How is arcing across the fuse link prevented?

Arcing across the fuse link is effectively prevented by using an arc-extinguishing filler. In the context of fuses, when a fuse link melts due to an overload or short circuit, there can be a tendency for an electrical arc to form at the point where the link has melted. This arc can potentially cause further damage or hazards.

The arc-extinguishing filler is a special material that is designed to absorb the energy of the arc and quench it rapidly, thereby isolating the fault and ensuring a safe disconnection of the circuit. This filler is typically made of a mix of materials that can absorb heat and cool the arc quickly, reducing the risk of arcing and allowing the fuse to operate effectively and safely.

The other options do not address the specific mechanism of preventing arcing. Increasing the voltage could actually exacerbate the situation by making arcing more likely. A metal casing might provide some physical protection but does not directly extinguish arcs. Reducing the load could help overall, but it doesn't prevent the arc from forming once a fault has occurred. Thus, using an arc-extinguishing filler is the most effective and direct method for preventing arcing across the fuse link.