Knob and tube wiring is a pretty significant problem (but very common) in older homes.Knob and tube get its name from the knobs that support the wires and attach them to the studs and the tubes that the wire goes through as it goes through a hole in the studs. In the third picture down you can see a knob and a tube for a wire running in a basement.
The first problem with knob and tube is the connections. If you look at the top picture, you can see two parallel lines. This is the knob and tube wiring. You can also see wires that come from the middle and hook up with the parallel wires. These wires go to a light in a ceiling.The connection is made by stripping some of the insulation off of the parallel lines and wrapping the other wires around it. Then the joint is wrapped in tape to cover the exposed wire. This would be the lumps you can see where the wires meet. This method does not make a good connection because as the wires heat up and cool down with regular use, it creates slight spaces between the wires. As a space gets bigger it begins to arc and cause more heat which causes the wire to expand and contract more and creates more space. If the joint is bad enough, it could do this to the point of starting a fire. If the joint was made well in the first place it will only heat up a little more than it should. And that brings us to the next problem.
The second problem with knob and tube wiring is the excess heat causes the insulation to become brittle. The bad joints made in knob and tube wiring causes them to heat up. The insulation was never meant to get that hot and as a result it becomes brittle over time. This wouldn’t be a problem unless it is disturbed. If left alone, the insulation will not move and therefore not crack. If someone tries to move it for whatever reason, the insulation may crumble and fall off of the wire leaving exposed live wires.
The third problem with knob and tube wiring is that there is no ground wire. If you look at the bottom picture you can see a new piece of wire. It has a black wire, a white wire and a ground wire. The knob and tube wires are all black but they function like the black and white wires of the new wire. The ground wire is there as a safety. If the live wire ever accidentally touched a switch or receptacle box and there was a ground wire, it would be a short circuit and the fuse or circuit breaker would blow like it’s supposed to. If the live wire accidentally touched the box and there was no ground wire, the box would become live as well. If the box becomes live, if you were to touch the box or the screw that attaches the plug or switch to the box, you would get a shock. An extreme case would be if a Jacuzzi tub was connected without a ground wire, the entire tub could become live.
The last problem with knob and tube wiring is how hard it is to find it. People will often have their knob and tube rewired but contractors do not always get rid of all of it. The parallel lines seen in the first picture will be fed directly from the electrical panel. To get a new wire from the panel to the attic can be difficult and require making holes in the main floor and so some contractors just use the old wires. This of course does not provide a ground wire. In the second picture down you can see some knob and tube wiring attached to some slightly newer wire. The same happens with brand new wire as well. The knob and tube is attached to a new wire that goes to a switch or plug. So when someone looks at switches or plugs they will see new wires and assume the knob and tube is gone but in fact the problems associated with knob and tube are still there.
The only way to know for sure if the knob and tube is replaced is to look in the attic to make sure the new wires are not connected to any knob and tube wires. On the main floor you would need an electrician to check and make sure that there is a ground and that things are connected properly.When new wiring is run you will have the black, white and ground wire to every box. With knob and tube wiring, you will not always find the live wire with the neutral. And even if you do, there is not always any way to tell which is which. This can make trouble shooting very difficult and if the person doing it is not a professional, it may lead to serious mistakes.
If your home has been around for a long time with knob and tube and nothing in the walls or ceilings has been disturbed, you will likely be safe enough. With less and less insurance companies covering homes with knob and tube, ongoing renovations and the chance of things wearing out completely, knob and tube is a hazard worth getting rid of.