The likely cause is an overload. A fuse opens when the current exceeds its rating, and when the element overheats over time, it can break while the glass window remains clear. If a short circuit had occurred, you’d typically see more rapid, dramatic damage or scorching from the high fault current. A change in electrical value isn’t by itself a cause for a fuse to blow. So, the pattern described—clear window and a broken element—fits an overload condition where the circuit drew more current than the fuse could handle for a period.

Ground fault protection in a swimming pool circuit is designed to detect hazardous leakage and trip the circuit to prevent shock. It works by watching the current leaving on the hot conductor and the current returning on the neutral; under normal conditions these currents match. If some current leaks to ground — for example through a faulty insulation, a damaged cord, or a wet surface — the imbalance is detected and the protection device trips within milliseconds. This rapid shutdown is crucial in a pool environment because water makes it easy for a person to become a path to ground, greatly reducing the risk of electric shock. The other options don’t relate to safety from electrical shock around water; they involve aesthetics, efficiency, or access control, not protection from shock.

Ampacity is the maximum current a conductor can carry safely based on the insulation’s temperature rating and the wiring table used by the code. For copper conductors, you read the ampacity from a table that lists values by insulation type and size. Type TW is an older thermoplastic insulation with a 60°C temperature rating, so you must use the 60°C column. For a 600 MCM copper conductor, the 60°C column shows an allowable ampacity of 350 A. That’s why 350 Amps is the correct value here. The other numbers would come from higher temperature-rated insulation (75°C or 90°C), which TW does not provide. In practice, you’d also consider de-rating for multiple conductors in a raceway or ambient temperature different from the assumed baseline, but the base allowable for this type and size is 350 A.

A single-gang box is sized to mount one device on a wall, such as a switch or a receptacle. Its design centers on holding a single device and the conductors that feed it, with space limited to that one device’s mounting and terminations. When you need to install more than one device at the same spot, you’d use a multi-gang box (like a two-gang) or a deeper box to accommodate the extra devices. Meter boxes are a different, much larger type of enclosure, not a single-gang box. Splices need to be contained in a box with enough volume for the conductors and hardware, but the standard purpose of a single-gang box is to serve a single device, which is why that statement is the best description.