Is it normal for an S2 steel bit to just shatter after an arc?

You likely saw significantly more than 100 amps during that fault. Most residential main breakers have an **AIC rating** (Amps Interrupting Capacity) of 10,000 or 22,000 amps, meaning the utility transformer can dump an massive amount of current into a dead short before the breaker even thinks about tripping. For that millisecond the circuit was closed, you basically created a high-speed induction heater out of that S2 steel bit.

The shattering happens because S2 is a high-carbon tool steel designed for impact resistance and hardness, but it becomes incredibly brittle when it hits the temperatures created by an arc flash. When that current dumped through the bit, the metal underwent a violent thermal expansion. Because the core of the bit couldn't expand as fast as the surface where the arc initiated, the internal stresses literally blew the steel apart. It's less like a mechanical snap and more like a miniature explosion caused by the rapid phase change of the metal. Your Milwaukee survived because the motor and electronics are isolated from the chuck to a degree, and the high-current path probaly stayed localized between the bit and the grounded panel or the main lug. If the current had traveled through the tool's internal wiring, you'd be holding a melted plastic carcass right now. Most of the energy was dissipated in that flash of light and sound, which is why your bit looks like it was hit by a hammer.

Since you were working behind the main breaker, you were likely on the line side where there is no overcurrent protection other than the transformer fuse on the pole. You're lucky the bit vaporized and broke the connection. If it had fused to the busbar instead of shattering, the arc would have continued until the panel melted or the utility fuse blew. Toss that bit, check your panel busing for pits or carbon tracking, and definitely pull the meter next time you're working that close to the mains.

That is terrifying to think about. I’m still pretty new to this, but I always assumed the main breaker was like a master safety net for the whole box. When you say the "line side" has no protection other than the pole fuse, does that mean those lugs are always live even when the main switch is flipped to off? How can you tell exactly where the line side ends and the safe parts begin? I'd love to know how to spot those danger zones before I accidentally blow up my own tools.

Spot on. People really underestimate how much energy is sitting on those line-side lugs. One thing people forget is the magnetic expulsion force—when that much current hits, the electromagnetic field actually physically repels the conductor (or in your case, the bit) away from the busbar. It likely helped 'kick' your tool out of the fault zone, wich probly saved your hand from being part of the circuit. definately check the surrounding plastic insulators for any carbon tracking, becuase that soot is conductive and can cause a secondary arc later on even if the bit is gone.

I’ve been there, though it was a flathead screwdriver for me. After I blinded myself for a second and realized I still had all my fingers, the first thing I did was grab a Scotch-Brite pad and some electrical contact cleaner. You relly need to scrub those busbars where the arc hit. If you leave those little pits and carbon traces, they create hot spots that’ll eventually melt your breakers. I also started wrapping my bit extensions in two layers of heat shrink tubing right up to the tip. It’s saved my bacon twice when things got tight in a live sub-panel.

While the previous mention of magnetic expulsion is technically accurate regarding the mechanical separation, one must also consider the microstructural degradation of the remaining tool steel; specifically, the localized annealing that occurs even in the sections of the bit that did not overtly fracture. Since S2 tool steel relies on a very specific tempering process to maintain its high torsional strength and impact resistance, the extreme thermal excursion from an arc fault inevitably compromises the metallurgical integrity of the shank, making it a liability for any future high-torque applications. I whould strongly advise inspecting the chuck of your Milwaukee as well, as the plasma discharge often deposits microscopic beads of vaporized copper and steel into the ball detent mechanism, wich can lead to premature mechanical binding.