eat 105 chapter 3

Electrical Installation Designs

CHAPTER 3

CABLE SELECTION AND WIRING SYSTEM


CABLE SELECTION AND WIRING SYSTEM

� Cables are means by which electrical energy isdistributed from its source to its point of use.

� Defined: length of insulated single conductor or of twoor more such conductors each provided with its owninsulation which are laid up together.

� The insulated conductor or conductors may or not beprovided with overall covering for mechanical protection.

� A single core cable refers to a cable that has only oneinsulated conductor with its own cable sheath, and amulti core cable refers to a cable that has multiple coresof insulated conductors within one common sheath.


� A conductor may be defined as the conducting portion ofa cable, which consists of a single wire or group of wiresin contact with each others.

� The ability of a material to be a good or bad conductorof electricity depends on the composition of thatmaterial, i.e. its resistivity.

� Conductors may be divided into two groups:

a) solid conductor

b) stranded conductor

CABLE COMPONENTS: CONDUCTOR


� Cables are usually a classified according to the type ofinsulation used. The type of insulation to be used musthave the following properties:

� It should have a high specific resistance� It should have high dielectric strength� It should be tough and flexible� It should not be hygroscopic, i.e, it should not absorb

moisture from air or surroundings.� It should be capable of standing high temperatures

without much deterioration.� It should be non inflammable.� It should not be attacked by acids or alkalies.� It should be capable of withstanding high rupturing

voltages.

CABLE COMPONENTS: INSULATION


CABLE COMPONENTS: SHEATHING

� Sheath - a metallic component over the insulation of acable. Various metals may be used as the sheath of acable such as lead, copper, aluminium, bronze, steel,etc.

� The choice of sheathing material depends on itsenvironmental performance: ambient temperature;flexibility; resistance to abrasion, water, oil and otherchemicals; performance under fire conditions; andcompatibility with other materials with which a cable is incontact during its operational life.

� The sheathing material must also be chemicallycompatible with the other materials used in the cableboth during and after processing. While insulations arechosen primarily for their electrical characteristics,sheaths are selected on their physical properties.


� Thus, not all insulations are suitable sheaths. However,in general, insulation and sheath materials are similar:e.g. a thermoplastic sheath protecting thermoplasticinsulation.

CABLE COMPONENTS: SHEATHING


CABLE COMPONENTS: ARMOURED

� When cables are not installed in conduit or trunking,they may require armour, most commonly provided bygalvanized steel wire (GSW) helically applied in a singlelayer and known as `SWA' (single-wire armour).

� Most types of power cable require mechanical protectionand/or an earth conductor to carry fault currents. Formost distribution cables this is provided by SWA. Cableswith aluminium sheaths seldom require armour. Cableswith lead sheaths may be armoured with steel tape,which is cheaper, but SWA is preferred in the UK for theheavier, higher voltage cables for 10 kV and upwardsbecause it increases corrosion resistance and thelongitudinal strength of the cable for installationpurposes. Steel tape is normally protected againstcorrosion by bitumen, but if better corrosion resistance isrequired, the tape may be galvanised


Stranded Cable

Non Armoured Cable

Armoured Cable


TYPES OF INSTALLATION

The final choice of a wiring system must restwith those designing the installation and thoseordering the work, but whatever system isemployed, good workmanship and the use ofproper materials is essential for compliancewith the IEE Regulation 130-02-01. Thenecessary skills can be acquired by anelectrical trainee who has the correct attitudeand dedication to his craft.



PVC insulated and sheathed wiring systems areused extensively for lighting and socketinstallations in domestic dwellings. Mechanicaldamage to the cable caused by impact,abrasion, penetration, compression or tensionmust be minimised during installation(Regulation 522-06-01).



a) Conduit Installation – Steel conduit/GI or uPVC

b) Trunking Installation

c) Tray Installation

d) Others – IEE Regulation – Table 4A1 page 210-215


CABLE SIZE CALCULATIONS

The size of a cable to be used for aninstallation depends upon:

a) the current rating of the cable underdefined installation conditions and

b) the maximum permitted drop in voltage asdefined by Regulation 525-01(pg 100).


Some symbols and definitions will be used incable size calculations as:

Iz is the current carrying capacity of the cablein the situation where it is installed.

It is the tabulated current for a single circuitat an ambient temperature of 30°C.

Ib is the design current, the actual current tobe carried by the cable.

In is the rating of the protecting fuse or circuitbreaker.




I2 is the operating current for the fuse orcircuit breaker (the current at which thefuse blows or the circuit breaker opens).

Ca is the correction factor for ambienttemperature.

Cg is the correction factor for grouping.

Ci is the correction factor for thermalinsulation.

Cr is the 0.725 correction factor to be appliedwhen the semi enclosed fuses protect thecircuit.


Current Cable Rating

The factors which influence the current rating are:

a) the design current, Ib , the actual currentto be carried by the cable. The cablecurrent carrying capacity, Iz must be equalto or greater than the circuit design current,Ib;

b) the type of cable either PVC, PVC/PVC,MICC, copper conductor or aluminiumconductor (refer Table 5.2)

bzII ≥


c)

the installed conditions either clipped tosurface or installed with other cables in atrunking. The correction factor may beneed applying for installation condition isthe grouping correction factor, Cg given inTables 4B1, 4B2 and 4B3 (pg 217&218), ofthe IEE regulations and the correctionfactor to be used when cables are enclosedin thermal insulation, Ci. The Cg is notapplied applied if the cable is to be clippeddirect to a surface or not in contact withother cables.



Cont…c)Regulation 523-04 gives us three possible

correction values:where one side of the cable is in contact

with thermal insulation, we must readthe current rating from the column inthe table which relates to ReferenceMethod 4.

where the cable is totally surrounded overa length greater than 0.5m, we mustapply a factor of 0.5.

where the cable is totally surrounded overa short length, the appropriate factor isgiven in Table 52A of the IEERegulations should be applied.



d) the ambient temperature, the cableresistance increases as temperature increasesand insulation may melt if the temperature istoo high. The correction factor may be needapplying for this condition is the ambient orsurrounding temperature correction factor,Ca, which is given Tables 4C1 and 4C2 ofAppendix 4 of the IEE Regulations.



e) the type of protection – for how long will thecable have to carry a fault current? Choosethe type and rating of protective device (fuseor circuit breaker) to be used, In.

To ensure correct protection from overload, itis important that the protective deviceoperating current or the operating current forthe fuse or circuit breaker (the current atwhich the fuse blows or the circuit breakeropens), I2 is not bigger than 1.45 times thecurrent carrying capacity of the cable, Iz.



It is important to appreciate that the operatingcurrent of a protective device is always largerthan its rated value. In the case of a back-upfuse, which is not intended to provideoverload protection, neither of theserequirements applies.


bnzIII ≥≥

zII 45.1

2≥

znII ≤


When the semi enclosed fuses is used toprotect the circuit, the 0.725 correction factormay be need applying for this condition.



Here then are the eight basic steps in asimplified form:

• Determine the design current Ib.• Select the rating of the protection, In.• Select the relevant rating factors (CFs).• Divide In by the relevant (CFs) to give

tabulated cable current carrying capacity,It.

• Choose a cable size to suit It.• Check the voltage drop.• Check for shock constraints.• Check for thermal constraints



Cable Capacities of Conduit and Trunking

Single PVC insulated conductors are usually drawninto the installed conduit to complete theinstallation.

Having decided upon the type, size and number ofcables required for a final circuit, it is thennecessary to select the appropriate size of conduitto accommodate those cables.


The following three cases are dealt with:

Single core thermoplastic (pvc) insulated cables

1. In straight runs of conduit not exceeding 3m inlength Table 5A & 5B.

2. In straight runs of conduit exceeding 3m inlength, or in runs of any length incorporatingbends or sets. Table 5C & 5D.

3. In trunking. Table 5E & 5F.


Conduit Capacities



The tables in Appendix 5 of the On Site Guidedescribe a factor system for determining the sizeof conduit required to enclose a number ofconductors. The method is as follows:

1. Identify the cable factor for the particular sizeof conductor. (This is given in Table 5A forstraight conduit runs and Table 5C for cablesrun in conduits which incorporate bends.)

2. Multiply the cable factor by the number ofconductors, to give the sum of the cablefactors.


3. Identify the appropriate part of the conduitfactor table given by the length of run andnumber of bends. (For straight runs ofconduit less than 3m in length, the conduitfactors are given in Table 5B. For conduitruns in excess of 3m or incorporating bends,the conduit factors are given in Table 5D.)

4. The correct size of conduit to accommodatethe tables is that conduit which has a factorequal to or greater than the sum of the cablefactors.


PPKSE




Trunking Capacities

The ratio of the space occupied by all the cablesin a conduit or trunking to the whole spaceenclosed by the conduit or trunking is known asthe space factor. Where sizes and types of cableand trunking are not covered by the tables inAppendix 5 of the On Site Guide a space factor of45% must not exceeded. This means that thecables must not fill more than 45% of the spaceenclosed by the trunking. The tables of Appendix5 take this factor into account.


To calculate the size of trunking required toenclose a number of cables:


1. Identify the cable factor for the particular sizeof conductor (Table 5E).

2. Multiply the cable factor by the number ofconductors to give the sum of the cablefactors.

3. Consider the factors for trunking (Table 5F).The correct size of trunking to accommodatethe cables is that trunking which has a factorequal to or greater than the sum of the cablefactors.

eat 105 chapter 3

Education