Pittsburgh has a lot of knobs and tubes. And what is knob and tube? It’s a type of electrical wiring, the original wiring in houses built in the early part of the century. The name comes from the ceramic knobs that the wiring was secured to and the ceramic tubes that insulated the wires when they went through a joist or framing member.
Here are some of the knobs. In this case, the insulation is degraded. The energized (hot) and neutral wires are on separate knobs run usually in separate bays of the framing, so there is no way for arcing to occur between the wires–If not improperly modified.
Sometimes the knob and tube is only visible in the attic or knee wall, having been replaced in the basement. Insulation should not cover knob and tube wiring since it was not designed for it.
The tubes are visible in this photo in the lower left.
In the basement you may see something like this, where newer wiring is spliced at a junction box to the knob and tube that goes upstairs somewhere.
Here are knob and tube conductors in the basement that have been spliced without the protection of a junction box–Not good.
Can you see the wire going into the insulation? With knob and tube, the energized and the neutral wires run on separate bays of the framing, so they can never touch.
Frequently the old knob and tube wiring was removed everywhere except for the lighting, because it was more complicated to open up the floors or ceilings to rewire.