14-9.4.4.* Arc Tracking. Arcs may occur on surfaces of nonconductive materials if they become contaminated with salts, conductive dusts, or liquids. It is thought that small leakage currents through such contamination causes degradation of the base material leading to the arc discharge, charring or igniting combustible materials around the arc. Arc tracking is a known phenomenon at high voltages. It has also been reported in experimental studies in 120/240-V ac systems.
Electrical current will flow through water or moisture only when that water or moisture contains contaminants such as dirt, dusts, salts, or mineral deposits. This stray current may promote electrochemical changes that can lead to electrical arcing. Most of the time the stray currents through a contaminated wet path cause enough warming that the path will dry. Then little or no current flows and the heating stops. If the moisture is continuously replenished so that the currents are sustained, deposits of metals or corrosion products can form along the electrical pathway. That effect is more pronounced in direct current situations. A more energetic arc through the deposits might cause a fire under the right conditions. More study is needed to more clearly define the conditions needed for causing a fire.
