Trade Secrets

Telling a professionally qualified electrician that they're not capable of installing voice and data cabling without retraining must grate. There are no hazardous voltages with information technology (IT) cabling, so why can't a conscientious tradesman turn from heavy current to this new kind of child's play? The trouble is, it's not child's play and some very expensive lessons have been learned by electrical contractors unaware of the intricacies of voice and data cabling. Over the past few years several complete systems have had to be removed and reinstalled, wiping out much more than the profit on the jobs. And all because the installers had not been trained in the art of IT cabling.

But why should laying cables and installing outlets for telephones and computers call for entirely new skills? To put it simply, because it uses a different form of electricity.

Working with low voltages is hardly new to electricians. However structured cabling for IT moves into rather a different realm, as it carries both voice and data. It could be argued that the voice side is fairly low-tech, but the data signals are an entirely different ball-game. Data in a Category 5e system is transmitted at frequencies of up to 100 MHz, well into the vhf radio spectrum.

As radio tends to radiate, confining rf signals to a twisted-pair cable without leakage, without losing voltage level (signal strength) and without incurring cross-talk, is no mean feat. It's not impossible of course, but achieving correct signal balance between pairs calls for closely controlled geometry of the various conductors, which is as much a function of the installation as of the manufacturing process. In this way the correct signal impedance can be maintained and detrimental emissions and cross-talk avoided.

Bending data cables round too tight a radius or compressing the cable with plastics cable ties tightened too enthusiastically can alter its characteristics sufficiently to make them entirely useless for high-speed data. The sole remedy then is to rip them out and replace the lot.

Only good old-fashioned cable lacing will satisfy some very particular customers' specifications, but in the majority of cases hook-and-eye fastenings and hook-and-loop straps are the order of the day.

Keep it clean
Immunising cables against unwanted effects is another area in which power electricians are fighting entirely different battles from IT cabling installers. At mains voltage most problems can be resolved by earth bonding or providing clean feeds, whereas in the sub-1 V domain the battle is against unwanted interference signals.

Ingenious techniques mean that cross-talk within cables can be predicted, hence eliminated, electronically. Alien cross-talk, induced from other Cat 5e cables or adjacent power conductors, is different; it cannot be compensated for electrically, so it's vital that the cable's own defence mechanisms are not degraded by careless installation.

Data and telecommunication practice varies from power wiring also with regards to protective and functional earths. Protective earth connections that are adequate for user appliances can be inadequate for voice and data applications. This is for two reasons: safety; and signal integrity. For example, if the earth wire of a 50-pair communications cable is mistakenly grounded by a connection taken to the nearest 13 A mains outlet, if a fault occurs in the outlet, then the potential on the earth may rise to 240 V above ground.

Many telecoms installations rely on an earth connection for signalling, safely grounding surge protectors, or for grounding a braided screen that shields balanced signal conductors from extraneous interference. Each of these situations calls for a direct connection to earth with virtually zero dc resistance and ultra-low ac impedance. Using a socket as a functional earth is clearly inadequate; its impedance will be high to the signal frequencies used in audio and data and its resistance may well upset sensitive signalling functions.

Connection to a main electrical earthing point is permitted (a dedicated earth electrode is preferred) and BS 6701 recommends that a separate functional earth connection be made in each building.

Bad economics
The technical performance of an industrial power installation may not be harmed by the use of cheaper domestic-grade fittings in a few locations, but the same doesn't apply with data cabling. Category 5e specifications allow fairly broad tolerances that can degrade the performance of critical data networks significantly. Cost-cutting on fittings can cost dearly, either now in reduced throughput or in future when client requirements augment. It's far better to install manufacturers' systems, in which all components and installation techniques/training are warranted.

Another trap to avoid is saving money on dado trunking. As data cabling practice moves towards Cat 6, the space inside a single piece of trunking will give inadequate separation distance between data and power cabling, regardless of safety partitions. To avoid expensive future upgrades, it makes sense to install separate trunking for power and IT from the outset.