Electronics > Manufacturing & Assembly

Can we get away without using shielded cable?

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Faringdon:
OK thanks, and then we go back to the details of the reply #2 above...

Faringdon:
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.

SteveyG:

--- Quote from: Faringdon on July 22, 2021, 06:19:14 am ---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.

--- End quote ---

It depends entirely on the nature of the emissions. If you can avoid your cable being an antenna for whatever frequencies and harmonics are being generated by your converter then there's no need to use a shielded cable.

The first step is to understand your converter and design it with parts and in such a way that it doesn't cause a problem. Next would be to suppress the noise. Final is to shield the noise.

If you've got multiple converters, you need to do some analysis and/or testing to work out the best shielding strategy, but I would still try to suppress the emissions first.

dmills:
Generally a mixture of a little filtering at each end of the cable (Remember noise can come in from the load end as well!) gets it done providing the power supplies are reasonably clean to start with.

Loop area matters, so twisted pairs can be your friends.
For each and every signal in the design consider the current loop formed (ALL of the current loop formed) and seek to minimise its area.

Sometimes synchronising the power supplies is helpful (For reducing input ripple current if nothing else), and in extremis synchronising to a deliberately spread spectrum clock is a thing.

Remember that for an aerial to be effective it needs to be reasonably long compared to the wavelength, so there is little point from an EMC perspective in worrying about circulating currents at frequencies where the total loop length is less then about 1/10th wavelength.

Get and read "Electromagnetic compatibility" by H. Ott and "High speed signal propagation - advanced black magic" by H. Johnson, and be enlightened, it is money VERY well spent.

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