Is this 1940s solder and tape wiring safe or should I rewire everything?

Soldered and taped joints were the standard before wire nuts became the norm in the late 1950s. Back then, electricians would twist the wires together—usually using a **Western Union splice**—then dip them in a pot of molten solder before wrapping them in rubber tape and a layer of black friction tape. While these joints are mechanically very strong, the problem is the rubber and cloth insulation. After eighty years, it loses all its plasticizers and becomes incredibly brittle.

The reason your switch exploded is that the insulation likely crumbled the second you moved the wires to install the new sensor. When you pushed the switch back into the box, bare copper touched the metal side of the box and shorted out. Since you have old **armored BX sheath** wiring, that metal casing is supposed to act as your ground path back to the panel, but it's often a high-resistance path in these old installs. If the fault current was high enugh, it just arcs and blows up rather than tripping the breaker immediately. You are definitely at the point where a full rewire is the only safe move. Trying to 'clean up' those joints is a lost cause beacuse every time you disturb the wire, the insulation will flake off further back into the wall or the conduit. Since you already found a messy junction box in the attic, you have a clear starting point. Pulling new Romex is the right call here. It gives you a modern dedicated ground wire and insulation that won't disintegrate if you need to swap a device in the future. That open box in the attic is a major red flag, expecially if it's buried in loose-fill insulation. If those old solder joints start to fail or the friction tape falls off, any arcing will ignite that dust and insulation in a heartbeat. I’d keep that circuit off at the breaker til you can replace those runs. It’s a miserable job crawling around in an old attic, but it's better than risking a house fire over a hallway light.

Yikes, hearing that your switch actualy exploded is terrifying. I just bought a place from the early 50s and now I'm scared to even touch a dimmer. If I find those soldered things in my attic, is there a specific tool I need to cut that metal BX armor away safely? I've seen people mention a Roto-Split, but I'm clueless if that's overkill.

Since you're pulling new Romex, are you planning on fishing it through the same holes or do you have to cut into the drywall? I'm trying to figure out if a full rewire is even posible without basically gutting the whole hallway.

A Roto-Split isn't overkill, it’s a necessity if you don't want to lose your mind. Using hacksaws on old BX usally ends with a jagged mess. If you end up keeping any armored sections, don't forget the 'red devils'—those plastic anti-short bushings. They protect the wire from the sharp edges of the casing.

That crumbling cloth is a classic sign of early BX that's been run too hot for too long. Once it's toast, it’s toast. Since you're already in the attic, check the main trunk lines too. If the branch is that bad, the feeders might be ready to pop next.

Finding buried boxes in the attic is the absolute worst. They call those 'treasure chests' but they're relly just fire traps. If you're pulling new Romex, watch out for the fire blocking in the walls. Homes from that era ussually have solid wood chunks between the studs that make fishing wire a nightmare. You might end up having to cut some drywall patches anyway, so don't get too frustrated if a simple pull turns into a multi-day project. It's almost always more than just a quick swap once you start opening things up.

Since you mentioned the explosion and the breaker didn't trip fast enought, I'd take a hard look at your service panel while you’re at it. If you’re still on an old Federal Pacific or Zinsco setup, those are notorious for 'no-trip' failures during a dead short. Also, when you pull that new Romex, check the ceiling boxes. The heat from decades of oversized incandescent bulbs usually turns the last six inches of wire into charred toast. Even if the attic runs look okay-ish, the stuff directly above the fixtures is almost always the most dangerous part of the circuit.