A fault current causes the restricted portions of a dual-element fuse's short-circuit element to vaporize, and internal arcing commences. Which element vaporizes?

• A. Short-Circuit Element
• B. Load Element
• C. Arc Chamber
• D. Heat Element

Answer: (A)

When a fault current occurs in a dual-element fuse, the fast-acting short-circuit element is the part built to respond quickly to high current. Its restricted portions are designed to heat up intensely under a short-circuit condition and vaporize. This vaporization creates metal vapor and an arc, which is then contained in the arc chamber. The arc chamber provides a space for the arc to exist while the fuse link continues to heat and eventually opens, interrupting the circuit. The load element is meant to carry normal or mildly overloaded current and does not vaporize during a typical fault; the arc chamber isn’t a vaporizing element itself, and there isn’t a separate “heat element” component in this context. So the vaporizing element is the short-circuit element.