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Cable shields connected to 0V?

Flyback

Well-Known Member
Hi,
We have a system in a plastic enclosure comprising multiple PCBs interconnected by cables and looms. One PCB comprises the 4 SMPS's. There is an offline 20W SMPS on this PCB also. Our circuit ground is directly connected to earth. We need cable screens for all the cables. Do you agree that we must connect our cable screens to circuit ground at each end of the cable? The thing is, do you also agree that we need to put ferrite beads in the connection leading to the cable screen, in every case?
You see, the situation is that we do not have a separate chassis ground, because our enclosure is plastic...so therefore we cannot connect our cable screens to any chassis ground.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
If you have any loops (common connections or grounds) though different cables, you need ferrites to ensure there are no circulating currents.

That applies with or without screening; the screening itself can add to the loop circuit.

Whether it works best grounded at one end or both depends on the layout and other current paths.

It's the same principle as a "ground loop" with audio causing hum, as a loop connection via different cables acts as an inductive coupler.
It can radiate as well as receive. Ferrites break the loop at high frequencies and reduce radiated interference.
 

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