Knob-and-tube (K&T) wiring was an early standardized method of electrical wiring in buildings, in common use in North America from about 1880 to the 1940s. The system is considered obsolete and can be a safety hazard, although some of the fear associated with it is undeserved.
InterNACHI inspectors should always disclaim knob-and-tube wiring during their inspections.
Facts About Knob-and-Tube Wiring:
It is not inherently dangerous. The dangers from this system arise from its age, improper modifications, and situations where building insulation envelops the wires.
It has no ground wire and thus cannot service any three-pronged appliances.
While it is considered obsolete, there is no code that requires its complete removal.
It is treated differently in different jurisdictions. In some areas, it must be removed at all accessible locations, while others don't, but inspect it for safety reasons.
It is not permitted in any new construction.
How Knob-and-Tube Wiring Works: