Shielded Cable

[jgarner]

In plain english, could someone explained purpose of the shield in shielded cables and why it is connected at just one end. thanks

 

[BILLY101]

The ground-fault current path that is described in Art. 250.2 FPN and utility company grounds which carry current and other reasons ground current can flow through a structure, could include the shield of a shielded cable if it were bonded at both ends.
Sensitive electronic equipment cannot tolerate this current.

BILLY

 

[LarryFine]

In plain english, could someone explained purpose of the shield in shielded cables and why it is connected at just one end. thanks

Just to clarify, are you asking about power cables or audio/video and other signal wiring?

 

[jgarner]

Talking about VFD cable and Cable used for 4-20 ma

 

[weressl]

Talking about VFD cable and Cable used for 4-20 ma

Two different animals. One is shielded to prevent 'noise' generated by the ciruit itself from escaping and affecting other equipment - ASD cables - and the other is shielded to prevent external interference to change the low signal value carried within the cable. The two main 'noises' of concern is RF - radio frequency - and EM, or electromagnetic. The ASD cables require ground continuity from the driven equipment to the source, eg. grounded at both ends and the signal cables are left 'open' at the user end and grounded at one point - not necessarily at the end but at any termination points close to the source end - to avoid circulating ground currents that would be another source of interference yet.

 

[glene77is]

Two different animals. One is shielded to prevent 'noise' generated by the ciruit itself from escaping and affecting other equipment - ASD cables - and the other is shielded to prevent external interference to change the low signal value carried within the cable.

Weressl,
Check me on this please.
These are two types of Faraday sheilds.
Grounded at One End only, produces an Inductive Faraday shield.
Grounded at Both Ends, produces a Capacitive Faraday shield.

NEC requires the Capacitive Faraday shield on GEC conductors to the ground rod, if protective EMT conduit is used at all. This conducts the high frequency lightning pulse towards the real Earth reference for lightning discharges.
As you might have said it, "allows 'noise' to escape".

And the corollary,
If only one end were grounded, then the lightning pulse would see a 'choke'.

Wiki has this:
"The shield of a screened cable, such as the coaxial cable used for cable television, protects the internal conductors from electrical noise."
Which says that the Capacitive Faraday shield allows noise to escape to the grounding points.

Comments please {^.^}

 

[ELA]

A True Faraday shield is not reliant on "ground"

A True Faraday shield is not reliant on "ground"

The "ideal" cable shield would not be reliant on a ground connection to be effective.
It has been referred to as a "dumb bell" Faraday shield. It consists of a 100% coverage shield material using 360 degree connections at each end of the cable. Envision a "dumb bell".

The idea being that neither external nor internally generated noise currents can penetrate the shield.

In the real world -when the shield is less than 100% coverage then grounding comes into play. The effectiveness of various methods of shielding is dependent upon the offending frequency.

Here is an interesting read:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090030527_2009031335.pdf