Pages

20050921

Sparking outlets and wires esp knob and tube



EXCESSIVE THERMAL INSULATION: There are simple ways in which a fire can be created with an electric cord that is neither damaged nor subjected to a current in excess of its rated capacity—loop it up upon itself several times, or provide a high amount of external insulation, or both. Laboratory demonstrations have verified that ignition readily occurs; in one case, simply coiling the cord three times and covering with a cloth sufficed. A special form of this hazard occurs with the old knob-and-tube wiring, which was common in the US prior to World War II. This type of wiring uses two separate conductors which are not grouped into a cable, but are individually strung on widely-spaced porcelain knobs. The current-carrying capacity is dependent on there being unobstructed air cooling of the wires, and fires have occurred when the wires were buried in thermal insulation. 

STRAY CURRENTS AND GROUND FAULTS:Stray currents occur when circumstances cause current to flow through paths not intended to carry current. Ground faults are a well-known example. They can occur if a conductor is abraded or damaged and contacts metal siding, roofing, etc. Kinoshita et al. documented that only 5 A was required for ignition when a 3-conductor, PVC-insulated cable contacted a galvanized iron roof.  An unusual mode of ignition from a ground fault is where current flows through a gas line. The current causes overheating of the metal and eventually a failure occurs In cold climates, it is not rare for individuals to thaw a frozen water pipe by attaching a welding transformer and passing current through it. Fires have resulted due the very large currents that are involved. Sanderson studied a case where thawing activity did not ignite the house that was being worked on, but caused ignition in six neighboring houses fed from the same power utility connection.