Why does the power cable for 4-wire instruments have a shielding layer?
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It was found that in the project, both the power cables for four-wire instruments and those used to supply power to the on-site PLCs were designed by the design institute to have shielding layers What is the function of this shielding layer? Single-ended grounding or dual-ended grounding? I would appreciate your advice as well. Thank you!
The main function of the shielded power cable for four-wire instruments is to prevent electromagnetic interference and crosstalk. When there are interference sources such as strong electric fields, high-frequency signals, and electromagnetic waves around the cable, electromagnetic interference and crosstalk occur, leading to signal transmission distortion and affecting the stability and accuracy of the system. The shielding layer can effectively block the influence of these interference sources, ensuring the quality of signal transmission. The shielding layer can be grounded at one end or at both ends; the specific setting should be determined based on actual conditions. Generally speaking, if the interference source is severe, it is necessary to ground both ends in order to improve the shielding effect ； If the interference is low, single-ended grounding can be used to avoid introducing other problems. .
Preventing interference has two aspects: one is to remain unaffected by other electromagnetic fields, and the other is not to generate electromagnetic fields that interfere with other devices.
I think there are two reasons: one is to prevent him from interfering with other circuits, and the other is to simplify the types of cables used.
Hello, I’m fine. Hello to everyone too :victory:
Isolation, to eliminate interference between power supply and signals
The design institute’s understanding of the isolation measures specified in clause 8.3.4 a depends on whether the 220VAC power cable for the instrument is laid in the instrument’s low-voltage cable tray; if so, it is recommended to use the shielded flexible cable designed by the design institute； If not, in accordance with clause 8.3.4 bc, unshielded flexible cables can be installed in the strong current tray. To suppress interference, the shielding layer should be grounded at one end in order to direct the interference away; I hope this helps.
. Four-wire instruments have signals of type II, and their power supply is equivalent to type IV signals. Different types of signals must follow different wiring principles or employ effective isolation measures. Signal classification: Category I signals: low-level signals such as thermal resistor signals, thermocouple signals, millivolt signals, and strain signals. Class II signals: 0~5V, 1~5V, 4~20mA, 0~10mA analog input signals ； 4~20mA, 0~10mA analog output signals ； Level-type digital input signal ； Contact-type digital input signal ； Pulse quantity input signal ； A digital output signal for resistive loads with 24VDC and less than 50mA. Class III signal: A digital output signal for inductive loads at 24V~48VDC, or resistive loads with a current greater than 50mA. Class IV signal: 110VAC or 220VAC digital output signal. Among them, Class I signals are easily interfered with, Class II signals are also prone to interference, while Class III and Class IV signals become strong sources of interference at the moment of switch operation, interfering with nearby signal lines through the spatial environment. The feeders for Category IV signals can be routed as power lines. .
Anti-interference; Anhui Tiankang Instruments – cooperation is possible, contact Engineer Huang at 18225806021