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Effects of High Resistance on Electrical Receptacles
By Joe Sesniak, IAAI-CFI, IAAI-CI, CFEI, GIFireE
Loose electrical connections at screw terminals can create an increase in resistance, which promotes development of oxide layer(s) on the affected metals and localized heating. While the oxides are conductive (meaning the circuit will still “work”) its resistance is higher than that of the original metals involved (NFPA 921, 2014)[1]. The nature of the heating results in a locally high “watt density” and creates a potentially competent ignition source for proximal fuels(DeHaan, J., Icove, D., 2012)[2]. Recent literature, including works by Benfer and Gottuk (2013)[3], Korinek and Lopez (2013)[4] and Shea (2006)[5], provide detailed explanation of the chemical and physical processes of oxidation (copper I and copper II oxides) and corrosion associated with high resistance or “glowing” electrical connections. It is the visible effects of such localized high resistance heating on the receptacle terminals, and the persistence of these effects in a post-flashover fire environment, that are the subject of this paper.
In this research, glowing connections were created on multiple electrical receptacles to produce heat effects on only one line side terminal connection of each receptacle. The purpose of this experiment was not to determine how heat effects manifest themselves on the terminals of electrical receptacles and associated conductors. The focus of this study was to determine whether or not the known effects persist beyond flashover at a visually perceptible level. This information is of importance to the fire investigator in the field. The reader should note that this work is considered preliminary. Potential variables were minimized, such as having conductors terminated on all screw connections and having multiple receptacles with varying loads on the same circuit. Further testing is required to evaluate the significance of such variables. Nonetheless the results of this testing are notable.The “heat damaged” test receptacles were installed in metal junction boxes and exposed to a room and contents fire that transitioned through flashover. The compartment was not instrumented. The point of origin and fuel load arrangement was selected to expose the receptacles to varying levels and duration of heat intensity. The post-flashover persistence of the effects of a glowing connection was subsequently visually evaluated. The intent was to provide fire investigators a resource for the preliminary field evaluation of electrical receptacles as a potential ignition source.
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