Fires Originating in Branch-Circuit NM Cables Due to Installation Damage

Vytenis (Vyto) Babrauskas, Ph.D.
Fire Science and Technology Inc., Issaquah WA

Presented at International Symposium on Fire Investigation, 2014

A significant fraction of US structure fires originate in electrical wiring, and there is also reason to believe that these numbers may be systematically undercounted. The role of voltage surges and damaged insulation in creating the potential for fire is discussed. That damaged electrical insulation may lead to fire has been known for a century, yet details of the mechanisms by which this occurs have not been extensively studied. UL recently published a study on damage due to poor workmanship in stapling of cables, specifically cases where the insulation is damaged but the conductors are not split. This study establishes that damage which may not appear visually striking may result in dielectric failure; such failures can be a direct cause of fire. Case histories are presented illustrating how mechanical damage associated with stapling of NM cables can result in serious fires. Two preventive measures are described: (a) installation by use of staplers and not hammers, and (b) testing the dielectric withstand voltage of branch circuits after installation and prior to energizing. Both of these measures, if implemented, should reduce the prevalence of electrical fires. However, given the immense amount of fixed wiring installed in buildings, fires due to damaged insulation do not imply that current wiring methods are a low-reliability technology. Only when serious installation defects are present does fire become a foreseeable event.

Keywords : dielectric strength of cables; electrical fires; forensic failure analysis; electrical installation defects; NM cables; voltage surges.

Download the complete paper here