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Knob and tube wiring can be presumed to be the original electrical wiring in the home, is GENERALLY considered old and outdated by modern safety standards, and is inherently difficult to work with because of its design. It's the age of the system that generally creates most problems because many service professionals don't understand how a knob and tube system is designed to work, and they improperly splice or otherwise damage the system.
Knob and tube wiring gets its name from its design, using porcelain or ceramic tubes to protect electric wires that run through lumber framing and knobs to hold the wires when they run along or next to lumber framing. The connections for knob and tube wiring are open and visible, and the wires typically are spliced and connected with fibrous electrical tape. This type of wiring system was used up until shortly after World War II. IF it was installed properly, IF the insulation is in good condition, and IF it has not been abused by excessive splicing and connections, it can provide many more years of reliable service (however, home insurance companies don't like it). It is the wiring that has been abused and damaged that is the potential hazard. Any home with active knob and tube wiring should be examined in its entirety by a licensed electrician familiar and experienced with working with older electrical systems. |
In a modern three-wire system, there are three wires: a hot, a neutral, and a ground. The term "neutral" is a misnomer since there is nothing neutral about it. Electricity comes into the house on the hot wire and leaves on the neutral wire. A properly designed and properly operating electrical system must return electricity to where it came from (and we wonder why the electric company charges us for electricity if they get it all back!). If the electricity can't get back to where it came from, then it will try to go to the ground, hence the name for the ground wire. If there is no ground wire, then people and appliances provide a pathway to the ground if there is a short in an appliance or a problem with the electrical system. Electricity traveling to the ground through people is never a good idea. Since a knob and tube system has only two wires, meaning that there is no ground wire, it is difficult to splice into properly with modern wires that do include a ground. In modern electrical wires, there are actually three smaller wires inside of the large wire you see, whereas in a knob and tube system, the smaller wires are exposed. Consequently, it is easier to damage the knob and tube system, possibly resulting in electrical hot spots, arcing, and fires.
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