What is knob and tube wiring?

Wires are run through porcelain cylinders or “tubes” through holes in wooden floor joists. They will also have porcelain “knobs” to keep the wires from touching the wood. Frequently these runs are insulated with a rubberized cloth fabric. This is called knob and tube.
The concern with this type of wiring is that it is not grounded. That is, it does not have the third wire that will allow you to use any appliance or product that has a three-pronged plug. This means that there is a chance for shocks or fire.
Another factor is that the wires are run separately. In modern wiring all three wires are enclosed in a single cable and insulated with plastic, versus rubber. The knob and tube wiring insulation is more prone to breaking down and creating a greater risk.
If you are not sure about the wiring in your home, you should have it inspected. The knob and tube wiring is far less safe and has a greater likelihood of short circuiting and overheating. These will lead to fires.
Insulation over wiring – When insulation is installed over knob and tube wiring, this is cause for serious concern. There is a greater chance of fire breaking out. The rubber coating on the wire does not have enough surface to dissipate the heat. With the excess heat will come a spark and
fire.
Overuse – As opposed to the mid 20 th century, today we have far more electric and electronics drawing power, including multiple computers, washers, several tvs, sound systems, slow cookers, dryers, etc. The older system was not designed for our current usage.
Plug Alterations – Sometimes people will remove the third prong (ground) from a plug in order to make it work in a two holed socket. This is extremely dangerous and should never be done.
Faulty Rewiring – If an attempt was made to rewire a knob and tube system, it is very likely that appropriate replacement parts were not available. Makeshift parts or poor splicing is dangerous business.
Damage – This wiring is prone to damage through age, inadequate fixes or other issues. The porcelain can crack and the wires will sag and fray.
Brittle Insulation – The rubberized cloth will become brittle and will flake off leaving exposed wires.

Replacing Knob and Tube Wiring