What role does the cylindrical body contact knife play during the fuse blowing process?

First, let us examine the overload fusing process. When a sustained overload current occurs in a circuit, the fuse element within the fuse body begins to heat up abnormally. Since the overload current typically ranges from 1.3 to 2 times the rated current, the heating process is gradual; consequently, the pre-arcing time is relatively long-lasting anywhere from a few seconds to several minutes-depending on the severity of the overload. During the pre-arcing phase, specific sections of the fuse element reach their melting point first and begin to melt, accompanied by the generation of an electric arc. This phase is termed the pre-arcing time, and it constitutes the first key distinction between overload fusing and short-circuit fusing. In fuses featuring a Cylindrical Body Contact Knife structure, the design of the contact interface between the fuse element and the metal end cap influences thermal conduction efficiency, thereby affecting the duration of the pre-arcing time.

Once the electric arc is generated, the process enters the arcing time phase. At this stage, the arc-quenching filler material (typically quartz sand) within the fuse body rapidly comes into play; by absorbing the arc energy and cooling the arc channel, it extinguishes the arc within an extremely short timeframe, thereby completing the interruption of the current. The interval spanning from the generation of the arc to its complete extinction is defined as the arcing time. The total fusing time is the sum of the pre-arcing time and the arcing time. Under overload conditions, due to the relatively limited amount of heat generated, the total fusing time is typically quite long-potentially ranging from 1 to 4 hours. This phenomenon serves as a manifestation of the fuse's inverse-time characteristic: the more severe the overload, the faster the fusing occurs. In applications utilizing NH-type fuse links with knife contacts, this time-delay characteristic is particularly crucial, as it prevents nuisance tripping caused by transient inrush currents, such as those generated during motor startup.

Next, we analyze the short-circuit fusing process-the most demanding operational scenario for a fuse. When a short circuit occurs, the current can surge to tens or even hundreds of times its rated value; consequently, all the fuse elements within the fuse body reach their melting point and melt almost simultaneously and instantaneously, accompanied by the generation of a powerful electric arc. Compared to overload scenarios, the pre-arcing time is extremely brief, typically measured in milliseconds. For aR-type fast-acting fuses designed for semiconductor protection, the entire process-from the onset of the fault to the complete interruption of the short-circuit current-can be controlled to within 10 milliseconds. During this process, the material and structure of the fuse contacts are subjected to immense electrodynamic forces and thermal stresses; their quality directly determines whether the fuse can reliably interrupt the circuit.

The arc-extinguishing phase is likewise dominated by the arc-quenching filler material; however, since the energy of a short-circuit arc is far greater than that of an overload arc, the performance requirements for the quenching material are significantly more stringent. The quartz sand must absorb a massive amount of arc energy within an extremely short timeframe and establish sufficient dielectric strength to prevent the arc from reigniting. Once the interruption process is complete, the fuse element inside the fuse body has completely melted, creating a reliable insulating separation between the metal end caps. In the design of fuse knife contacts, the contact material typically consists of silver-plated copper or silver-plated brass, a choice that balances high electrical conductivity with resistance to arc erosion.

The selection of contact materials is of paramount importance during the fusing process. As the interface connecting the fuse to the external circuit, the contact resistance of the fuse terminals directly influences the amount of heat generated during normal operation. High-performance contacts frequently utilize silver-plated copper knife contacts, leveraging the high conductivity of silver to minimize contact resistance while relying on the high mechanical strength of copper to ensure mechanical reliability. Conversely, in applications requiring high-temperature endurance, silver-plated brass knife contacts are often preferred due to the superior heat resistance of the brass substrate. Furthermore, the use of silver-plated copper alloy end bells at the extremities of the fuse effectively prevents the end caps from deforming due to arc erosion following multiple circuit interruptions.