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What is the purpose of outer sheath in FTP cable, i.e. individual pairs already have the shielding and thus both crosstalk between the pairs and alien crosstalk should be eliminated?

TP cable types

In addition, how is screening(for example SF/UTP or S/FTP cable types) different from foiling(for example F/FTP type)?

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It appears to be the difference in the level of blocking EMI for the reason these different cables are made.

Copper braided shields are easiest to work with (crimping and connecting), but only cover about 70% of the area. Foil shields are more difficult to work with, but due to higher resistance, aluminum foil compared to copper braids aren't as good at blocking EMI. For greater EMI protection than either of them can provide alone, they're used together. If you're in a very noisy environment or you can't afford to have corrupted signals, you'll want to go the most expensive route. I can imagine vehicle connectivity being done with something like this as safety is a primary concern.

In summary, aluminum foil is the cheapest and worse protection from EMI. Copper braid comes next for usability and blocking ability. Lastly, both together offer the best protection.

For more details please see this reference.

Note that I assume foil screen is very similar to copper braid. It may be made out of aluminum though.

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  • \$\begingroup\$ Nice answer, I believe it completes mine (or mine completes yours) very well! +1 \$\endgroup\$ – Doombot Jul 11 '14 at 14:36
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About the use of an outer, general shield over the individually shielded twisted pairs, it should enhance the electromagnetic interference of the whole cable. Remember that the insulator around the cable, because of it's non-conductivity, is prone to accumulate electrical charges. Using a shield layer directly beneath it helps dissipate these charges. You could say they would dissipate on the shielded twisted pairs but a constant layer of shield put directly under the insulator is better.

About screening, I assume you speak about braids (inter-meshed layer of small conductor wires instead of a plain foil). If so, the reason is mechanical. The use of braids enhance the flexibility of the shielding, in regard with that of an aluminum foil. As an analogy, it is (kind of) the same principle than using multistrand wires in an electrical circuit vs using a full wire. It bends more easily that way and cables subject to movements might be required to be braided instead of foiled, depending of the norm used (braids are more expensive than foils so they allow to use foil when it is ok).

Edit: As @horta mentions on it's answer, braids usually have a lower resistance (more conductive material or "area section" in them) than foils (thinner, less "area section"), which is better is better to dissipate electrical charges.

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