# Derating of Electrical Contacts with Varying Surface Roughness
## 2 Materials and Methods  
In order to analyze the impact of surface roughness on the derating parameters of electrical contacts, three samples of M12 turned contacts were prepared. The samples were made from the same conductive material CuZn42, following the same geometric and technological specifications, but they differed in the surface roughness of the contact area, expressed as R\({}_{x}\). Two roughness values were selected for the tests:  
* R\({}_{x}\) = 2 \(\mu\)m: representing a high-quality surface that minimizes micronotches and maximizes the actual contact between the components. * R\({}_{x}\) = 10 \(\mu\)m: representing a surface of much lower quality, with larger irregularities that reduce the effective contact area. The selection of R\({}_{x}\) = 2 \(\mu\)m and R\({}_{x}\) = 10 \(\mu\)m as reference values for surface roughness is based on established industrial and metrological classifications, where lower values indicate smoother, more refined surfaces, and higher values signify increased surface roughness. According to standardized manufacturing and metrology guidelines (e.g., ISO 4287, DIN 4768), surfaces with R\({}_{x}\) \(\approx\) 2 \(\mu\)m are typically associated with precision machining, grinding, or polishing, indicative of high-quality surface finishes. In contrast, R\({}_{x}\)\(\approx\) 10 \(\mu\)m corresponds to rougher surface textures commonly produced by casting, milling, or rough turning, representing lower-quality finishes. In precision engineering applications, such as those in the aerospace, medical, and high-performance mechanical sectors, an R\({}_{x}\) value of 2 \(\mu\)m or lower is desirable, as it contributes to reduced friction, enhanced wear resistance, and improved dimensional accuracy. Conversely, in general manufacturing, surfaces with R\({}_{x}\)\(\approx\) 10 \(\mu\)m are often sufficient for non-critical applications, structural components, or parts intended for subsequent finishing processes [28]. Each sample was prepared using precise turning techniques and checked for compliance with the target R\({}_{x}\) parameters using an optical profilometer. The samples were then cleaned to remove any machining residues, such as oils and fine chips, that could interfere with the test results. The initial contact resistance of each specimen is measured using a low DC current, 16A as recommended for this type of contact cross section, with a four-wire measurement technique. This step ensures accurate resistance readings by minimizing the effects of lead and contact resistance. The derating test follows the guidelines in DIN EN 60512-5-2 [29]. Initial setup is based on the electrical contacts being placed in a test circuit with a variable DC power supply. Thermocouples are attached to the contact surfaces to monitor the temperature rise. The contacts are tested under controlled conditions in a temperature- and humidity-regulated chamber. A stepwise increasing current is applied to the contacts, starting from about 50% of the maximum rated current (Imax). The temperature of the contact surfaces is monitored throughout the test. The current is progressively increased until the temperature stabilizes or reaches the maximum allowable limit. After determining the maximum current (Imax) that the contact can carry without exceeding the temperature limit, the derating factor of 0.8 (as per DIN EN 60512-5-2) is applied. This means the recommended operational current is 80% of the maximum rated current. The tests aimed to determine the impact of surface roughness on the maximum allowable operating parameters (e.g., current and temperature) of the M12 contacts. The analysis was to include changes in contact resistance, heat generation, and surface stability under electrical overload conditions. The samples were subjected to:* Contact resistance measurements at room temperature. * Thermal tests at various current values to determine the derating values for different R\({}_{2}\).