What happens in cable shield when it's not connected?

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Kingston

Kingston

Well-known member
Joined
Nov 1, 2005
Messages
3,716
Location
Helsinki, Finland
This question has bothered me for quite some time.

What happens to cable shield when it's not connected to anything, but when wires inside it are conducting? Floating, but doing what exactly? Shielding still, but what?

And I don't mean just audio, but in general.

Mike
 
It most likely couples the interference to the two inner conductors (capacitively and inductively). In some cases the non connected shield is better than no shield but in some cases it can be worse too.

So it is better to connect the shield. But you should think carefully where to connect it (and dump the interference). Sensitive PCB signal ground is probably not the best alternative...
 
audiox said:
It most likely couples the interference to the two inner conductors (capacitively and inductively).
Click to expand...
Yes it does! A non-connected shield is almost no shield at all. Cable shields are efficient at shielding against electrostatic interference (capacitively coupled), but pretty inefficient against magnetic (inductively coupled), unless it is a piece of iron tube!
In some cases the non connected shield is better than no shield but in some cases it can be worse too.
Click to expand...
Having the shield not connected at ONE end is sometimes beneficial, but BOTH ends never.
 
On "balanced" lines, it still shields, not as well.

Unbalanced lines with a ground conductor, it is barely better than nothing.

Unbalanced lines which use shield for return will be full of crap.
 
PRR said:
On "balanced" lines, it still shields, not as well.
Click to expand...

To expand on that slightly, for properly balanced interfaces it will collect and spread the noise within the shield, then retransmit it common mode to the two audio lines. In a high RF environment with slow unprotected input circuitry this could result in rectification like TV suggests.

Not ever a good idea IMO.

JR
 
to the layman he is saying that the ungrounded shield acts like a transformer lamination, concentrating all the stray flux into one place, right next to your audio.

then the first stage makes it into KFRC the Big 610, with doctor don rose in the mornings,

hey that reminds me, they are going to fix the bay bridge right now.

 

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