Knob-and-tube wiring is no longer used in new construction. However, since you might be considering an older house, the building might have knob-and-tube wiring throughout or in portions of the structure. A knob-andtube wiring system uses porcelain insulating knobs, tubes, and flexible nonmetallic tubing for the protection and support of single-insulated conductors. (See FIG. 12-9.)
Obviously you cannot see the type of wiring behind the walls. However, very often there are exposed wires in the unfinished attic or basement. If there are exposed sections of knob-and-tube wiring, the outer insulation covering should be checked for broken and open sections. The insulation is often dry and brittle and chips easily. If there are exposed conductors, the exposed areas must be covered with electrical tape as a safety measure. Knob-and-tube wiring, although obsolete, is considered safe, providing that no modifications are made to that portion of the electrical system. If any changes or extensions are made, rewiring of that branch circuit might be necessary. All modifications to knob-and-tube wiring must be made by a licensed electrician. In addition, the outlet receptacles of a knob-and-tube-wired system are not grounded. Consequently, as a precautionary measure, they should be used only with appliances that do not require grounding.