Your IT Road Map : Data Cabling and InstallationPW Data Group
Data Cabling Pathways
For efficient outlay of Data Cabling the cable must have a defined route that will protect it according to the environment in which it exists. No standard exists that details exactly what kind of cable pathway is required in different circumstances and generally engineers are expected to use common sense to ensure the cable is adequately protected. What is clear however that the cable routes cannot be abandoned to chance with installers allowed to pick any random route from A to B. There is also the commercial consideration that well-engineered cable management systems cost money and the end-users must decide how much cable management they want to pay for and the consequences of under-investment in this area.
General rules for Data Cabling Installation
There are some general rules that apply to the installation of all cable bundles, regardless of containment type, and they are:
Cable ties must not be too tight. Any cable within a tied bundle must be able to be moved through that tie with slight resistance. If the cable cannot move through the tie at all then it is too tight. Category 6 and optical cables cannot stand the same heavy-duty lashing as power cables. The tie must not be too thin either or it will start to cut into the sheath of the cable. Ideally the tie should be larger than 5mm wide.
Data Cables must not be forced around bends tighter than that specified by the manufacturer. Manufacturers will often specify six to eight times the cable diameter as the cable bend radius. This could range from 30mm for 5mm diameter Category 5 cable to 48mm for 6mm diameter Category 6 cable. ISO 11801 2nd Edition calls for 50mm bend radius for four-pair cables and fifteen times the diameter for cables with more pairs. This is a good generic specification to enforce upon cable contractors and should be easily achieved in main backbone and horizontal cabling runs. The last piece of cable jammed behind the Telecommunications Outlet however is always more problematic as there is never enough space available. At this point installers must ensure that they at least achieve the cable manufacturer’s minimum bend radius requirements.
Cable bundles must not be too big. There is no exact or correct figure for the amount of cables allowed in any one bundle but experience shows that a figure of between 24 and 48 cables is the optimum.
Cables must not be dragged around or across sharp edges.
Power cables must cross data cables at right angles and should be separated from each other by a bridge.
Types of Data Cabling pathways
Data Cable routes marked on Floor
Apart from nothing at all, this is the simplest form of cable containment. It can be appropriate when cable can be laid directly on a relatively smooth concrete floor which in turn is covered by a false floor. The assumption here is that the cable is protected from damage by virtue of the floor suspended above it. Cables should be loosely bundled together with cable ties which can be nylon or Velcro-type.
Data Cable routes on with cable-mat
Cable-mat is spongy matting material that can be laid directly on rough concrete screed or wire basket tray to smooth out the mechanical load applied to points on the underside of the cables in contact with the floor or wire tray. Opinion is divided as to whether this is really necessary or not. It would only be required on a concrete floor if the finish was extremely rough.
Data Cable Conduit
Conduit is an enclosed tube that can be made of metal or plastic. It will provide maximum protection to the cable but will cost more to install and take longer to pull the cable into it. The cable should not take up more than 50% of the available cross-sectional area of the conduit. EN 50174 says that access to trunking, ducting or conduit systems should be available at intervals of no greater than 12 metres. ISO 18010 Information technology. Pathways and Spaces for customer premises cabling, gives a maximum figure of 30 metres between pull points and not more than two 90°; bends in the conduit. The inside bend radius of the conduit must not be less than six times the inside diameter of the conduit.
Data Cable Tray and Trunking
Tray and trunking may be totally enclosed, but in which case the term duct or ducting is more often used, especially when the containment is permanently or semi permanently enclosed.
There are many forms of Cable Trays including:
- Solid-bottom cable tray
- Perforated or trough cable tray
- Spine cable tray – centre rail construction
- Wire tray – welded wire construct
- Mesh cable tray – wire or plastic mesh
- Wireway – fully enclosed with gasket cover
- Cable runway – no side panels, cable is tie-wrapped to base tray
Which one is best really depends upon how much one wants to spend. Wire basket tray is suspected by some to cause problems with Category 6 cables and some manufacturers recommend cable mat to be placed in the bottom of the tray. No evidence to support this view ever seems to have been published. on the other hand manufacturers of cable trays have published technical documents to prove that wire tray is not a problem for high performance data cable.
The cable should not be piled higher than the side walls of the trough for any kind of cable tray. EN 50174 gives details of the earthing and bonding requirements of metal cable trough to ensure best EMC protection and in particular the requirements of joining two pieces of the trough together. A simple metal strap between the two may be fine for a safety earth but offers little in the way of EMC protection. For best EMC protection the shape of the trough must be maintained across each jointed section. A supplementary earth wire should follow the cable tray and connect to it every 15 to 20 metres.
Power cables and IT cables should not be randomly mixed in the tray or trough but segregated with as much lateral separation as possible. Cables should be bundled loosely together in the tray with cable ties in bundles of no more than approximately 48 cables. If the weight of the cable is supported in the horizontal then fixing to the tray needs be at intervals of about 1 to 1.5 metres. In the vertical plane the fixing should be approximately every 500 mm.
As with conduit, cable troughs should not be filled to more than about 50% of their theoretical maximum capacity. Remember that when calculating cable cross sectional areas, the area taken is not πr² but the diameter of the cable squared. This is because each cable takes up the equivalent of a square of sides the same as the diameter of the cable, so a 6mm diameter cable takes up 36 square millimetres of space.
A move to Augmented Category 6 will have a major effect on cable containment sizes, e.g. a move from 6 mm Cat 6 cable to 9 mm Cat 6A cable could take up 225% more space.
Alien crosstalk is sometimes mentioned in relation to cable bundles. Alien crosstalk is electromagnetic energy leaking from one cable (or indeed any source) and being picked up by another adjacent cable. In such circumstances it is the six cables in immediate contact with the victim cable in the middle of the six, which will cause nearly all of the effect. Cables from further out in the bundle will have a very much smaller effect. For this reason the size of the bundle has little bearing on alien crosstalk problems.
The main factor in increasing alien crosstalk is the presence of long parallel runs of the cables. These should be avoided if possible. In practice, individual four-pair cables do not run exactly parallel after they have gone around a few corners. However, Augmented Cat 6 unscreened cables, bundled cables and Pre-terminated cable bundles will tend to suffer the most from this phenomenon. If installers wish to use a pre-termination method then they must obtain assurances from the cable manufacturer that their cables are suitable for bundling and pre-termination.
With all cable trays care must be taken to ensure that right angled bends in the tray do not end up forcing cables to go below their minimum allowed bend radius. Modern cable tray designed for data communications and optical fibre cables now build-in pre-manufactured corners. EN 50174 requires an access space of 150mm minimum above a cable tray to allow access.
Suspended Data Cables
One method of containment is to suspend cables from hooks attached to the ceiling. This method is popular in the USA but not often seen in Europe. If the size of the cable bundle is not too large then there is nothing wrong with this method. These hooks are often referred to as J-hooks. J-hooks should have a broad base to support the cable and should support the cable at least every 500 mm.
Room Perimeter Data Cable Pathways
Room perimeter pathways are usually surface mounted trunking systems attached to the wall at desk level, sometimes called dado trunking, or at floor level, skirting trunking.
These constructions are usually made of extruded white PVC and may be made up of several internal compartments. The internal compartments are for power and communications circuits.
Some forms of trunking have metal separators between the compartments to improve EMC performance, and sometimes the plastic itself is metallised to achieve a similar, though not so effective, result. At appropriate distances, power and telecommunications outlets are placed in the front of the trunking.
These should be located within three metres of where users are likely to want to make a connection.
A common mistake is to pick very slim trunking for aesthetic reasons. However when it comes to installing all the power and data cable there is not enough room. One manifestation of this is crushed cables and reduced bend radii behind the outlet block, which usually extends deep into the trunking itself. Users must calculate the internal volume they require in the trunking after considering the density of outlets required, where they are going to make the drop to the rest of the horizontal cabling and the restrictions of EN 50174 concerning proximity of power and data cable.
Similar considerations must be given to the use of power poles that have exactly the same function as the perimeter pathway. All power poles must be securely mounted so that they cannot be knocked over.
There are usually local or national regulations that require the maintenance of a fire stopping system where cable trays, conduit or trunking etc. pass through one fire zone to another. In the UK it is covered under Building Regulations Part B. In such instance there will have to be some physical fire stopping substance that stops flame or smoke or even the passage of air that may feed a fire further up the building.
This may be done by several specialised materials now available, and ordinary plaster or filler or polyurethane foam should not be used. There are numerous caulks, foams, putties, blankets, pillows, collars and blocks etc. that are made just for the job and specialist manufacturers and contractors should be consulted. One piece of terminology used is intumescent foam. This means foam that will expand in the presence of heat to block off an air passage.
Data Cabling Management in the Comms Cabinet
Fixed cabling entering the rear of an equipment rack must be properly organised and secured with each cable individually labelled. Earthing connections must be made between any metallic cabling tray entering the rack and the rack itself.
Where single racks are installed, the vertical cable managers should be at least 150 mm wide. Where a row of two or more racks is installed, consider mounting 250mm wide vertical cable managers between racks, and 150 mm wide vertical cable managers at both ends of the row. The cable managers should extend from the floor to the top of the racks.
The same standard calls for the following for horizontal cable management.
In the entrance room, main distribution area and horizontal distribution areas, horizontal cable management panels should be installed above and below each patch panel. The preferred ratio of horizontal cable management to patch panels is 1:1.
The vertical cable management, horizontal cable management, and slack storage should be adequate to ensure that the cables can be neatly dressed and that bend radius requirements specified in ANSI/EIA/TIA-568- B.2 and ANSI/EIATIA-568-B.3 are met.