Explanation on Knob And Tube Wiring
Knob and tube wiring was popular from the 1920′s until the 1950′s. In fact, if you see a home that predates the 1920′s, and it has knob and tube wiring, then that was probably an upgrade. Seriously, an elderly friend lives in a home of that vintage and she remembers how exciting it was to go from gas lights to wiring — which was the knob and tube. Knob and tube is not necessarily unsafe, however due to the age of it, and the uncertainty as to the quality of some connections, it has earned that reputation. In fact, many insurance companies will no longer insure knob and tube wiring.
Those that do tend to charge much higher premiums. From what I see in the field, usually the biggest problem with knob and tube, as far as safety, has less to do with the original system and more to do with the way people have cut into it, modified it and butchered it over the years. For example, seldom do I find an attic or crawl space, where this style wiring is most likely to be visible, that does not have newer wires spliced in to the knob and tube with exposed wire nuts, no junction boxes, and often multiple locations with hacked wires and terrible splices. Sometimes the wire spliced in is the wrong gauge, so it will overheat, or maybe those butchering the system have used aluminum wire which corrodes when it is hooked to the dis-similar metal of the knob and tube wiring. Heat is the result. Often the tape around such splices is melted.
If you have never seen knob and tube, and want to know how to identify it: It consists of two strands of wire (the insulation is cloth-like, and not plastic, if original), run a foot or so apart from one another. The tubes are porcelain insulators that shield the wire as it goes through wood, usually a joist. Holes are drilled and the tubes slide in the joists and the wires go though the tubes. The knobs are porcelain insulators that screw in and have a hole the wires can wrap in and around. They are used at corners when turns are made and that is where you see the original junctions or splices.
Original junctions should have been twisted together, soldered and taped — no junction boxes back then. If a good soldering job was done, these old connections are often sound. But, if not, resistance can build at any points where the wires are not tightly connected and that can lead to overheating or fire danger. That brings up another point, to keep knob and tube cool, the experts suggest that it should not have attic or under floor insulation in place over the top of it. If there is insulation, the knob and tube needs to be free of it.