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                                                                       Forensic Engineering:
                            Testing of Hostile Electrical Connections in a Residential Clothes Dryer

                                                      by Roger L. Owens, P.E. (NAFE 412F)

                                                                               Introduction

A fire occurred in a residence that was being utilized as a day care center. All parties involved in the initial investigations into the fire cause agreed that the fire originated in the laundry room in the vicinity of the electric clothes dryer. The plaintiff’s expert opined that the dryer was defectively designed and hence caused the fire. This writer was retained by the manufacturer of the dryer and asked to determine the most probable failure mode and to test an exemplar dryer in order to validate those observations. The ignition point appeared to be at a high resistance electrical connection at the dryer heater box involving both copper and steel. (See Figure 1)


Figure 1

Initial examination of the clothes dryer at the residence revealed that the dryer heater circuitry connection had been modified. One of the male prongs had been removed, the insulating material stripped back and the #10 AWG stranded copper wire was partially inserted and held in place by a common steel wood screw. (See Figure 2)

The conductor strands and the wood screw appeared heavily heat stressed and appeared to be a viable ignition source. A few inches away black electrical tape was deformed by heat but not fully combusted. (See Figure 3) This connection was duplicated on an exemplar dryer test stand and under operating conditions ignited lightweight fabrics.
Figure 2

                                                                             Discussion
Electricity in any amount flowing through any material produces heat. The earliest quantification of this effect was by Joule in 1845, the familiar expression Power = I2R (current squared multiplied by resistance). This formula is a satisfactory model for normal cylindrical conductors, but is only qualitatively true for electrical arcs, vacuum tubes and transistors, and conduction through electrolytics and ionized gases. In this case heating occurs at a connection (where a 10 gauge stranded copper conductor was connected to the Ni-Chrome heater inside of a clothes dryer).
Figure 3

Connections occur in a bewildering variety of materials and mechanical geometries, but of course, the objective is to have a connection that will have a low resistance (high conductance) and an adequate current-carrying capacity for a particular circuit. Connections may be soldered, crimped, screwed, push-in, punch-down, and so forth, and may include materials such as copper and copper alloys, brass, bronze, aluminum and aluminum alloys, steel, etc., and any of these materials may be plated with tin, silver, solder, brass, platinum, cadmium, and others. The failure modes of a connection are either an open circuit or an increased resistance at the connection.

The reason for a particular failure due to increased resistance will vary from one example to the next, but could be caused by chemical change due to oxidation of one or more materials at the connection, metallurgical changes, such as grain or hardness growth, or mechanical failure, such as cold flow or fatigue. Mechanical stress is also a factor. External environmental agents, such as heat, humidity, and caustic chemicals could also be exacerbating factors. I2R is appropriate for calculating the heat generated in a failing connection, but due to the uncertainties of the thermodynamics and heat transfer calculations, the resulting temperature of the failing connection can vary over a wide range.

The clothes dryer involved in this case was of the electric type, which means that an electrical motor-driven induction fan pulled heated air through a rotating clothes drum. A 240-volt, 5200-watt electrical heater heated the air. The connection of the copper appliance conductors to the heater was by means of a specially designed pin and socket arrangements. The pin was crimped to the copper appliance conductor and the socket was crimped to the Ni-Chrome wire of the electrical heater. The pin inserted into the socket with an interference fit. The two single-phase connections were housed in a porcelain holder. As designed, the connection is adequate for the designed power level.

There was no pin in the improper connection found in the dryer recovered from the fire scene. Instead, the wire had been pushed into the connection socket and a wood screw was used to hold it in place.

                                                                              Protocol
An exemplar clothes dryer was obtained from a reseller. The applicable parts were removed and bench-mounted for easy access. The connection made to the heater was duplicated in the manner that the post-fire investigation had documented. (See Figure 4)
Using any of several techniques artificially aged this connection. These techniques included passing current through the connection using a variance and step-down transformer and by heating the connection with a hand-held heat gun. Exposing the components of the connection to various corrosive chemicals also artificially aged the connection. The bench-mounted dryer components were then “cycled,” which means the

Figure 4
exemplar bench-mounted dryer assembly was run at operating voltage, allowing enough time for the unit to reach equilibrium or failure, whichever came first. During each cycle, the temperature of the modified connection was monitored.

The temperature was monitored using a hand-held IR thermometer and by a type K thermocouple. When the temperature recorded at the connection started escalating a cotton handkerchief was placed over the screw and the system was re-energized.

                                                                  Discussion of results
The exemplar connection exhibited an increase in internal resistance when it had been artificially aged using internally generated heat and externally applied heat. The connection temperature was monitored to reach 185 to 220°F within a few cycles with an eventual maximum temperature recorded of 1038°F. (See Figure 5)


Figure 5

As the power through the connection cycled on and off, the connection appeared to age and the temperature recorded at the connection increased. This phenomenon of aging has been observed in prior testing of connections performed and discussed in an article titled Forensic Engineering Analysis of Copper/Brass Contacts that was published in the NAFE Journal in December 2001.

Artificially aging the connection with a drop of household bleach dramatically increased the temperature at the connection. This procedure was particularly effective if the application occurred when the connection was hot. A cotton handkerchief was draped over the connection in an attempt to duplicate a fallen piece of fabric or clothing. The system was then energized as before and the handkerchief caught on fire. (See Figure 6)
Figure 6

                                                                        Conclusion
The test project demonstrated that fabric ignition temperatures could be achieved by the utilization of a common steel wood screw as a connector for the dryer heater circuit. The aging process at the connection that caused the high resistance was primarily a function of electrical and thermal cycling with the introduction of a caustic mist that would be found in a typical laundry room. The project also demonstrated that the most probable cause of the fire was a modification by the owner at the electrical connection to the dryer heater.

The author wishes to acknowledge that Joe Stainton, BSEE, P.E. was of tremendous assistance in setting up and monitoring the experimental project.

 

 

 
 
Forensic Engineering Analysis and Daubert Challenges to Prosthetic Fastener Failure

Testing of Hostile Electrical Connections in a Residential Clothes Dryer

Forensic Engineering Analysis of Copper/Brass Contacts
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