Latent Faults In Fuses: Causes Of Welding Defects And Non-destructive Testing Solutions

In the protection chain of the power system, drop out fuse often plays the role of the last line of defense. Recent failure analysis of a batch of returned products revealed that the root cause of many protection failures was not circuit overload, but rather a microscopic defect in the welding joints of the fuse wires inside the fuses. These defects are often difficult to detect during factory withstand voltage testing, but gradually worsen with temperature cycling after the equipment is put into operation.

Defect Mechanism and Failure Manifestations

Welding defects mainly manifest as "incomplete soldering" or insufficient fusion between the fuse wire and the terminal cap. According to test data, the contact resistance at this connection interface will abnormally increase. When the fuse carries normal operating current, localized overheating occurs at the defective area. The accumulated heat causes the solder joint to remelt or the fuse wire metal to undergo electromigration. In actual substation operation, such poor contact has caused severe voltage fluctuations in PT cabinets and even led to malfunctions of protection devices. Severely defective fuses will melt prematurely at rated current, causing unnecessary power outages.

Analysis Categories and Identification Characteristics

From a microscopic perspective, welding defects can be categorized into three typical forms:

• Lack of Fusion: The fused wire metal and the silver-plated end cap fail to form a metallurgical bond, resulting in obvious interface gaps.

• Porosity and Slag Inclusions: Gases fail to escape during welding, forming voids and reducing the effective conductive cross-sectional area.

• Oxidation Inclusions: Incomplete surface cleaning before welding leads to the formation of an oxide film at high temperatures, increasing contact resistance.

Full Inspection Plan and Process Control

Given the concealed nature of welding defects, relying solely on sampling or visual inspection is insufficient to meet quality control requirements. We introduced high-frequency X-ray non-destructive testing technology to internally inspect each batch of fuses. This technology clearly reveals the fusion state of the fuse and end cap, accurately locating areas of poor welding that are difficult to detect using traditional methods. Combined with image processing algorithms, the inspection system can automatically identify the boundaries of the effective welding area, enabling quantitative assessment of welding quality. For the welding of copper terminals, we simultaneously optimized the welding parameter curves to ensure sufficient heat input for adequate wetting of the fuse metal and substrate, eliminating the risk of lack of fusion at its source.