The decision to use Unscreened or Screened cables will depend on a number of factors including the budget available, the performance of the cabling system, the electrical environment in to which the cabling will be installed (the level and type of Electromagnetic Interference (EMI)), the type of cable containment, the proximity of data cables to power cables and the availability of adequate points of earthing within the building to be cabled.
There are two different types of screen:
Overall Screen - This is the screen that sits around the outside of all four of the pairs. It performs two functions, one to prevent the emission of signals (noise) out of the cable and, two, to protect the signals travelling within the cable from external interference. This screen may be either in the form of an aluminium foil or a tin plated copper braid.
Pair Screens - These foil screens are wrapped around each of the individual pairs within the cable and again perform two functions. The first is to prevent interference from the signal travelling in one pair of wires from affecting the signal travelling in another pair of wires in the same cable sheath (referred to as Near End Crosstalk or NEXT). The second function is to prevent the emission of signals (noise) out of the cable, where they could interfere with signals travelling in other, nearby cables (referred to as Alien Crosstalk).
In general, the higher the frequency of signal travelling down the cable the greater the need for screening. This is because, as the frequency rises, the signal travels further and further away from the centre of the core of the copper wire to the point at which it travels as an electromagnetic field around the core, which at this point becomes basically an aerial. At very high frequencies the signals can actually migrate outside of the cable sheath, when Alien Crosstalk becomes a potential problem.
Unscreened cables are cheaper than their screened alternative simply because they require fewer materials and manufacturing processes. The majority of Cat 5e and Cat 6 UTP cables installed in the UK are unscreened as the frequencies of up to 250MHz do not tend to create issues with Alien Crosstalk. Screening really only need be considered in the case of Cat 5e and Cat 6 if the cables are to be run in areas where there is the potential for EMI and the risks can not be mitigated through physical separation between the cables and the EMI source or through the use of metallic conduit that will, when properly earthed, provide an overall screen for the cables passing through it.
Cat 6A cables may be unscreened or screened. If they are unscreened then some mechanical measures need to be taken to minimise the risk of Alien Crosstalk. This is usually done through the use of special, non-circular sheath extrusions, e.g. oval or triangular. These help to ensure that cables do not lie parallel to each other over extended distances but they are generally more expensive than their screened counterparts and require the use of expensive and time consuming Alien Crosstalk testing at the time of installation. Screened Cat 6A cables tend to have a smaller overall diameter than the unscreened types which means that more cables can be installed in a given size of containment and cable bend radii are smaller.
Cat 7A cables are always screened, with a S/FTP construction, as they have the potential to carry signals with frequencies of up to 1000MHz, creating a very high risk of Alien Crosstalk if no screen were used.
In conclusion, there are no hard and fast rules as to when or where unscreened or screened cables should be used and each installation should be assessed on a case by case basis.