When is a Shielded cable a UTP cable?

Screened v Unscreened cable

It’s often asked if a screened cable can be used in an unscreened application. To answer this question, legislation and standards should be reviewed. The first place to start is with AS/CA S-008 which requires any screen to be electrically conductive and to have a drain wire present. The aim of the drain wire is to provide a method for allowing the screen to be connected to earth. There are two reasons for requiring the screen to be grounded. One is to allow it to function as a screen by setting the screen at a potential that is close to earth. This will knock out most interference that comes from a low frequency source such as power mains.  Although, at very high frequencies, typically above 1Gbit, the necessity to connect to ground is debatable. Above this frequency, a noticeable lowering of interference can be experienced so long as the screen surrounds the cable. The second reason is to protect both personnel and equipment that may come into contact with the screen. Without the screen being grounded, there is a risk that voltages of a dangerous nature may present themselves on the screen. This may be by induction from one or multiple sources (eg: adjacent cables or lights),  from contact with a voltage source due to either cable damage or faulty equipment, or a voltage being purposely applied to the screen as part of a fault finding or cable identification process. When such an energized screen comes into contact or close proximity with an object at a different potential it may result in a spark or high current spike that can either damage equipment (failure of router or storage device) or shock personnel with a possible potential for serious injury (eg: if the person has a pacemaker, heart condition etc). To prevent this from occurring, AS/CA S-009 requires that all drain wires be earthed – there is a whole section of the standard that addresses exactly how this is to be done.

Faster bit rates, more interference

Given what we know, why would we want to have a screen that is not connected to earth? The answer comes from our modern LAN cabling. As we push the bit rates and communication signals to higher and faster rates, the actual signal is becoming an increasingly stronger source of interference. In addition, the use of lower level signals in order to minimise power usage, and the implementation of multilevel coding schemes to increase the bit rate of the signal, mean that the signal itself is also more susceptible to interference. This is putting additional requirements on the cabling resulting in the inclusion of Alien Crosstalk requirements (interference originating from outside the cable) on systems operating at 10Gbit and beyond, typically Cat6A, Class EA and higher. The net result is that where unscreened or UTP cabling was suitable in the past, the technology is pushing for a screened solution instead. That is not to say that an unscreened cable option is not possible. It is provided that the alien cross talk requirements in the cabling/permanent link/channel are met. This is where the problem arises. In order to meet the Alien Crosstalk requirement, the easiest option is to screen the cable. This can however, have implications in terms of the installation.

Alien Crosstalk requirements

By experience, most people consider an unscreened solution to be easier to install. However, if the screen is omitted in the cable, there needs to be some other mechanism to control the alien crosstalk between adjacent cables. The conventional approach is to make the cable jacket thicker. A thicker jacket increases separation between adjacent cables which weakens the interference between cables and brings the alien crosstalk under control.  So whilst a traditional unscreened cable may be considered easier to install, it may be larger than an equivalent performing screened cable. As the space in ceilings, wall cavities and at the rear of a rack is considered so important, a cable that can meet the alien crosstalk requirement, but still remain small is ideal. For high frequency interference, as experienced in alien crosstalk, a drain wire is not necessarily required to be grounded to reduce the interference to the point where the design can meet the alien crosstalk requirement. So why not just use a screened cable but not connect the screen? Even better still, why include the drain wire in the first place?

RCM requirements

In Australia, LAN cable falls under the guise of communication cabling. It is required to comply with the RCM requirements and associated Federal legislation. That calls for LAN cables to be compliant with AS/CA S-008 which is the governing specification for customer cabling products such as cables and connectors. Within this standard, clause 5.6.7 calls for any continuous metallic shield in a cable to be electrically conductive and where this shield is of a foil construction, a drain wire shall also be provided. One would think it would be simple, if a continuous foil screen is present, then a drain wire is required, and if a drain wire is provided, then it should be grounded.

What is a screen?

Within the requirements however, there is no firm interpretation of what a screen is, within the standard. That determination is currently left to the person declaring compliance with the standard. If someone wanted to include a foil screen in the cable so that they could keep the cable size down, but not have the complexity of having to install a screened solution, they could declare that the foil is not a screen but an electrostatic or other component. This negates the need to provide a drain wire by saying that the metal foil is not a screen. With limited access to an authority that can make a ruling on this approach, the choice is left up to the individual cable manufacturers and importers.

The screened v unscreened solution

However, the issue of protection to equipment and personnel is not guaranteed, when the intepretation of what is a screen, is left ambiguous. So we can end up with an installed cabling system that has an ungrounded continuous metallic coverings which are claimed not be screens running throughout a building. They are left open to the risk of high induced or applied voltages resulting in possible undiagnosed equipment failure and injury to maintenance personnel. How do we avoid that? The best bet is to either install a properly grounded screened solution. Alternately, we can install a properly designed unscreened cabling system that is alien crosstalk compliant and that does not rely on the use of ungrounded or unterminated continuous metallic foils  or screens in the cable to meet the required alien cross talk requirements. Garland have both options covered. For unscreened cable solutions we offer UTPL6AHF  a U/UTP design, and for screened solutions, we offer FTPL6AHF which is a F/UTP design.