Cable comforts

All of these methods provide support for cables and varying degrees of protection and accessibility.

The need for protection against mechanical damage is obvious, but support is important too. The weight of a reel of copper cable tells you that the cabling is heavier than some people think and will sag and deform if not properly supported along its length. The same heaviness means that cables in the bottom of a tray can easily be crushed by excessive weight from conductors above them.

This kind of sagging and crushing might be tolerable with mains wiring, but it is definitely not in structured cabling, for two reasons. The geometry, lay and spacing of wires in structured cabling are extremely important to their correct operation. The individual conductors are run in a rotating twist to minimise mutual induction and external interference, while the distance between conductors determines their characteristic impedance. Any distortion of the correct spacing will alter the nominal 100 Ohm value, leading to signal reflections that cause mismatch and data errors.

Crushed feelings
Crushing is a common cause of cable distortion, as is taking cables around sharp bends, which can upset the twist. It’s vital to realise that as well as baseband audio signals, these cables also carry radio frequency energy. Category 5e cables operate at up to 100 MHz and Category 6 at 250 MHz, well into the very high frequency (vhf) region. At these frequencies the copper conductors will act as antennas if not restrained; the balanced twist is one of the mechanisms that prevents these signals leaking and the pick-up of interfering signals.

For these reasons bend ratios are very important. The rule for pulling in new cables is to have no bend smaller than eight times the cable’s diameter. At the final installation stage the minimum is four times the diameter, but never less. Right angle corners and hairpin bends are not allowed.

Sags and bends are not the only causes of cable deformation; mistreatment is another. For every type of cable there’s a correct pulling force; tugging too hard can lead to damage. Low smoke, zero halogen (lszh) cable can be very temperature sensitive and consequently quite stiff if left in a van overnight. For easy handling and flexibility it should be allowed to warm to the room temperature prior to pulling.

It’s not uncommon either to see installers forcing cables into trunking and then hammering down the capping to make everything fit. Not uncommon, but a clear sign of poor workmanship nonetheless. Cost cutting is the cause: either the client won’t pay for additional trunking or the estimator won the contract by under-specifying the task. Even if the system works properly at first, the chances are that it will fail soon afterwards, leading to callbacks that cost more to fix than doing the job properly in the first place. Also, installing trunking without a millimetre to spare for expansion in the long run costs more than specifying the job properly initially.

Deep fill danger
When it comes to conduit, trunking and cable trays, deep fill is not desirable; it raises the risk of crushing and creates problems for running additional cables in future. For fill rates the rule of halves applies, so on new installations you should fill your trunking or other containment device to 50% of its maximum capacity. This leaves half the remainder (25% of capacity) for future expansion, with the other half (the final 25%) empty.

The logic is pretty unassailable; by never filling the containment to maximum you avoid the need to batter down cables to make them fit. There is always space for the odd cable in a real emergency and you have clear warning when it’s time to install additional trunking.

Never be afraid to demonstrate what’s feasible and what’s not to customers. If they ask you to install 400 cables in a cable tray 225 mm-wide and 50 mm-deep, or seven data cables inside a 25 mm circular conduit, you can show them that this is impossible even if their consultant has told them otherwise. Packing cables too densely always leads to problems eventually and the cost of rectifying disasters is always higher than avoiding them. This is particularly true if a replacement Category 6 system is a future possibility, since like-for-like, Category 6 cables are up to 50% larger in cross-sectional area.

Understandably, customers are under great pressure to cut costs by selecting the lowest tender; they probably have no option in this respect. If you sense you are in danger of being undercut, then it’s your job to explain why the rival solution will cost far more in the long term.

Unfortunately, many new-build systems and major upgrades are placed in the hands of systems integrators with a background in power wiring and little or no perception of structured cabling’s special needs. This is an education task facing the industry as a whole.

Bonds and ties
Many rules of power wiring apply equally to structured cabling, such as the choice between closed conduit or capped trunking and open cable trays. The former gives better protection and the latter easier access, so your choice depends on the need to protect from tampering.

If you are using sealed conduit as opposed to capped trunking, a rule of thumb is to never have more than two bands without including a turning box with a lid for easy access. Otherwise, pulling through may be a strain and a risk hazard to the cables.

All metal containment systems must be electrically bonded to the customer main earth terminal by an earth conductor complying with BS 7671: 1992. Metal cable trays must be linked at every butt joint with a flexible copper strap that allows up to 1 V rms potential difference between it and true earth at the main earth terminal.

Trunking is particularly suitable for use where power cables and structured cabling need to run in the same channel for reasons of space or economy subject to compliance with BS 7671 segregation regulations. PVC trunking ranges in size from 15 x 15 mm up to 150 x 150mm and larger; it is quick and easy to install. Steel trunking affords better protection but is harder to install; it ranges in size from 50 x 50 mm up to 300 x 300 mm. PVC conduit comes as lengths of robust tubing, in set sizes 16, 20, 25 and 32 mm. It is relatively easy to install, requiring no more than basic tools, either surface-mounted or flush in walls. Draw wire or ropes should be left in all conduits to allow future cables to be added.

When it comes to cable ties Krone recommends the use of either hook and loop types (Velcro or similar) cable ties or the Krone Mille-Tie, which uses a patented intelligent grip memory polymer technology, which is pre-set to a pressure that cannot damage Category 5e, 6 or fibre cables.