Why can't the shielding layer of shielded cable be grounded repeatedly?

- Jun 05, 2020-

Electric cable (212)Electric cable (272)



Shield grounding is usually handled in two ways: shield single-ended grounding and shield double-ended grounding


① The single-end grounding of the shielding layer is to directly ground the metal shielding layer at one end of the shielded cable, and the other end is not grounded or is grounded through protection.


Single-ended grounding is to eliminate the electromagnetic interference by suppressing the potential difference. This grounding method is suitable for short-length lines, and the induced voltage corresponding to the cable length cannot exceed the safe voltage.


② Double-ended grounding is to connect both ends of the metal shield of the shielded cable to ground.


In the case where the shielding layer is grounded at both ends, the metal shielding layer will not generate an induced voltage, but the metal shielding layer will be affected by the interference magnetic flux and will cause the shielding circulation to pass through. If the potentials of location A and location B are not equal, a large potential will be formed Circulation, circulation will have a negative attenuation effect on the signal.


Why can't the shielding layer of shielded cable be grounded repeatedly?


There are three ways of system grounding: floating ground, direct grounding and capacitor grounding.


For the PLC control system, it is a high-speed low-level control device, which should be directly grounded. Due to the influence of the distributed capacitance of the signal cable and the filtering of the input device, the frequency of signal exchange between the devices is generally lower than 1MHz, so the PLC control system ground wire adopts one-point grounding and series one-point grounding.


The centralized PLC system is suitable for parallel one-point grounding. The center grounding point of each device cabinet is led to the grounding electrode with a separate grounding wire. If the device spacing is large, a series of one-point grounding method should be used, and a large-section copper bus bar (or insulated cable) should be used to connect the center ground point of the cabinet of each device, and then connect the ground bus bar directly to the ground electrode.


When the signal source is grounded, the shielding layer should be grounded on the signal side; when it is not grounded, it should be grounded on the PLC side; when there is a connector in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multi-point grounding must be avoided. When the shielded twisted pair cables of multiple measuring points are connected to the multi-core twisted pair overall shielded cable, the shielding layers should be connected to each other, and after insulation treatment, select the appropriate grounding point for single-point grounding.


Summary: 


1: There is no potential difference between the grounding points at both ends. If there is a potential difference, there will be a weak current, which makes the shielding layer actually become a ground wire. 


2: The shielded wire grounded at both ends works at the site where the high-frequency interference is serious. The line capacitance formed between the shielding layer and the internal signal line is coupled to the signal loop, which will seriously affect the signal misjudgment. 3: Various speed governor and PLC instructions make it clear that the signal wire shield must be grounded at one end and the ground should be on the controller side.