Symmetry testing and analysis of CAN bus communication cables for mining

The electromagnetic environment in underground coal mines is complex, and interference such as the start-up and shutdown of high-power equipment, harmonics of power cables, switching transients, and dense deployment of wireless communication devices mainly couple into communication cables as common-mode signals, posing a serious threat to the stable operation of safety monitoring systems. The symmetry of communication cables is a key parameter that determines their resistance to common-mode interference. The current inspection scheme for coal mine safety monitoring systems uses the physical structure (i.e., whether the cable is twisted) as the criterion for symmetry determination and treats CAN signal interconnection lines as asymmetric lines by default for testing. This approach potentially conflicts with GB/T 17626.5-2019 "Electromagnetic compatibility—Testing and measurement techniques—Surge immunity test", which defines symmetric lines based on Transverse Conversion Loss (TCL). To address this issue, CAN bus cables widely used in coal mine safety monitoring systems were selected as the research object. The TCL test method was used to systematically evaluate the symmetry properties of three types of structures: unshielded parallel wire bundles, unshielded twisted pairs, and shielded twisted pairs. The results showed that the differential-mode-to-common-mode conversion loss of all tested cables exceeded 20 dB, meeting the definition of symmetric lines and indicating that there was no inherent correspondence between the symmetry of cables and physical structure. Accordingly, it was proposed that electromagnetic performance indicators such as TCL should replace the physical structure as the basis for symmetry determination, and it was recommended that mining communication cables be uniformly tested as symmetric lines in electromagnetic compatibility tests, including surge immunity, conducted radio-frequency immunity, and conducted emissions, to better match the actual underground interference environment.