The wiring process of secondary circuits within a low-voltage distribution cabinet has a decisive impact on electromagnetic compatibility (EMC), and its design rationality directly affects the stable operation of the equipment in complex electromagnetic environments. As the core component for signal transmission and control within the low-voltage distribution cabinet, the wiring of secondary circuits must balance signal integrity, interference immunity, and equipment safety. Any manufacturing defects can trigger EMC problems, leading to equipment malfunctions or damage.
The routing and spacing of wiring are the primary factors affecting EMC. If control cables in the secondary circuits are arranged parallel to the high-voltage busbar or with insufficient spacing, signal coupling will occur due to mutual capacitance and inductance effects, resulting in conducted interference. For example, when control cables are arranged perpendicular to the high-voltage busbar, the magnetic field coupling strength can be reduced to less than 1/10 of that when arranged parallel. Therefore, process specifications require that control cables be routed as perpendicular to the high-voltage busbar as possible. If this is not possible, the spacing must be increased, and further isolation should be achieved through metal partitions or shielding layers.
The method of fixing and bundling wire harnesses also has a significant impact on EMC. If cable harnesses are not layered for fixing or are bundled too widely, relative displacement can occur under vibration or temperature changes, leading to cross contact between signal and power lines. This crossover forms a loop, generating an induced electromotive force under electromagnetic fields, interfering with sensitive signals. Standards require that the horizontal spacing of cable harness fixings not exceed 300mm and the vertical spacing not exceed 400mm, and that the bundling tape be made of non-magnetic material to avoid introducing additional interference.
Grounding of shielded cables is a critical aspect of electromagnetic compatibility (EMC) design. If the shielding layer is not grounded at one end or is poorly grounded, circulating currents will be generated within the shielding layer due to electromagnetic induction, exacerbating interference. For example, in inverter control circuits, shielded cables must be grounded at a single point at the control cabinet end, and the grounding resistance should be less than 0.1Ω to ensure that high-frequency interference signals are discharged to ground through the shielding layer. Furthermore, the connection between the shielding layer and the grounding terminal must use cold-pressed terminals or welding processes to avoid shielding failure due to excessive contact resistance.
Separating signal and power lines is an effective way to reduce electromagnetic interference. In secondary circuits, if low-voltage signal lines (such as instrument signal lines) and high-voltage power lines are laid together, magnetic field interference caused by harmonic currents in the power lines can lead to signal distortion. Manufacturing specifications require that signal lines and power lines be laid in separate trays. If they must share a tray, metal partitions must be used for isolation, with a spacing of at least 200mm. For high-sensitivity equipment (such as PLC input modules), twisted-pair or shielded cables must be used for signal lines to further suppress common-mode interference.
The selection of terminal blocks and wiring terminals also has a significant impact on electromagnetic compatibility. If terminal blocks do not use flame-retardant insulation materials or the spacing between wiring points is too small, short circuits may occur due to arcing or creepage, generating electromagnetic pulses that interfere with other circuits. Specifications require terminal blocks to have an arc-proof design, and each wiring point should only be connected to one wire. In special cases where two wires need to be connected, dedicated wiring clips must be used to ensure reliable contact. Furthermore, the arrangement of terminal blocks should follow the principle of "from left to right, from top to bottom" to avoid connecting signal lines across areas.
The grounding system design in the secondary circuit is fundamental to ensuring electromagnetic compatibility (EMC). Low-voltage distribution cabinets require an equipotential bonding network, connecting all metal components (such as cabinets, equipment housings, and shielding layers) to the grounding busbar via low-impedance conductors to eliminate common-mode interference caused by potential differences. For example, in a TN-S system, the PE and N lines must be strictly separated, and the PE line cross-section must be no less than half the cross-section of the phase line to ensure rapid discharge of fault current. Furthermore, the grounding busbar must be made of copper, and the connecting bolts must be coated with conductive grease to prevent grounding failure due to excessive contact resistance.
The details of wiring processes have a cumulative effect on EMC. For example, the stripped length of wires must be strictly controlled; excessive length can introduce interference from exposed parts, while insufficient length may cause arcing due to poor contact. The bending radius of the wires must be no less than three times the wire diameter to prevent insulation damage due to mechanical stress. Multi-strand wires should be cold-pressed or tinned to prevent corona discharge caused by burrs. Improper handling of these details can lead to a decrease in overall EMC due to the "weakest link" effect.
