POOR ELECTRICAL CONNECTIONS: PHYSICAL FEATURES, MATERIAL CHARACTERIZATION, AND NEWLY IDENTIFIED CHARACTERISTIC TRAITS, BEFORE AND AFTER FIRE EXPOSRE

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1 POOR ELECTRICAL CONNECTIONS: PHYSICAL FEATURES, MATERIAL CHARACTERIZATION, AND NEWLY IDENTIFIED CHARACTERISTIC TRAITS, BEFORE AND AFTER FIRE EXPOSRE Chris W. Korinek, P.E., Timothy C. Korinek, P.E., Synergy Technologies, LLC, USA International Symposium on Fire Investigation Science and Technology (ISFI), Chicago, IL, 2018 ABSTRACT Overheating due to a poor connection (OPC) and the resultant characteristic traits depend upon the materials and configuration of the two conductors in contact, the waveform of the electric current, and the amplitude of the current. This study was performed on OPCs with copper to copper and brass to brass conductor connections and the resultant materials and their surface and internal characteristics were analyzed. These characteristics were compared to other published characteristic traits documented in NFPA 921, prior to and after a simulated flash-over fire exposure. 1 Correct diagnosis of an electrical system to determine that an OPC had occurred in the field is needed to determine if an OPC was the ignition source. This study was our second effort after our initial study of OPCs with iron and copper conductors. 2 For our setups, one connection in open air or with limited insulation covering the conductors was used. INTRODUCTION AND SETUP An OPC can occur when two conductors in series come into contact with a light contact force and a small contact area. Portions of an OPC can reach temperatures high enough to glow (600 to 1,400C) and the temperature of the glowing molten oxide (the filament) is a function of current. 3 A connection can loosen over a short or an extended period of time, can operate intermittently, and will operate without a protection device stopping the OPC. The initial tightness of the connection, thermal expansion and contractions, corrosion, stresses, manual alterations, and vibration can affect how the connection may loosen over time. Figures 1A, 1B, and 1C show three common configurations that have been observed to cause OPCs when they are in loose contact; a brass male connector member inserted into its female counterpart and two solid or stranded abutted copper conductors that had previously been separated. While some testing was done with a male and female connection, the vast majority of tests were performed with two identical abutted conductors, to best visualize the OPC at all stages, study, and document the overheating in a configuration as symmetrical as possible. Figure 1 Figure 1B Figure 1C The materials used in this testing were all commercially available and included blades from fieldinstallation plugs made from brass, drawn, and punched to shape, and solid and stranded (19) copper wire: #12 AWG, Type THHN, Nylon-coated, 600V, 105C rated. The two conductors were brought into light contact as part of the neutral conductors in series with a resistive load. All three connection configurations Page 1 of 12

2 were tested using both 120V AC and 12V DC power sources. Often, multiple make and break contacts were needed to begin to create an oxide that would dissipate significant wattage and overheating. A small vibrating motor arrangement was often utilized to achieve the makes and breaks of the connection. ELECTRICAL OVERHEATING TEST RESULTS Figures 2A, 2B, and 2C show typical photos of an OPC occurring with these connections in series with the loads at 120V AC. For the OPCs in a 120V AC circuit, the glowing portion of the oxide was most often in the form of a narrow band or filament that changed locations all around the bridge circumference. Both ends of the filament moved about at or near the point at which the base metal had changed to a solid, full-thickness oxide. This filament has been called a worm in other literature. The entire mass of oxide is termed the bridge. Figure 2A shows brass plug blades carrying 2.2A, Figure 2B shows stranded wire carrying 2A (note entire bridge glowing), and figure 2C shows solid wire carrying 1.8A. Figure 2A Figure 2B Figure 2C Figures 3A and 3B show an OPC occurring and carrying 7.2ADC (and after opening and cooling) with the connections in series with a load using a 12V DC power source. The left side of the bridge is connected to the positive battery terminal such that electrons flow from right to left ( - to +). The initial break in the wires was at the arrow and the bridge only grew in the + direction. The right side of the filament stayed in its position at the top of the bridge. Virtually no thermal damage was observed on the conductor on the right side of the OPC. Table 1 shows the test data from the 92 samples tested for this paper showing the six combinations of configuration, materials, and voltage and waveform. Not all data was taken for all samples. + - Figure 3A Figure 3B Table 1 Conductor configuration Materials Voltage, waveform Current (Amps) Voltage Drop Max Watts Time (min.) No. of runs Plug blades Brass 120V AC Plug blades Brass 12V DC Str. wires Copper 120V AC N/A Str. wires Copper 12V DC N/A Solid wires Copper 120V AC Solid wires Copper 12V DC During the OPC, small flashes of light were often observed coming from the ends of the filament, as is seen in Figure 4 (solid wires, 120V AC, 1.7 A). These flashes of light are incandescent molten globules of oxide that can be deposited onto adjacent materials in the form of spatter. A closer look at spatter from an OPC (Fig 21) shows that it has the surface texture of the oxide and a skid mark that points to its source of ejection. Page 2 of 12

3 Figure 4 There are two means by which the OPC can sever, a melt-open or fracturing of the oxide. Most often in our tests the oxide fractured after solidifying. Figure 5 shows a brittle fracture in the oxide that occurred after the current stopped flowing and the material cooled and became thermally stressed. Figures 6A and 6B show the conductors that had melted open while the AC current was still flowing. Fig. 6A shows glowing rounded ends and 6B shows the cooled and solidified rounded ends. The OPC damaged wire ends (mates) were both located on the grounded conductors which is different from (parallel) arcing which results in damaged mates to both the grounded and ungrounded conductors. If an OPC is formed in the ungrounded conductor, it is understood that the overheating and characteristic traits would be identical to that observed for an OPC in the grounded conductor. Figure 5 Figure 6A Figure 6B POST-FIRE EXPOSURE AND MATERIAL ANALYSIS The conductors were then exposed to fire conditions inside a Quadr-fire Classic Bay 1200, 37.6kBtu/hr., pellet stove flame using wood pellets. The temperature profile included an approximately 5 minute peak of 900C followed by a continual fluctuation between C for 90 minutes. The figures below are macro photographs that display some notable features observed amongst the variety of configurations of OPC s tested. They are pictured in a vertical orientation; however, the tests were carried out with the conductors in a horizontal orientation. A dotted line border around the image indicates a postfire image. A solid border indicates a pre-fire image. Figure 7 is an as-produced AC sample with solid conductors. The oxide bridge is more than 1 inch long, and fractured on each end close to the wire base material due to cooling and subsequent handling after the test. However, a small length of oxide remained adherent to each wire end. Figure 8 is an as-produced alternating current sample with stranded conductors. The current through the OPC was maintained at 2A for a period of 20 minutes, then a change in loads caused a current of 13A. After operating for a period of 20 minutes at 13A, the OPC became unstable and melted open. Note, that one end of the open connection has the appearance of copper melting caused by arcing, and the other end has the appearance of copper melting due to fire or alloying. Figures 9 and 10 are the same sample before and after being exposed to a fire environment. This sample was produced using alternating current and stranded conductors. The oxide bridge cracked before fire exposure, and the formerly molten filament can be seen in the center of the bridge. Overall, the fire exposure did not change the surface features or shape. Figures 11 and 12 are two different samples produced with direct current and solid wires. In Figure 11, the sample is in the as-produced condition. In Figure 12, the sample has been through fire exposure. Other than a layer of oxide scale forming on the positive side of the Page 3 of 12

4 fire exposed sample, the appearance of the oxide bridge remains the same. Figures 13 and 14 show two different samples produced with alternating current and brass plug blades. In Figure 13, the sample is in the as-produced condition. In Figure 14, the sample has been exposed to a fire. Overall, the surface features of the oxide bridge remain after the fire exposure. Fig 7 Fig 8 Fig 9 Fig 10 Fig 11 Fig 12 Fig 13 Fig 14 SEM ANALYSIS OF EXTERNAL SURFACE FEATURES Figures 15 and 16 show SEM images of the sample previously shown in figure 7. The features of the former liquid filament, sometimes referred to as a glowing worm, are visible on the surface, as indicated by the black arrow. Figure 16 shows a high magnification image of the filament. When the filament solidifies, dendrites grow from the edges and meet at the centerline, as indicated by the black arrow. Figure 17 also shows a high magnification SEM image of the filament surface on a sample produced with direct current, and the dendritic structure is also present. Figures 18 and 19 are SEM images of a filament before and after fire exposure, respectively. There is a region of equiaxed grains at the centerline of the filament, and a dendritic structure on each edge. The surface structure of the filament was not significantly altered by the fire. The reason for the equiaxed grains in the center may be due to the cooling rate of the filament. Figure 15 Figure 16 Figure 17 Page 4 of 12

5 Figure 18 Figure 19 A prominent feature of the brass to brass OPC s were the presence of molten oxide spatter on the blades next to the oxide bridge. This spatter accumulates gradually over time. Figure 20 shows a blade from an asproduced brass to brass OPC. The oxide bridge fractured off of the blade after the test was stopped. Some of the spatter is spherical in shape, while other particles leave behind a tail or skid mark opposite to the direction of the particle origin. A spatter particle of this type is shown in figure 21. Eleven brass to brass plug blades were examined after fire exposure to determine if the characteristic spatter with skid marks remained after the fire. The presence of spatter was readily identifiable in nine out of the eleven blades. Figure 22 shows spatter with skid marks after fire exposure. The effects of handling and cleaning of the fire-exposed blades was tested by brushing two blades with a toothbrush for 30 seconds. Also, two blades were ultrasonically cleaned for 2 minutes in ethanol. The brushing removed essentially all of the spatter on one blade and a significant amount on the second blade. Spatter could readily be detected on both of the ultrasonically cleaned blades; however, the amount of spatter was less on both samples after the cleaning. Figure 23 shows spatter with skid marks, found after ultrasonic cleaning. Figure 20 Figure 21 Figure 22 Figure 23 Page 5 of 12

6 MATERIALS CHARACTERIZATION AND METALLOGRAPHY The chemical compounds produced in the copper to copper and brass to brass OPC s were determined by performing x-ray diffraction (XRD). Several samples from each type of OPC were combined and ground into a powder for analysis. The instrument used was a Bruker model D8 Discovery X-Ray Diffractometer. The results are given in Figures 24 and 25 below. The copper to copper OPC s contained primarily Cu 2O (also referred to as: Cuprous oxide, Copper(I)oxide, or Cuprite). The Copper to copper OPC s also contained a small amount of CuO (also known as: Cupric oxide, Copper(II)oxide, Tenorite). The Brass to brass OPC s contained Cu 2O, ZnO (also referred to as: Zinc oxide, Zincite), and a small amount of CuO. Figure 24 Figure 25 The stereomicroscope images in the Figures 26, 27, and 28 below show characteristic patterns that were observable on some of the samples produced with alternating current. Figure 26 is a macro photo of the sample as-produced. Figure 27 is a micrograph of the same sample after being mounted, ground down to the midsection, and polished to produce a flat longitudinal cross-section. This microscope image was taken using reflected light illumination with a ring light. The regions indicated by the white circles are copper colored and the bulk of the oxide mass appears dark gray. Figure 28 was taken with crossed polarizing filters in the microscope optics. The regions indicated by the white arrows appear deep red under this illumination. The color of Cu 2O is cited in the literature as ranging from gray to red. 4 Cu 2O is semitransparent, and the red color is produced by internal reflection within the material, and crossed polarizing filters accentuates this feature. 5 Figure 26 Figure 27 Page 6 of 12

7 Figure 28 The microscope image in Figure 29 below shows the interface of the wire and oxide bridge at higher magnification. This is the region inside the white circle in Figure 27. Figure 30 is at 200X magnification, and the microstructure consists of segmented regions of different phases of material. Both of these images were taken with crossed polarizing filters in the microscope optical path. The color of the banding ranges from a copper color close to the wire, to a deep red color caused by the internal reflection of the Cu 2O. The worm-like shape of the microstructure suggests these areas are representative of the actual conducting filament at a given point in time. Figure 29 Figure 30 Figure 31 below is an optical micrograph taken at a magnification of 500X with no polarizing filters. This region is located within the arrows in Figure 30 and shows that the segments are composed of small globs of metallic copper dispersed in an overall matrix of Cu 2O. The globs are copper colored, the matrix is gray colored, and the globs were further confirmed to be copper by performing energy dispersive spectroscopy (EDS) in the SEM. Figure 32 below is an optical micrograph of the same sample after the cross section was exposed to fire and subsequently re-mounted and re-polished. This image was taken with crossed polarizing filters. The magnification is 500X, and the overall microstructure remains the same. Conversion of some of the oxide back into metallic copper was observed on many of the samples, which is possibly due to a reducing atmosphere in the test stove, and/or a reaction of oxide and carbon in favor of carbon monoxide. Figure 31 Figure 32 Page 7 of 12

8 Figure 33 is a SEM image of a longitudinal cross-section of a brass to brass OPC before exposure to the fire atmosphere. In Figure 33, the oxide bridge is the gray mass in the center and the brass base metal is light colored at the top and bottom. Figure 34 is a 50X optical micrograph of the bridge for the same OPC sample after being exposed to the fire atmosphere, and subsequently re-mounted and re-polished. The darker colored areas are primarily ZnO and the lighter colored regions are primarily Cu 2O. The microstructure has a high degree of banding due to the motion of the molten filament throughout the connection. Figure 33 Figure 34 Figure 35 below is a higher magnification (350X) SEM image of the polished cross section shown in Figure 33. The banding between the Cu2O and ZnO microstructure can easily be seen. Figure 36 is an optical micrograph of the post-fire cross-section shown in Figure 34. The magnification is 200X and little change in the microstructure was found. Figure 37 is a transverse cross-section of a brass to brass OPC oxide bridge. The filament region is shown in the center of the image. Figure 35 Figure 36 Figure 37 Page 8 of 12

9 The three SEM images below show the as-polished cross-section of the sample shown earlier in Figure 8. Figures 38 and 39 shows the region of last liquid to solidify before the connection melted open. The transition from a thin conducting filament to a much larger molten mass was due to the higher current applied. Figure 40 shows a high magnification image of the microstructure in the last liquid to solidify. The white branching structures are primary Cu 2O dendrites, and the finely dispersed white phases are a eutectic composed of Cu 2O and CuO. Figure 38 Figure 39 Figure 40 Page 9 of 12

10 The information in Table 2 is in addition to that in contained in the current edition of 2017, section and will be submitted to the 921 Committee for insertion into future 921 publications. Table 2. Characteristic Traits of OPCs Proposed for Addition to NFPA 921 (continued - next page) Characteristic Additional Example Image(s) from Poor Connection Damage Trait Details Fracture face of oxide, base metal, or combination thereof Has been observed after an intact connection fractures after the overheating (Photos) Spatter of oxide (L=Photo R=SEM image of X-sect) Remnant of filament (surface or interior) (New) (L=Photo R=SEM image of X-sect) ceases. (New) Molten oxide ejects and solidifies on adjacent material and may leave skid mark. (New) May be visible on the exterior of the bridge or in the microstructure in a Metallographic Cross-section. (New) *Banding (surface or interior) (New) (L=photo R=SEM image of X-sect.) *Damaged mates in series (Photos) Segments of solidified oxide formed when an area melts, glows, and then solidifies. Appears similar to welding banding. (New) During an OPC the damaged areas are both in series with the load. (New) Page 10 of 12

11 * Sharp demarcation between damaged and undamaged areas (Photos) Round, smooth shape of artifact (Photos) *Copper drawing lines visible outside the damaged area (Photo) High internal porosity when viewed in a cross-section (oxide) (Photo) *Rough, gray area or shiny silvery area (Photo) Same trait as for (parallel) arcing, per Can occur if OPC melts open. Same trait as for arcing, per and for alloying per Same trait as for (parallel) arcing, per Same trait as for (parallel) arcing, per Same trait as for alloying, per *Brittle (Photo) Same trait as for alloying, per Notes: * denotes trait was present on 100% of OPCs for this and previous research. A dotted line border around the image indicates a post-fire image. A solid border indicates a pre-fire image. L=left, R=right Page 11 of 12

12 DISCUSSION AND RECOMMENDATIONS: 1. Use caution when cleaning potential OPC artifacts. Ultrasonic cleaning for 2 min. has been successful. Do not use a fiberglass brush. If brushing is used, use a method that abrades the surface minimally states, There is no clear way of visually distinguishing alloying from the effects of an overheating connection. This is consistent with our results regarding some traits. Therefore, a lab examination may be required to analyze the damage. 3. If an artifact has less fire exposure than in this study, the artifact may more resemble the pre-fire artifact. 4. Prior to the fire exposure, the difference between the conductor color was generally distinguishable from the gray oxide. After 90 min. of fire exposure, the color difference was less apparent. 5. Evidence of an oxide after OPC is fundamentally different from (parallel) arcing. An oxide bridge is not an arc mark nor copper melting due to (parallel) arcing. An oxide bridge is caused by oxide growth and glowing liquid oxide, and is not caused by a plasma. 6. Subsequent arcing and arcing through char can cause changes to the damage patterns shown. 7. The presence and location of evidence of an OPC should be included in an arc map. CONCLUSIONS 1. The overheating process and the damage to the conductors for OPCs between copper and between brass conductors were studied and documented. 2. Stranded and solid conductors were used and AC and DC current waveforms were utilized. 3. External and internal structures were analyzed to identify and document the characteristic traits found. 4. If present, many of these characteristic traits can be recognizable in the field either with the naked eye or low magnification (up to 10x). Higher magnification with light or SEM microscopy may help to confirm these traits. 5. Internal traits do require a metallographic cross-section and light or SEM microscopy analysis. 6. Eleven new characteristic traits were identified and have been proposed for addition to NFPA The fire exposure caused no or minor changes to the characteristic traits discussed. 8. Artifact cleaning recommendations were made, based upon actual testing to evaluation how various cleaning methods caused or did not cause significant damage to certain characteristic traits. ABOUT THE AUTHORS Chris W. Korinek, P.E. is a Forensic Electrical and Mechanical Engineer who Founded Synergy Technologies LLC in Chris has Bachelor degrees in both Mechanical and Electrical Engineering. He consults for insurance carriers, attorneys, manufacturers, utilities, and government entities in the fields of electrical overheating, fires, explosions, water damage, mechanical damage, and product design and testing. He has also worked for 21 years at three major manufacturers as a Senior Project Engineer performing Product and Process Design, Quality Assurance, and Engineering Management. Timothy C. Korinek, P.E. is a Forensic Materials Engineer who has been employed by Synergy Technologies. Timothy has a Bachelor degree in Materials Engineering and is pursing a degree in Mechanical Engineering. Timothy has worked as a Forensic Engineer or Forensic Technician since Timothy consults for insurance carriers, attorneys, manufacturers, utilities, and government entities in the fields of mechanical and electrical failures, fractures, corrosion, electrical overheating, fires, explosions, water damage, mechanical damage, and testing. ENDNOTES 1 National Fire Protection Association (NFPA) 921, Shea, IEEE Holm Conference on Electrical Contacts, Glowing Contact Physics, Korinek, Fire and Materials, Pre and Post-flashover Characteristics of an Electrically Overheated Poor Connection between Copper and Steel, ASM, Metallographic Technique for Copper and Copper Alloys, Metals Handbook, Introduction to Practical Ore Microscopy, 1989 Page 12 of 12