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Screened VS Unscreened Cables

Screened VS Unscreened Cables

Both screened and unscreened network cables have been used for many years, and they both offer different benefits. Shielded cabling originally became popular mainly in Germany, Switzerland, Austria and France, and the rest of the world supported unshielded cables. Now, it has been found across the globe, that although both types of cable work perfectly at lower data rates, screened cables have shown their superiority through their performance over higher data rates.

What’s the difference between a screened and unscreened cable?

Screened Cables

Unscreened Cables

A screened cable has an extra layer above the insulation, covered by a metallic screen and an outer cover.

An unscreened cable is not surrounded by any screen. The cable has an outer cover which has been directly forced above the insulation.

Benefits

Benefits
  • They reduce the effects of electromagnetic interference which can affect performance.
  • Suitable for industrial application
  • Smaller cable diameters
  • Cheaper alternative due to fewer materials being needed
  • More ideal for office applications
  • Work perfectly over lower data rates

Disadvantages

Disadvantages
  • More expensive solution
  • Liable to Alien Crosstalk interference

The decision to use screened or unscreened cables will depend on several factors including:

  • The available budget
  • The overall performance of the cabling system
  • The electrical environment which the cabling will be installed into (the level and type of Electromagnetic Interference (EMI))
  • The kind of cable containment
  • The adjacency of data cables to power cables
  • The accessibility of adequate points of earthing within the building to be cabled.

What are the different types of screening?

There are two different types of screening, overall and pair screens.

Overall screens sit around the outside of all four pairs of wires. They perform two functions, preventing the emission of signals (noise) out of the cable, and protecting signals travelling through the cable from external interference. Aluminium foil or tin-plated copper braids are both materials used in overall screens.

Pair screens are made from foil, wrapped around each of the individual pairs within the cable and also perform two functions. They are used to prevent interference from the signal travelling in one pair of wires from affecting the signal in another pair in the same cable sheath. This is referred to as Near End Crosstalk or NEXT. Pair screens also prevent the emission of signals (noise) coming out of the cable, where they are at risk of interfering with signals travelling in other nearby cables. This is referred to as Alien Crosstalk; electromagnetic noise that can occur in cables that run alongside other signal carrying cables).

When is best to use screened or unscreened cables?

The higher the frequency of the signal travelling down the cable, the larger the need for screening. This is as a result of the frequency rising, causing the signal to move further away from the copper wire core, to the point at which it travels as an electromagnetic field around the core. At very high frequencies the signals can migrate outside of the cable cover when Alien (cable to cable) Crosstalk becomes a potential problem.

Unscreened cables are cheaper than the screened alternatives, due to them requiring fewer materials and manufacturing processes. The majority of Cat5e and Cat6 UTP cables installed in the UK are unscreened, as the frequencies of up to 250MHz don’t tend to create issues with Alien Crosstalk. Screening only needs to be considered in the case of Cat5e and Cat6 if the cables are being run where there is the potential for EMI. This particularly requires consideration if the risks can’t be mitigated through physical separation between the cables and the EMI source. Another need for consideration is when metallic conduit is being used that will, when correctly earthed, provide an overall screen for the cables coming through it.

Cat6A cables may be unscreened or screened, although mechanical measures need to be taken if they are unscreened, in order to minimise the risk of Alien Crosstalk. This is usually done using special, non-circular sheath extrusions, e.g. oval or triangular. These help to ensure that cables don’t lie parallel to each other over extended distances.  They are commonly more expensive than their screened alternatives and require the use of costly and time-consuming Alien Crosstalk testing at the time of installation. Screened Cat6a cables usually have a smaller overall diameter than the unscreened types, which allows more cables to be installed in a particular size of containment and the cable bend radii is smaller.

Cat7A cables are always screened, with an S/FTP construction. This is because they can carry signals with frequencies of up to 1000MHz, generating a higher risk of Alien Crosstalk if no screen is used.

So, which types of cables should I use?

In conclusion, there are no standardised rules as to when or where unscreened or screened cables should be used. As long as you are aware of the differences, each installation should be assessed on a case by case basis to decide which would be best. Both offer different benefits, and these should be considered when making the decision.

For all your screened and unscreened cable solutions, contact Netshop today, and one of our dedicated account managers will be here to help.