and lower moving contacts lose the tension constraint of the fuse element. The upper moving contact slides off under the elastic force of the upper fixed contact spring piece, and the entire fuse tube flips and drops around the lower shaft under its own weight and spring force, forming a clearly visible open point. This mechanical drop action ensures reliable circuit disconnection.

III. Key Design Points

• Arc Extinguishing Tube Lining Material: Directly affects gas production and arc extinguishing effectiveness, the key to interrupting performance
• Fuse Element Tension: Both excessive and insufficient tension affect contact reliability, requiring precise control
• Elastic Design of Upper and Lower Fixed Contacts: Must maintain good contact pressure during long-term operation
• Fuse Tube Drop Mechanism: Must reliably operate after fuse melting, without jamming

Although drop-out fuses have a simple structure, the design of each component has undergone rigorous engineering verification, embodying the design philosophy of "simple and reliable" power equipment.