Guide on Cable Shielding

8/12/2019 Guide on Cable Shielding

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20 | advancing information transport systems | www.bicsi.org

When telecommunications cables are routed nextto large electromagnetic elds, surplus voltage andcurrent can be induced on them. If the power levelof the electrical cable is large enough, the electrical

noise can interfere with operation and performance ofthe telecommunications applications running on thecabling. Electrical and data systems designers must befamiliar with this phenomenon and ensure that the two

systems can work in harmony.For analog voice communication, electromagnetic

interference (EMI) can create psophometric noise, whichdegrades transmission quality.

In data communication, excessive EMI reducesthe ability of distant receivers to effectively detect datapackets. The result of this inability to detect data packetsis an increase in network congestion and network traf cas a result of errors and due to packet retransmissions.

Sources of Coupling Between Electricaland Data Cables The coupling between power lines andtelecommunications cables may be a result of one ormore of the following types of couplingconductive,capacitive or inductive coupling. Conductive coupling is the transfer of energy bymeans of physical contact. This type of coupling is alsoknown as direct coupling. In a commercial and industrialfacility cabling installations, the incidence of conductivecoupling is typical when the grounding and bondingsystems utilized for power and telecommunicationssystems are not appropriately isolated from each other.

Capacitive coupling is the transfer of energy fromone circuit to another by means of mutual capacitancebetween circuits. This coupling can be intentional oraccidental. Capacitive coupling can develop betweentelecommunications and power cables that run inparallel for long lengths through a building or otherstructure. The capacitance between two cables or conductors is

caused by coupling between the power and data circuits.The value of the capacitance will vary with and dependupon the distance between the power and data circuits.The value of this capacitive coupling will be less for largedistances and more for short distances. To reduce thevoltage noise level from the capacitive coupling betweencables, either the impedances can be increased or thecapacitance can be decreased. Screened twisted-paircabling (ScTP) can be utilized to shield the cables fromthe circuit with the noise. This screening of the cable willreduce the value of capacitance.

For situations where it is not an option to increase

the impedance or decrease the capacitance, screenedcabling may be the only option.

Inductive coupling refers to the transfer of energyfrom one circuit component to another through a sharedmagnetic eld. A change in the current ow throughone conductor or cable can induce a current to owin another conductor or cable. This coupling may beintentional or unintentional. The common buildingtransformer works based on this type of coupling. When current ows in a circuit while feeding a loadin the system, it develops a magnetic ux proportional to

As data transmission speeds increase, separation of power and

data cables is more critical. BY KEITH LANE

Determining theAppropriate Separation oData and Power Cables

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the c urrent that is owing in the circuit. This magneticux may induce noise voltage into a nearby conductor.

This can result in a current in the data or voice circuit.This type of coupling is very common between data andpower conductors in commercial facilities.

The layout of the conductors and the space betweentwo cables establishes the strength of the inductivecoupling. The use of metallic or nonmetallic raceway orcable tray or other pathways can affect the amount ofinduced elds that affects the data or voice cables. The strength of the magnetic eld is directlyproportionate to the current in the disturbing cable andinversely proportionate to the distance between thetelecommunications and power cables. As illustrated inthe examples below, power cables with higher powerlevels (kilovolt-ampere [kVA]) will require greaterseparation between voice and data cables. In order to minimize the effect of inductive couplingbetween circuits, it is essential to safeguard cablegeometry for the complete cable length and to keepadequate separation between power and data cables.

Design for Proper Separation of Cables When considering the effects from interferencebetween power and data cables, electrical and datasystem designers must consider all of these effectstogether. This combined effect comes from conductive,capacitive and inductive coupling. These combinedeffects can be very destructive to data and voice signals Providing the proper separation between electricaland data systems is essential. Too little separation andthe 60 hertz (Hz) noise from the electrical system caneffect the transmission of the data signals. The projectcould be impacted signi cantly on the cost side from toomuch separation. There are many sources for a designer or engineerto identify the appropriate separation between theelectrical and the data systems. The two main sourcesshould be the National Electrical Code (NEC ) and BICSIsTelecommunications Distribution Methods Manual (TDMM) .The TDMM references standards from ANSI, TIA and EIA. ANSI/TIA/EIA569-A indicates that the installedseparation of both the telecommunications cable andthe electrical cable should be governed by the applicableelectrical safety code. The NEC (NFPA 70) , Article 800.133(2005 NEC) indicates the separation requirements. Thissection of the NEC speci es the following: Communica-tion wires and cables shall be separated at least 50 mm(2 in) from conductors of any electric, power, Class 1,nonpower limited re alarm, or medium power networkpowered broadband communication circuits.Two exceptions are noted in the NEC:

Exception #1: Where either (1) all of the conductors ofthe electrical light, power, Class 1, nonpower limited realarm and medium power network powered broadbandcommunications circuits are in a raceway or in metalsheathed, metal clad, nonmetallic sheathed, type

AC, or type UF cables, or (2) all of the conductors ofcommunications cable are encased in raceway.

Exception #2: Where the communications wires andcables are permanently separated from the conductorsof electrical light, power, Class 1, nonpower limited realarm, and medium power network power broadbandcommunications circuits by a continuous and rmly

xed nonconductor such as porcelain tubes or exibletubing, in addition to the insulation of the wire.

Electrical and data system designer and engineers

should remember that NEC is primarily written for safetypurposes; it is not intended to make recommendationsfor optimum performance of communication systems.The 50 mm (2 in) separation should be viewed as asafety issue only, not driven by performance issues of thesensitive data systems. There are many excerpts about how importantseparation is. Network cable solutions supplier Siemonrecommends the following separation for pathways andspaces based on the power levels of the power cable.

Unshielded Twisted-Pair n For less than 3 kVA: 50 mm (2 in) for pathways and

50 mm (2 in) for spacesn For > 3 < 6 kVA: 1.5 m (5 ft) for pathways and 3 m

(10 ft) for spacesn For > 6 kVA: 3 m (10 ft) for pathways and 6 m (20 ft)

for spaces

Screened and Shielded Cables n For less than 3 kVA: 0 mm (0 in) for pathways and

0 mm (0 in) for spacesn For > 3 < 6 kVA: 0.6 m (2 ft) for pathways and 0.6 m

(2 ft) for spacesn For > 6 kVA: 0.9 m (3 ft) for pathways and 0.9 m

(3 ft) for spaces

The separation requirements for screened andshielded cables are obviously not as signi cant as forunshielded twisted-pair (UTP) cable, but the cost for thiscable would exceed standard UTP cable. The decision toeither utilize the more expensive cable or to ensure thatseparation requirements are met must be weighed by thedesigner to ensure that the most effective methods orcables are utilized.


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Examples of Separation Calculations To illustrate the required separation using theSiemon model, the power level of a 20 amp circuit at 120V = single phase with 10 amps of load:10 amps * 120 Volt = 1.2 kVA The recommendation would

be for 50 mm (2 in) of separation for both pathways andspaces. If the power cable was fed at 208 volt single phasefrom a 30 ampere breaker with 15 amperes of load, thetotal kVA would be as follows: 15 amps * 208 V = 3.12 kVA. For this example, the recommendation would be for1.5 m (5 ft) for pathways and 3 m (10 ft) for spaces forUTP and 0.6 m (2 ft) for both pathways and spaces forscreened or shielded telecommunications cable. For a nal example, assume a 5 horsepower motor at240 volt single phase. The total full load amperes are 28:28 amps * 240 V = 6.72 kVA. For this example, therecommendation would be for 3 m (10 ft) for pathwaysand 6 m (20 ft) for spaces for UTP and 0.9 m (3 ft) forboth pathways and spaces for screened or shieldedtelecommunications cable. By utilizing the proper physical separation distances,the data system designer can still avoid EMI with theuse of UTP cabling. In most design situations whereproper physical separation can be maintained betweenpower and data systems, UTP cabling is the ideal cablingmedia. On the other hand, in situations where minimumseparation distances cannot be maintained for UTPcabling, screened twisted-pair (ScTP) or shielded shieldedtwisted-pair (SSTP) cable can be utilized.

Conclusion Installing cabling with no consideration of potentialsources of EMI can be harmful to network systems

performance and data transmission quality.Shielding, barriers and the use of optical ber

also reduce separation requirements. Optical bertransmitters are devises that include lasers or LED sourcesand do not emit or receive EMI. With immunity to bothEMI and radio frequency interference (RFI), optical beris a more suitable solution for certain applications. Circuit imbalance, the presence of harmonics andthe physical separation of the wires (i.e., bus duct hasmore separation between the phases than pipe and wire,and metal clad cable with its twisted wires has the leastseparation) determine the actual EMI emitted from thepower cables. Harmonics present in the electrical systemrepresent higher frequencies (orders of magnitude above60Hz) and can more negatively affect data systemsthan current traveling at the fundamental frequency.Unfortunately, many of ce or data center environmentswith high volumes of data cables house a large number

of computers and other switch mode power suppliesthat cause harmonics to be re ected into the electricalpower system. The data system parameters will also determinethe amount of bit error rate (BER) and amount ofcrosstalk and noise allowable. Data systems with highertransmission speeds will be more adversely affected byEMI. Therefore, as transmission speeds of data systemscontinue to increase, the design engineer must be moreconcerned with maintaining the separation betweenthe systems. The design engineer should be aware of all coderequirements and issues of good design practice aswell as an understanding of the type of power andcommunication systems involved in a project priorto determining the appropriate separation. The intent

of this article is to illustrate some of the issuesinvolved in making this determination. n

References: 1. The NEC 20052. Siemon white paper, Electromagnetic Interference

Keith Lane, P.E., RCDD/NTSSpecialist, TPM, LC, LEED A.P.

Keith Lane is a principal and partner ofLANE COBURN & ASSOCIATES, LLC,which offers complete electrical engineeringincluding electrical and data systemsinfrastructure design. Keith can bereached at +1 206.499.5221 or at [email protected] or at www.lanecoburn.com.

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Guide on Cable Shielding