Thanks for your comments.

In my experience, what you may be describing is an overheating "high-resistance" electrical connection. If one connection is poor (due to corrosion on the contacts, loss of spring tension, etc.) excess heat will be generated at that connection when the electrical current is flowing, particularly if it is a heavy load (like a heater or a toaster). That excess heat can cause pyrolization of the insulation leading to tracking and eventual ignition of nearby combustible materials. And particularly where there are multiple connections and only one really bad one, the metal in the contacts in the bad one may show evidence of long term overheating, different than the appearance of the other connections, even after the fire.

Thus, I believe the most common root cause of what you described is a high-resistance connection, not necessarily electrical arcing per se. However, if the over-heating of the contact reachs the melting temperature for the conductor metal, then arcing is almost certain to occur (although ignition of combustible materials often occurs far below the melting temperature for the conductor metal).

Mike Learmonth

Michael A. Learmonth, B.A.Sc., M.B.A., M.Eng., J.D., LL.B., C.F.E.I., P.Eng.
Senior Associate, Giffin Koerth Forensic Engineering and Science
40 University Avenue, Suite 800
Toronto, Ontario, Canada M5J 1T1
Phone: 416-368-1700 ext. 225
Fax: 416-368-5576