## 6 Discussion  
### Contact Surface Inspection  
The copper and brass contacts that were used in the experiment were examined using electron scanning microscopy (SEM, 6010 LV, JEOL Ltd., Tokyo, Japan) and light microscopy (Measurescope UM2, Nikon Co., Ltd., Melville, NY, USA) to investigate the damage from the initial arcing contact. All of the copper samples used in the SEM were treated with a high concentration of hydrochloric acid (HCl) to remove any copper oxides from the surface. The acid was able to remove both types of oxides, copper(I) oxide (Cu\({}_{2}\)O) and copper(II) oxide (CuO), from the surface and to expose the damaged copper beneath. Hydrochloric acid does not react with pure copper. On the other hand, the brass contact surface was not treated with acid since brass can react with hydrochloric acid and form a zinc chloride solution that can damage and alter the brass surface. Figure 8 shows a comparison between a new copper surface and one damaged by the initial arc. The damage from a small initial arc is sufficient to reduce the effective contact area of the conductor. Figure 9 shows a comparison between a new brass contact surface and one damaged by the same electrical discharge setting as the copper sample. The brass sample exhibits less contact deterioration from the initial arc, although the damage is still largely visible and features a large patch of black residue. It is thought that a single small arc can inflict enough damage to reduce the overall current carrying capacity of the contact area, as is shown in Figure 10. However, in actual practice, it is not possible to determine the area of the contact surface that carries the current due to its microscopic scale. Any small movement by a human operator, either accidentally or from purposefully readjusting the setup, can drastically change the point of contact, thus altering the results. When the contact surface is subjected to repeat arcing, the average contact area may wear down leading to a reduction in the current carrying capacity and causing an over-current state. Figure 7: FFT results for the vibration velocity waveform. (**a**) FFT result for an arc fault present at 8.33 A. (**b**) FFT result for the normal state when both contacts are held firmly together. Notice that only the 50 Hz component is left, which may be caused by nearby vibrations from the power supply when there is no arc-fault state. Figure 8: The copper contact surface. (**a**) The new copper contact surface under a microscope. (**b**) The same copper contact surface after being subjected to an electrical arc from normal contact. The current that passed through the contact was 4 A. Note that the copper surface loses its mass after the contact arc. Figure 9: The brass contact surface. (**a**) The new brass contact surface under a microscope. (**b**) The same brass contact surface after being subjected to an electrical arc from normal contact. The current that passed through the contact was 4 A. Note that brass surface leech others copper contact and form bulges of oxides on the surface. Figures 11 and 12 show a comparison between a new copper contact surface and a surface that has been damaged by a single contact electrical discharge of 4 A. Oxide formations were not observed on the surrounding copper material. The copper was leeched by the oxide at the contact surface and subjected to the electrical discharge. By taking the space of the affected area and effectively digging out the copper, the affected surface was replaced with its oxide counterpart. This type of erosion [21] is predominant when the electrical arc is energetic enough to remove a portion of the contact surface from the base material. Brass contact surfaces are much more resistant to electrical discharge wear, as is shown in Figures 13 and 14. The result has the same pattern as what was observed by [PERSON] [22] and [PERSON] [23]. Figure 11: A new copper contact surface before it was subjected to an electrical arc, viewed under an electron microscope at 100\(\times\) magnification.