Screens vs. Shields vs. Braids

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Screen, Shield and Braid - they all are sometime used for same meaning. The way I see the differences are:

Screen: The term screen is used to keep the electrical signals of the cables separated from outside electromagnetic, electrostatic and radio frequency interference. This is like putting a screen between the cable signal and the outside noise signal.

Shield: A metallic or semi-conductor layer used around the cable or conductor for the screening purpose.

Braid: Braid is one of the styles of cable shield. It is the style of using the shield filaments with interwoven in cylindrical form.

So, we can say 'the braided shield is used for screening the cable from outside electro-magnetic interference'.

The changing magnetic field of the cable signals produce voltage across the lengths of the shields and if both ends of the shield are grounded, a circulating current flows through the shield. This causes power loss and additional temperature increase of cable insulation. To prevent the circulating current, only one side of the shield is grounded.

See the previous threads:

http://cr4.globalspec.com/thread/44817

http://cr4.globalspec.com/thread/38838

- MS

Thanks msamad for your reply. Now I understood. But I have some dubt that some say that shield should be grounded at one end and some say that it should be grounded at both end. I have seen grounding of Power cable sheild (33Kv) I dont know the reason behind it. Another is when we should use twisted pair cable and when we should use co-axial cable?

<...co-axial cable...> is generally used for signals that are of small voltage and current, such as radio and television reception. The signal is usually on the centre conductor, the outer forming a cylindrical "Faraday cage" to protect the signal inside it. However, there other areas where this style of cable has applications such as <...33kV...>, where it is necessary to prevent the currents flowing to be picked up as interference in other nearby circuits by way of electromagnetic coupling between them.

<...twisted pair cable...> is the usual choice for current signals, the principle being that superimposed electromagnetic interference will affect both conductors equally, and can be easily filtered out. It is also the cable of choice where multiple signals that are galvanically isolated from each other need to travel within the same overall sheath, not having a common conductor to all circuits attached. However, it is because of the immunity of current signals to interference that they could, in principle, travel down the proverbial "two bits of wet string" rather than in any special cable. The shield can prevent interference getting in, regardless of the signals' inherent resilience.

<...grounded at both end[s]...> can introduce circulating currents within the circuit protective conductor and/or shield, and is best avoided; an example of the problems that can occur would the the introduction of "mains hum" onto circuits carrying audio voltage signals, such as microphones and pick-ups.

What the <...we...> does with cabling can be found in the facility-specific and applicable cable installation standards, which have been withheld from the forum and cannot be seen from here. A discussion about them with a local Mentor is the recommended way forwards.