Tài liệu KRONE - Expensive Lesson docx

EXPENSIVE LESSON top tips 1 Expensive Lesson Going back to the classroom can be a touch demeaning for time served tradesmen with a proven professional understanding of their job. All the same, for electricians moving into the profitable and expanding world of structured cabling, there are some vital new lessons that will cost dearly if ignored. Dyson Booth, KRONE’s installation trainer, conducts a master class for turning wised-up wiremen into equally competent IT cablers. Telling a professionally qualified electrician he’s not capable of installing voice and data cabling without retraining must grate. It’s rather like informing a portrait painter he wasn’t fit to repaint the garage door — nothing short of an insult. There are no hazardous voltages with IT cabling, so why on earth can’t a conscientious tradesman turn his hand from heavy current to this new kind of child’s play? 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 re-installed, wiping out much more than the profit on the jobs, and all because the installers had not been trained in the deceptively similar art of IT cabling. But why should laying cables and installing outlets for telephones and computers call for entirely new skills? Because you’re dealing with a different form of electricity, to put it simply! Similar but different Working with low-voltages is hardly a new experience for electricians — fitting alarms, bells and intercoms is bread and butter for many installers and has been for 100 years or more. With structured cabling for Information Technology (IT) applications we’re moving into rather a different realm, however, as it carries both voice and data. You can argue that the voice side of things is fairly low-tech and I wouldn’t necessarily disagree, but the data signals are an entirely different ballgame. Data rates in the typical Category 5e system are transmitted at frequencies of up to 100MHz, well into the VHF radio spectrum where FM radio broadcasts are to be found. We all know that radio tends to radiate, meaning that confining those radio-frequency signals to the confines of a twisted pair cable without leakage, without losing voltage level (signal strength), and without incurring crosstalk between the various signals is no mean feat. It’s not impossible of course but achieving correct signal balance between pairs in the cable calls for closely controlled geometry of the various conductors – which is every bit as much a function of the installation technique as of the manufacturing process. In this way you can maintain the correct signal impedance and avoid detrimental emissions and crosstalk. Fussy or what? Bending data cables round too tight a radius or compressing the cable with plastic cable-ties tightened too enthusiastically can alter its characteristics sufficient to make them entirely useless for high-speed data. The sole remedy then is to rip them out and replace the lot. In fact the need to avoid crushing, stretching and over bending cable (and upsetting the geometry) is so vital that in the best quality work, plastic cable ties are not the preferred choice. 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-1V domain the battle is against unwanted interference signals, both from adjacent data cables and from entirely separate interference sources. Ingenious techniques mean that crosstalk from other pairs in the same cable can be predicted and hence be eliminated electronically. Alien crosstalk signals, induced from other Category 5e cables or from adjacent power conductors, are different; we cannot correct for them 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 regard to Protective and Functional Earths. It’s important to realise that protective earth connections that are entirely adequate for providing a safety earth connection for user appliances can be inadequate for voice and data applications. This is for two reasons: safety and signal integrity. An example of the former is where the earth wire of a 50-pair communications cable is mistakenly grounded by a connection taken to the nearest 13-amp mains outlet. If a fault occurs in the mains outlet, the potential on the ‘earth’ wire may rise to 240V above ground. Many telecomms installations rely on an earth connection for signalling (the earth being used as a return conductor), for 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 d.c. resistance and ultra-low a.c. impedance. Using the earth of the nearest mains socket as a Functional Earth is clearly inadequate; its impedance will be high to the signal frequencies used in audio and data applications and its resistance, though only a few ohms, 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). From here an independent telecomms earth cable of at least 1.5 sq mm cross sectional area must be taken to all voice and data devices requiring a functional earth connection. Bad economics The technical performance of an industrial power installation will 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 such as data outlets, cable couplers and patch cords can cost dearly, either now in detectable reduced throughput or creating a future problem when customer requirements augment (assuming you’re giving a 15 to 20 year warranty). It’s unwise using components that are not from top-rank manufacturers and far better to install ‘manufacturers systems", in which all components, cable and installation techniques/training are warranted by the manufacturer. EXPENSIVE LESSON top tips 2 Another trap to avoid is saving money on dado trunking in offices and elsewhere. Architects prefer you to use a single slimline housing but they have no comprehension of EMC (electro-magnetic compatibility) issues. As data cabling practice moves towards Category 6, the space inside a single piece of trunking will not allow adequate separation distance between data and power cabling, regardless of safety partitions. To avoid expensive re-provision in future, it makes sense to install separate trunking for power and IT needs from the outset—notify the client of the potential pitfalls of not doing so. Further information • Wiring Code (parts 1 & 2). Office of Telecommunications (OFTEL), 020 7634 8888. • BS 6701 (Telecommunications cabling). British Standards Institution, sales office 01908-320033. • Telephone Installation Handbook, by Steve Roberts. Newnes, ISBN 0-7506-3427-8. • Implementing Structured Wiring Schemes, by Karl Tryner. KRONE Technique (UK) Ltd. EXPENSIVE LESSON top tips 3 CHECKLIST: SIX EASY MISTAKES THAT COULD COST YOU DEAR • IT cabling too tightly bent, distorting cable geometry. • Cable ties applied too tight, distorting cable geometry. • Screening shields severed and rendered ineffective by clumsy crimping. • Using inferior outlets and patch-cords (total system performance can be degraded). • IT and power cables laid too close together, causing data corruption and potential safety risk. Install separate trunking instead. • IT cable ground conductors bonded to mains earth, producing potential safety risk. All the above will result in acceptance test failures, subsequent claims or even death—and may require re-installation from scratch. EXPENSIVE LESSON top tips 4 KRONE's one-day Electrical Conversion training course is only £275.00. Before Patch cable nominal 100 Ohm Impedance After Cable after disturbing twists increased characteristic impedance Cable prior to disturbing twists 100 ohms 200 ohms 1" pipe 1 / 2 " pipe 1" pipe An impedance mismatch reflects energy like water is reflected here . 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. 0190 8-3 20033. • Telephone Installation Handbook, by Steve Roberts. Newnes, ISBN 0-7 50 6-3 42 7-8 . • Implementing Structured Wiring Schemes, by Karl Tryner. KRONE