design of electrical services for buildings 4th edition 1

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Design of Electrical Services for

Buildings


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Contents

Preface to third edition vi

Preface to fourth edition viii

1 Accessories 1

2 Cable 29

3 Wiring 42

4 Cable rating 9

! Circuits ""

Distribution #

" $ighting 1%3

# Po&er 12#

9 Protection 131

1% $ightning 'rotection 212

11 Design e(a)'le 2!


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Cha'ter 1

Accessories

*ntroduction

+ro) the user,s 'oint of vie& the electricit- service in a building consists of light

s&itches. soc/ets. cloc/ connectors. coo/er control units and si)ilar outlets0 Such fittings

are collectivel- /no&n as accessories this na)e ca)e about because the- are accessor-

to the &iring. &hich is the )ain substance of the installation fro) the designer,s and

installer,s 'oint of vie&0 o the). the &a- the outlets are served is the )aor interest. but

it is uite secondar- to the user &ho is concerned onl- &ith the a''earance and function

of the outlet0 *n the co)'lete electrical installation of a building the &iring and

accessories are interde'endent and neither can be full- understood &ithout the other a

start has to be )ade so)e&here ho&ever. and in this boo/ it is 'ro'osed to consider

accessories first0

S&itches

A s&itch is used to )a/e or interru't a circuit0 5or)all- &hen one tal/s of s&itches one

has in )ind light s&itches &hich turn lights on and off0 A co)'lete s&itch consists of

three 'arts0 here is the )echanis) itself. a bo( containing it. and a front 'late over it0

he bo( is fi(ed to the &all. and the cables going to the s&itch are dra&n into the bo(0 After

this the cables are connected to the )echanis)0 o carr- out this o'eration the electrician)ust 'ull the cables a&a- fro) the &all sufficientl- to give hi)self roo) to &or/ on the

bac/ of the )echanis)0 6e then 'ushes the )echanis) bac/ into the bo( and the length of

cable that he had to 'ull out fro) the &all beco)es slac/ inside the bo(0 *t is therefore

i)'ortant that the bo( is large enough to acco))odate a certain a)ount of slac/ cable at the

bac/ of the )echanis)0

Standard bo(es for recessing &ithin a &all are 1. 2!. 3! and 4")) dee'0 So)eti)es

the &iring is done not in the de'th of the structural &all. but &ithin the thic/ness of the

'laster0 +or use &ith such &iring. bo(es are )ade 1)) dee' 7'laster de'th bo(es80 *t is

often necessar- to install &iring and accessories e('osed on the surface of &all0 +or sucha''lications surface bo(es are )ade &hich are both )ore robust and neater in a''earance

than bo(es &hich are to be recessed in &alls and )ade flush &ith the surface. although

the- are )ade to si)ilar de'th0 -'ical bo(es of both t-'es are sho&n in +igure 1010


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Design of electrical services for buildings 2

+igure 101 Bo(es 7Courtes- of 0:0Electric $td8

;oc/er o'erated s&itches are illustrated in +igure 1020 *t has a roc/er &hich is 'ivoted

at its centre and &hich carries a s'ring


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Accessories 3

+igure 102 S&itch )echanis)s

here is a )a(i)u) current &hich the contacts of an- 'articular s&itch can )a/e or

brea/. and a )a(i)u) voltage that the contact ga' can &ithstand0 A s&itch )ust not be

'ut in a circuit &hich carries a current greater than that &hich the s&itch can brea/0 ost

)anufacturers )a/e s&itches in standard ca'acities. the lo&er being rated at !. . 1!. or

2%A and the higher rating of 4!A for control of instantaneous sho&er units0Discharge lights are an inductive load. and the induced voltage surge &hich occurs

&hen an inductive load is bro/en )ust be ta/en into account in selecting a s&itch for.


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sa-. fluorescent lighting0 *t &as for this reason that so)e of the older s&itches had to be dereat Britain the )aorit- of soc/et outlets

intended for do)estic or co))ercial use are BS 133 soc/ets. and are designed to acce't 13A

'lugs0 hese 'lugs have three rectangular 'ins and the soc/ets have three corres'onding

rectangular slots to ta/e the 'ins0 Each 'lug also has a fuse inside it. so that each a''liance

has its o&n fuse at the feeding end of its fle(ible cable or cord0 his 'rotects the cable or

cord. and the fusing arrange)ents of the building &iring need'rotect onl- the 'er)anentfi(ed &iring of the building0

6o&ever. there )a- be older installations still in e(istence and 'lugs and soc/ets for

use &ith the) are still being )anufactured0 he older fittings. all have round 'ins and

soc/ets0 he- are rated at 2A. !A and 1!A0 he 1!A 'attern is still used in the ;e'ublic

of South Africa0 he s'acing of the 'ins and soc/ets are different for the different ratings0

his )a/es sure that a 'lug of one rating cannot be inserted. even &ilfull-. into a soc/et

of a different rating0 Plugs and soc/ets rated at 2 and !A are available in both t&o and

three


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Accessories "

s&itch )echanis)s built into soc/et outlets for this 'ur'ose are of the sa)e t-'e as thoseused for lighting s&itches0 *t is 'ossible to leave a 'lug half in and half out of a soc/et sothat on older t-'es of 'lug. 'arts of the bare 'ins are left e('osed0 *f the soc/et is'er)anentl- live the e('osed 'art of one of the 'ins is live and in this half


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Design of electrical services for buildings #

+igure 10! Soc/et outlets 7Courtes- of0:0 Electric $td8

electricall- heated to&el rails. in ones &here connection of such eui')ent is

'er)itted the installation of )ains


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Accessories 9

+igure 10 +used connection units

7Courtes- of 0:0 Electric $td8

+igure 10" Shaver outlet 7Courtes- of

0:0 Electric $td8

Shaver outlets

he use of shaver outlets is described in Cha'ter 90 he outlet itself consists of a t&o


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Design of electrical services for buildings 1%

Coo/er control unit

Electric coo/ers ta/e a )uch larger current than )ost other do)estic a''liances0 he-

therefore reuire heavier s&itches than those used for lighting or in soc/et outlets0

oreover. it is usuall- convenient to have a soc/et outlet near the coo/er in addition tothe coo/er s&itch itself0 Coo/er control units are. therefore. )ade &hich have a 4!A

7so)eti)es onl- a 3%A8 s&itch &ith outgoing ter)inals for a 'er)anent cable connection

to the coo/er and &hich also contain an ordinar- 13A s&itched soc/et outlet0 he coo/er

s&itch is double 'ole. that is to sa-. on o'ening. it disconnects both 'hase and neutral

lines. and the unit also has a substantial ter)inal for the circuit 'rotective conductors0

+igure 10# Coo/er control unit

7Courtes- of 0:0 Electric $td8

A coo/er control unit is sho&n in +igure 10#0 Again units are available for both flush andsurface fi(ing0 he unit is )ounted &ithin eas- reach. to the side of the coo/er so that theo'erators can s&itch off the coo/er uic/l- in an e)ergenc- &ithout 'utting the)selvesin danger0 he cable fro) the unit to the coo/er is usuall- hidden in the &all and co)esout at lo& level behind the coo/er0 A s'ecial fle( outlet cover is )ade to fi( on the

surface of a bo(. &hich is let in flush &ith the &all to )a/e a neat outlet fro) the &all tothe coo/er0 he fle( outlet is nor)all- su''lied as a loose 'iece &ith the coo/er control

unit0

Bo(es

he use of bo(es for housing s&itches and other accessories has alread- been described0

he sa)e bo(es are used for conduit installations0 When &iring is done b- dra&ing cable

through conduit. access )ust be 'rovided into the conduit for 'ulling the cable in0 Also

&here the 'aths of cables branch t&o or )ore conduits )ust be connected together0 +or

both these reasons. a bo( of so)e sort is needed for use &ith conduit. and the t-'e of bo(

used is the sa)e as that used for housing s&itches0 As stated in the section on s&itches.

bo(es are available for recessing in &alls. recessing &ithin the narro& de'th of 'laster


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Accessories 11

onl- or for fi(ing to the surface of &alls0 Where a large nu)ber of conduits is to be

connected to the sa)e bo(. the bo( is )ade longer in order to acco))odate the) side b-

side0

*t can be seen in +igure 101 that the bo(es have a nu)ber of circles on the)0 hese are

called /noc/


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= outlets

6ousing design toda- has to acce't that ever- flat. )aisonette or house &ill have a

television &hich )a- reuire connection to an outdoor aerial0 *t

+igure 101% = outlets 7Courtes- of

0:0 Electric $td8

&as co))on to 'rovide a co))unal aerial s-ste) &hich serves all d&ellings on an

estate fro) a single aerial0 he chief reason for doing this is that it avoids the ugliness of a

large nu)ber of aerials. all of different 'atterns. 'ut u' close to each other b- different 'eo'le0

*t has the further advantage that one 'o&erful aerial erected in a carefull- chosen 'osition can

give better rece'tion than the aerials &hich individual occu'iers install0 his signal )a- be

fanned out to individual d&ellings through a )ains su''lied booster0 errestrial channels)a- be accessed through satellite aerials0

*f a co))unal aerial s-ste) is installed. it beco)es necessar- to run a television aerial

cable fro) the aerial to an outlet in each d&elling. or hotel bedroo)0 here has to be a

suitable ter)inal in the roo). and this ta/es the for) of a soc/et ca'able of acce'ting the

coa(ial 'lugs used on the end of aerial cable0 An outlet of this /ind is sho&n in +igure

101%0 Since a television set also needs a 'o&er su''l-. it is usual to 'rovide a )ains

soc/et outlet near the aerial outlet0 ?ne )anufacturer )a/es a co)bined unit having an

aerial soc/et and 13A soc/et outlet &ithin one housing0

+or radios &hich reuire both an aerial and an earth connection. s'ecial t&o


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Accessories 13

+igure 1011 ele'hone outlets7Courtes- of 0:0 Electric $td8

be connected later0 he onl- essential reuire)ent is an o'ening through &hich standard

tele'hone cable can be brought out neatl-0 A 'late &ith a suitable outlet &hich fits into a

standard bo(. is sho&n in +igure 10110

he )ore )odern 'ractice is to connect each tele'hone set to the 'er)anent

installation via a teleco) soc/et and 'lug in the :. a B 'attern in used. &hich isslightl- different to the S 'attern0 he soc/et for)s 'art of a lid &hich scre&s onto a

standard conduit bo( at the agreed outlet 'ositions0 An outlet of this /ind is sho&n in

+igure 10110

Cloc/ connector

S'ecial outlets are )ade to &hich electric cloc/s can be easil- connected0 A t-'ical one is

sho&n in +igure 1012 and can 'erha's be considered as a s'ecial


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&iring is not suitable for such a large current and )ust. therefore. have its o&n 'rotection

at the 'oint at &hich the su''l- to it branches fro) the )ain circuit0 he necessar-

'rotection is 'rovided b- the fuse in the connector0 he front of the connector has an

o'ening through &hich the cloc/ cable can be ta/en out to the cloc/0 *n )ost cases. the

cloc/ connector is )ade flush &ith the &all and the cloc/ is subseuentl- fi(ed over it06o&ever. surface connectors are available. and in this case the cloc/ &ould be fi(ed ne(t

to the connector &ith a short length of cable run on the surface of the &all bet&een the

cloc/ and connector0 With the develo')ent of uart batter- cloc/s. cloc/ connectors are

seldo) used0

$a)'holders and ceiling roses

*n 'ublic buildings the lu)inaires are fi(ed as 'art of the electrical installation0 *nhousing. the choice of the la)'shade or lu)inaire is usuall- left to the o&ner or tenant and

is )ade once the d&elling is occu'ied0 Plain la)'holders are. therefore. 'rovided &hich

&ill acce't ordinar- 1%%W and 1!%W tungsten bulbs. and &hich usuall- have a ring or s/irt

to &hich a nor)al la)'shade or si)ilar lu)inaire can be attached0 he to' of the

la)'holder scre&s do&n to gri' the fle(ible cable cord on &hich it is sus'ended fro) the

ceiling0 -'ical la)'holders are sho&n in +igure 10130

he fle(ible cord on &hich the la)'holder is sus'ended 'erfor)s t&o functions0 *t

carries the electric current to the la)'. and it su''orts the &eight of the holders. la)' and

shade0 *ts 'h-sical strength is. therefore. ust as i)'ortant as its current carr-ing ca'acit- and

it has to be selected &ith this in )ind0 At the ceiling itself. the &iring in 7or on8 the ceiling)ust be connected to the fle(ible cord0 he connection is )ade b- )eans of a ceiling rose.

&hich is illustrated in +igure 10140 *t consists of a circular 'lastic housing &ith a ter)inal

bloc/ inside and a bushed o'ening on the underside &here the fle(ible cord to the

la)'holder can co)e out of the rose0 *n installations &hich have the )ain &iring inside the

ceiling. this &iring enters the rose through the bac/ or to' of the rose &hen the )ain

&iring runs e('osed on the surface of the ceiling. it enters the rose through a cut


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Accessories 1!

+igure 1013 $a)'holders


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+igure 1014 Ceiling roses

*n so)e situations. it is undersirable to have the la)'holder hanging on the end of a

fle(ible cable &hile there is no obection to having the la)' at ceiling height0 *n such

cases. one )a/es use of a batten la)'holder. &hich is illustrated in +igure 101!0 *t

co)bines the ter)inal bloc/ of the ceiling rose &ith the la)'holder in one fitting. and it

can be scre&ed directl- to a standard circular bo( on the ceiling0 A batten la)'holdercould also be used to fi( a light to a &all. but the la)' &ould 'roect 'er'endicularl-

fro) the &all0 he angled batten


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Accessories 1"

+igure 101! Batten la)'holders

+igure 101 Angled batten la)'holder

surface behind it0 Standard co)'onents are available for this and are /no&n as 'attresses0

A bo( for use &ith a circular soc/et outlet is sho&n in +igure 101". &hich also gives a

s/etch sho&ing the use of the bo( &ith surface conduit0 he inclusion of the bo( )a/es it

'ossible for the cables to enter the soc/et outlet fro) the bac/. &hereas &ithout it. there

&ould be an untid- unction of the conduit &ith the botto) of the soc/et outlet0 *t is no&

fairl- standard 'ractice in co))ercial situations to install trun/ing &hich incor'orates

the soc/et outlets0


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Design of electrical services for buildings 1#

+igure 101" ounting bo(

+igure 101# sho&s a 'attress for use &ith batten la)'holders0 +igure 1019 sho&s a

different t-'e of 'attress0 his is useful &ith so)e )odern building )ethods in &hich the

&iring is installed in a s'ecial s/irting0 he s/irting is at floor level. but this is too lo& for

soc/et outlets and the latter are. therefore. a little above the s/irting so that at each outlet.

cables have to rise a s)all vertical distance0 he 'attress sho&n 'rovides a neat and

convenient &a- of doing this0 *t has also been /no&n to ha''en that in the course of

erecting a ne& building. an electrical outlet is &rongl- 'laced0 +or e(a)'le. a heating

'i'e to a radiator )a- run right in front of the bo( left to ta/e a soc/et outlet0 he t-'e of'attress sho&n in +igure 1019 is a neat &a- of e(tending the &iring to an adacent

'osition. &here the alternative )ight be to de)olish large 'arts of a &all alread- built in

order to give access to conduit buried in it. as the onl- )eans of e(tending that conduit0

$aboratories

$aboratories in schools. universities and industrial establish)ents often need s'ecial

services &hich are not reuired in other areas0 he )ost co))on electrical service of this/ind is an e(tra lo& voltage su''l-0 his is usuall- obtained fro) a stabilised su''l- unit

&hich is 'lugged into the bench )ains soc/et. the soc/et being installed on angled bench

trun/ing0 +or higher current a''lications. a transfor)er fro) the )ains. &hich can either

'rovide a fi(ed secondar- voltage or be of the ta'


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Accessories 19

+igure 101# Pattress

+igure 1019 Pattress


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+igure 102% Bench trun/ing 7courtes- of

0:0 Electric $td8

rectifier is often incor'orated so that an e(tra lo& voltage d0c0 su''l- is )ade available at

the sa)e ti)e as an e(tra lo& voltage a0c0 su''l-0 $aborator- benches su''lied through

angled bench trun/ing. as sho&n in +igure 102%. are 'rovided &ith s'ecial ter)inals to

connect eui')ent to the laborator- su''lies &hich )ust be shrouded so that there are no

live conductors e('osed to touch as the laborator- eui')ent is being connected0

Connectors

*t is often necessar- to oin cables together0 *n the &iring of buildings this is rarel- done

b- soldering0 >ood soldered oints can be )ade in factor- conditions. but the conditions

e(isting on a building site. and the ualit- of &or/ that can be done under such

conditions. are such that oints )a- not be sufficientl- reliable0 Also. the ti)e ta/en to

)a/e the) &ould 'ut u' the cost of the electrical service considerabl-0 Cri)'ing the

cables is a )ore cost


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Accessories 21

+igure 1021 Connector bloc/

the t&o cables0 oints and ter)inals )ade in this &a- )ust be available for ins'ection0 *n

addition to this. all oints and ter)inations )ust be enclosed &ithin a nonenerall-. the ris/

arises because fla))able va'ours or dusts are 'resent in the at)os'here0 +or e(a)'le.

in coal )ines there is al&a-s the 'ossibilit- of )ethane a''earing in sufficient concentration

to ignite or burn0 *n such cases an- electrical eui')ent in the area subect to ris/ )ust be

s'eciall- designed to reduce that ris/0

he )ere flo& of electricit- &ill not ignite a va'our unless the te)'erature beco)es

too high0 he te)'erature can be /e't lo& b- adeuate siing of the cables so that this is

not a 'roble) as far as the installation is concerned0 he surface te)'erature of )otors.

lu)inaires and other electrical eui')ent )ust. ho&ever. be considered0 =a'our can also

be ignited b- a s'ar/ at a ter)inal or s&itch or as a result of )echanical da)age causing

a s'ar/ or local hot s'ot0 here are various &a-s of designing eui')ent to reduce the

ris/s in haardous areas and these are no& covered b- British Standards &hich arehar)onied &ith Euro'ean standards if the national standards do not e(actl- )atch0 *f the

national standards are identical. then the- &ill be designated as a Euro


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nder the Euro'ean AEF directive 199992EC. on the ini)u) ;euire)ents for*)'roving the Safet- and 6ealth Protection of Wor/ers at ;is/ fro) E('losiveAt)os'heres,. it is necessar- to consider both the t-'e and )agnitude of the ris/0 he)agnitude of the ris/ is the 'robabilit- of a dangerous concentration of fla))able

va'our. and haardous areas are classified into three ones according to the li/elihood of sucha concentrationG

1 Hone % 7AEF categor- 1> I>asJ8 A 'lace in &hich an e('losive at)os'here

consisting of a )i(ture &ith air of fla))able substances in the for) of gas. va'our or)ist is 'resent continuousl- or for long 'eriods or freuentl-0

2 Hone 1 7AEF categor- 2> I>asJ8 A 'lace in &hich an e('losive at)os'here

consisting of a )i(ture &ith air of fla))able substances in the for) of a gas. va'our or

)ist is li/el- to occur in nor)al o'eration occasionall-03 Hone 2 7AEF categor- 3> I>asJ8 A 'lace in &hich an e('losive at)os'here

consisting of a )i(ture &ith air of fla))able substances in the for) of a gas. va'our or)ist is not li/el- to occur in nor)al o'eration but. if it does occur. &ill 'ersist for a short

'eriod onl-0

+or dust- at)os'heres the follo&ing definitions a''l-G

1 Hone 2% 7AEF categor- 1D IDustJ8 A 'lace in &hich an e('losive at)os'here in the

for) of a cloud of co)bustible dust is 'resent continuousl- or for long 'eriods or

freuentl-0

2 Hone 21 7AEF categor- 2D IDustJ8 A 'lace in &hich an e('losive at)os'here in the

for) of a cloud of co)bustible dust in air li/el- to occur in nor)al o'erationoccasionall-0

3 Hone 22 7AEF categor- 3D IDustJ8 A 'lace in &hich an e('losive at)os'here in the

for) of a cloud of co)bustible dust in air not li/el- to occur in nor)al o'eration but.

if it does occur. &ill 'ersist for a short 'eriod onl-0

*f there is no li/elihood at all of a fla))able at)os'here the area is a safe one0

he )agnitude of the ris/ &hich deter)ines &hich one an area is in de'ends on such

things as the 'rocess 'roducing the fla))able gas or va'our or dust cloud. the rate of

'roduction in relation to roo) sie. the ris/ of lea/age and the distance of the area fro) the

source of the haardous )aterial0 hese factors are assessed b- a safet- s'ecialist &hodesignates the one classification of areas on a site and it is not usual for the electrical

designer to have to do this hi)herself0

he t-'e of ris/ de'ends on the 'ro'erties of the gas. va'our. or dust concerned0 +or

gases. dangerous substances are. accordingl-. classified into four grou's. de'ending on the

)ini)u) ignition energ- of the gas and on the abilit- of a fla)e e)erging fro) a narro&

oint to ignite itG

1 >rou' *. for &hich the t-'ical or re'resentative gas is )ethane. is reserved for )ininga''lications onl- and is therefore of interest onl- to )ining electrical engineers0

2 >rou' 11A is for gases &ith 'ro'erties si)ilar to 'ro'ane and reuire )ore than2%%Koules of energ- to ignite0

3 >rou' **B is for gases &ith 'ro'erties si)ilar to eth-lene 7L%Koules to ignite80


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Accessories 23

4 >rou' **C7L2%Koules to ignite8 is for the )ost haardous gases. of &hich the t-'ical

one is h-drogen0 hese categories relate to the )ini)u) ignition energ- in Koules

reuired to cause ignition. at the )ost volatile gas air )i(ture0

*t &ill be noted that the ones are nu)bered %


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E5 !%%21 Electrical a''aratus for 'otentiall- e('losive at)os'heres0 -'es of 'rotectionn,. sho& the classes &hich are used to designate the )a(i)u) 'er)itted surfacete)'erature0 *t is 'ossible for eui')ent having an- t-'e of 'rotection to have an-

te)'erature classification. although one &ould not e('ect intrinsicall- safe eui')ent to

have a lo&er class than ! or 4 &hile it is difficult for other t-'es of eui')ent toachieve classes ! or 0 *t should be noted that there is no real relationshi' bet&een

)ini)u) ignition te)'erature and )a(i)u) surface te)'erature 'er)itted0 6-drogen isa class **C gas but its

able 101 Eui')ent t-'es

Hone -'e of 'rotection

% ia

1 ia.ib.d.e.'

2 i. ia. ib. d. e. '. 5

able 102 e)'erature classification

Class a(i)u) surface te)'erature MC

1 4!%

2 3%%

3 2%%

4 13!

! 1%%

#!

surface ignition te)'erature is 10 Each gas data )ust be chec/ed thoroughl- before

s'ecif-ing eui')ent0

Electrical eui')ent suitable for use in haardous areas has to be )ar/ed &ith the

'rotection it affords0 he )ar/ co))ences &ith the letters E(. &hich have beeninternationall- agreed to indicate e('losion 'rotection. continues &ith the t-'e. and

&here relevant. the grou' for &hich the eui')ent has been certified. and concludes &ith the

te)'erature classification0 An e(a)'le of )ar/ing &ould thus be E( d **B !0

Eui')ent &ill also be )ar/ed &ith ate( )ar/ings CE %%%% ** 2 > CE suitable for use in

the Euro'ean Co))unit-. %%%% the certif-ing test house registration nu)ber. **

e('losion 'roof. 2 categor-. and > suitable for gas0

5earl- all the accessories described in this cha'ter. including s&itches. soc/et outlets and

bo(es. are available in versions &ith various classes of haard 'rotection0

Distribution eui')ent and lu)inaires. &hich are discussed in Cha'ters and ". are alsoavailable in a variet- of t-'es of 'rotection0


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Accessories 2!

Enclosures

he enclosure of an- eui')ent serves to /ee' out dirt. dust. )oisture and 'r-ing fingers0his is a se'arate )atter fro) 'rotection against e('losion a 'iece of electrical

eui')ent )a- have to be )ounted outdoors and be 'rotected against the &eather &herethere is no ris/ of e('losion. or it )a- be indoors in a 'articularl- dust- but

nonfla))able at)os'here0

An internationall- agreed s-ste) has been develo'ed to designate the degree of

'rotection afforded b- an enclosure0 *t consists of the letters *P follo&ed b- t&o digits0

he letters stand for *nternational Protection and the digits indicate the degree of

'rotection0 he first digit. &hich )a- be fro) % to . describes the 'rotection against

ingress of solids0 he second digit. &hich )a- be fro) % to #. describes 'rotectionagainst ingress of liuids0 *n both cases. the higher the nu)eral the greater the degree of

'rotection0 he definitions of the levels of 'rotection are given in ables 103a and 103b

&hich are based on BS E5 %!290 S'ecification for degrees of 'rotection 'rovided b-

enclosures 7*P code80

his )ethod of classification can be a''lied to all the eui')ent described in this

cha'ter and the distribution eui')ent and lu)inaires discussed in Cha'ters and "0

hus an enclosure &hich is rain'roof )ight be designated 1P23 &hereas one that is

et'roof &ould be 1P!!0

able 103a Protection of 'ersons against contact&ith live or )oving 'arts inside the enclosure and

'rotection of eui')ent against ingress of solid

foreign bodies 7'rotection against contact &ith

)oving 'arts inside the enclosure is li)ited to

contact &ith )oving 'arts inside the enclosure

&hich )ight cause danger to 'ersons8

+irst Degree of 'rotection

Characteristicnu)eral

% 5o 'rotection of 'ersons against contact &ith live or )oving 'arts inside theenclosure0

5o 'rotection of eui')ent against ingress of solid foreign bodies0

1 Protection against accidental or inadvertent contact &ith live or )oving 'arts

inside the enclosure b- a large surface of the hu)an bod-. for e(a)'le. ahand. but not 'rotection against deliberate access to such 'arts0

Protection against ingress of large solid foreign bodies0

2 Protection against contact &ith live or )oving 'arts inside the enclosure b-fingers0

Protection against ingress of )ediu)


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3 Protection against contact &ith live or )oving 'arts inside the enclosure b-tools. cables or such obects of thic/ness greater than 20!))0

Protection against ingress of s)all solid foreign bodies0

4 Protection against contact &ith live or )oving 'arts inside the enclosure b-tools. cables or such obects of thic/ness greater than 1))0

Protection against ingress of s)all solid foreign bodies0

! Co)'lete 'rotection against contact &ith live or )oving 'arts inside theenclosure0

Protection against har)ful de'osits of dust0 he ingress of dust is not totall-

'revented. but dust cannot enter in an a)ount sufficient to interfere &ith

satisfactor- o'eration of the eui')ent enclosed0

Co)'lete 'rotection against contact &ith live or )oving 'arts inside theenclosure0

Protection against ingress of dust0

able 103b Protection of eui')ent against ingress of

liuid

Second characteristic Degree of 'rotection

nu)eral

% 5o 'rotection0

1 Protection against dro's of condensed &aterGdro's of condensed &ater falling on the enclosure shall have no har)fuleffect0

2 Protection against dro's of liuidGdro's of falling liuid shall have no har)ful effect &hen the enclosure istilted at an- angle u' to 1!M fro) the vertical0

3 Protection against rainG&ater falling in rain at an angle u' to %M &ith res'ect to the verticalshall have no har)ful effect0

4 Protection against s'lashingGliuid s'lashed fro) an- direction shall have no har)ful effect0

! Protection against &ater


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Cha'ter 2

Cable

*ntroduction

Electricit- is conve-ed in )etal conductors. &hich have to be insulated and &hich also

have to be 'rotected against )echanical da)age0 When the conductor is insulated to

)a/e a usable 'iece of eui')ent for carr-ing electricit-. it beco)es a cable0 his

no)enclature )a/es a convenient and logical distinction bet&een a bare conductor and

insulated cable. but in 'ractice the ter)s conductor, and cable, are in fact used

interchangeabl- and it is onl- the conte(t &hich )a/es clarified &hat is being referred to0 We

shall tr- to avoid confusion and shall discuss conductors first and the insulation a''liedto the) after&ards0

Conductors

he co))onest conductor used in cables is co''er0 he onl- other conductor used is

alu)iniu)0 Co''er &as the earlier one to be used. although alu)iniu) has the

disadvantage of being )uch &ea/er than co''er0 Conseuentl- BS ""1 states that the

)ini)u) 'er)issible cross


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*nsulation

Ever- conductor )ust be insulated to /ee' the) a'art. /ee' the flo& of current &ithin theconductor and 'revent its leaving or lea/ing fro) the conductor at rando) along its

length0 he follo&ing t-'es of insulation are in use0

her)o'lastic P=C

Pol-vin-l chloride is one of the co))onest )aterials used b- )an toda-0 *t is a )an)ade

ther)o


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Cable 31

and cable )ade &ith this construction is ver- useful for fire alar)s0 her)osetting rubber of1#%MC is used in hotlass

>lass fibre has good heat


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P=C is no& used for the larger 'o&er and sub


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Cable 33

concerned onl- &ith the construction of the rising


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+igure 204 ;ising )ains

&here the- run horiontall- at high level along the &alls of &or/sho's0 Plain connectors

can be fi(ed at short intervals and short cables run fro) each set of connectors to a s&itch

fuse fi(ed on the &all i))ediatel- belo& or above the trun/ing0 he s&itch fuse can then be

connected to serve a )achine near it on the floor of the &or/sho'0

*CC

hese letters stand for )ineral insulated co''er covered this t-'e of cable is also /no&n

as )ineral insulated co''er sheathed. &hich is abbreviated to *CS and as )ineral

insulated )etal sheathed. abbreviated to *S0 his last descri'tion )a- refer to

alu)iniu) sheathing. as &ell as to co''er sheathing alu)iniu) &as used as a sheathing

so)e ti)e ago0 *t )a- be encountered during refurbish)ents0 All versions of this t-'e of

cable consist of single


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Cable 3!

+igure 20! ineral insulated co''er

sheathed cable

*S cable is e(tre)el- robust and. &hen 'ro'erl- installed. has an indefinite life0 *tcan be used outdoors and for such use is usuall- su''lied &ith an overall covering of

P=C0 *t is then /no&n as *S P=C sheathed0 Since P=C is e)brittled &ith ultravioletlight. this P=C covered cable should not be installed &here it &ill be e('osed to directsunlight0 his is not as drastic a restriction on its use as )a- a''ear since it is 'robabl-un&ise to run an- cable &here it is so e('osed that direct sunlight can reach it0 *n an-

such situation. it &ould also be too vulnerable to da)age b- vandalis) and fro) ani)als0

Because of its robustness. *S reuires no further 'rotection. but &ill not &ithstand

being struc/ b- shar' obects0 *t is. therefore. )ore easil- built into the structure of a

building than other cables. nearl- all of &hich )ust have so)e for) of enclosure around

the)0 Because it has an indefinite life. there is no need for facilities to )a/e it 'ossible to

re&ire the installation0 +or both these reasons. it can often be used &here no other cable

&ould be entirel- satisfactor-0 *S cable can carr- a higher current than other cables&ith the sa)e sie conductor because the insulation can &ithstand a higher conductor

o'erating te)'erature0 6o&ever. its current carr-ing ca'acit- de'ends on &hether it is

bare. P=C


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Sheathing

We have described ho& a cable is )ade fro) a conductor &ith insulation around it0

Electricall-. this is all that is needed to )a/e a device to carr- electricit- fro) one 'lace to

another. but if the cable is to survive in use. it )ust also &ithstand )echanical da)age. and

the insulation. &hich is enough to achieve electrical 'rotection. is seldo) strong enough

to give adeuate )echanical 'rotection0 So)ething further. therefore. has to be 'rovidedover the insulation. and it can either be )ade an integral 'art of the cable or 'rovided b-

entirel- se'arate )eans0

+or e(a)'le. the steel ar)our of a her)o'lastic P=C cable. &hich has been

described above. gives )echanical 'rotection and is 'art of the structure of the cable0

Si)ilarl-. the )etal sheathing of *S cable gives the cable all the )echanical'rotection that is needed0 5either of these cables could be used &ithout its built


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Cable 3"

;ubber

But-l rubber and silicone rubber cables are usuall- sheathed &ith thic/ but-l and silicone

rubber res'ectivel-0

+le(ible cord

+le(ible cord is the na)e given to a 'articular t-'e of cable0 *t is one &hich is fle(ible

and in &hich the cross


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here are also fle(ible cords )ade &ith silicone rubber insulation &ith a covering of

braided varnished glass fibre0 +le(ibles &hich have glass


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Cable 39

*n order to give better 'erfor)ance in fires. alternative )aterials have been develo'ed0

?ne t-'e of cable for enhanced fire 'erfor)ance has F$PE insulation &ith cablear)ouring bedded in a co)'ound &hich has lo& s)o/e and fu)e 'ro'erties and issheathed in the sa)e co)'ound0 +ro) its lo& s)o/e and fu)e 'ro'erties it is designated$S+ it consists of inorganic fillers in 'ol-)ers such as eth-l vin-l acetate and eth-lene'ro'-lene rubber0 he construction is illustrated in +igure 2090

?ther cables have EP; 7eth-lene 'ro'-lene rubber8 insulation &ith a sheath ofelasto)er &hich is heat and oil resistant and fla)e retardant. and is therefore designated6?+;0 hese cables are available &ith and &ithout a cable ar)ouring bet&een theinsulation and the sheath0

*t &ill not co)e as a sur'rise that the better the 'erfor)ance of the cable. the )ore

e('ensive it is0 Conseuentl-. it is i)'ortant to consider carefull- the a''lication and

'recise reuire)ents0 *t has onl- been 'ossible here to give an indication of

s'ecial'ur'ose cables available0 *n 'ractice. final selection cannot be )ade &ithout

discussion &ith )anufacturers and su''liers0

Co


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'laced over the insulation to screen the conductor fro) un&anted signals. and this cover

is in its turn 'rotected b- an overall sheath of non


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Cha'ter 3

Wiring

*ntroduction

o the average user the onl- i)'ortant 'art of the electricit- service is the outlets at

&hich he received electricit-0 o the engineer concerned &ith designing or installing the

service. the s-ste) of cables &hich lin/s these outlets to each other and to the su''l-

co)ing into the building is ust as i)'ortant and 'erha's even )ore so0 *n 'ractice. the

electrical service is a co)'lete interde'endent s-ste) and the 'ractical engineer thin/s of it as

a &hole. but. as &ith the teaching of an- subect. one has to brea/ it do&n into 'arts in order

to e('lain it in an orderl- fashion &hich &ill )a/e sense to a student &ith no 'revious

/no&ledge of the subect0

*n this cha'ter. &e shall consider different &a-s in &hich cables can be installed in a

building0 he calculation of the sie of 'articular cables &e shall leave to Cha'ter 4 and

the selection and grou'ing of outlets to be served b- one cable &e shall leave to Cha'ter

!0 +or this cha'ter. &e assu)e that &e /no& &here cables are to run and discuss onl- ho&

to get the) into the building0 his as'ect of the electrical service can forconvenience be called )ethods of installation,0

A )ethod of installation consists of ta/ing a suitable t-'e of cable. giving it adeuate

'rotection and 'utting it into the building in so)e &a-0 he subect can. therefore. be

fairl- logicall- considered b- considering t-'es of cable. )ethods of 'rotection and

)ethods of installation0 he t-'es of cable available and in general use have been

described in Cha'ter 20 he 'rotection against )echanical da)age given to cable is

so)eti)es 'art of the cable itself. as &ith P=C insulated P=C sheathed cables. and

so)eti)es 'art of the )ethod of installation. as &ith conduit s-ste)s0 *t can be )ore

confusing than hel'ful to ta/e a logical sche)e of things too rigidl- and. rather than deal&ith 'rotection in a cha'ter of its o&n. &e are dealing &ith it 'artl- in the 'revious

cha'ter and 'artl- in this. according to &hether it is associated &ith the cable or &ith the

)ethod of &iring0

*t is 'robabl- true to sa- that one of the co))onest )ethods of installing cables is still to'ush the) into conduit and &e shall devote )ost of our attention to this0


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Wiring 43

Conduit

*n a conduit s-ste) the cables are dra&n into tubing called conduit0 he conduit can be

steel or 'lastic0 Steel conduit is )ade in both light gauge and heav- gauge. of &hich

heav- gauge is )uch )ore freuentl- used0 *n both cases. it can be )ade either b-e(trusion or b- rolling sheet and &elding it along the longitudinal oint0 he latter is

s'ecified as &elded conduit and the for)er as sea)less0 Sea)less conduit is generall-

regarded as the better ualit-0 he different sies of conduit are identified b- their

no)inal bore and in the case of electrical conduit the no)inal bore is al&a-s the sa)e as

the outside dia)eter of the tube0 hus 2%)) light and heav- gauge conduits both have

the sa)e outside dia)eter and conseuentl- )ust have slightl- different inside dia)eters0

his is the o''osite of the convention used for 'i'es for )echanical engineering in &hich

the no)inal bore usuall- corres'onds )ore closel- to the inside than the outside

dia)eter0 Electrical conduit is s'eciall- annealed so that it )a- be readil- bent or set&ithout brea/ing. s'litting or /in/ing0

6eav- gauge conduit is nor)all- oined together b- scre&ed fittings there is a

standard electrical thread &hich is different fro) other threads of the sa)e no)inal

dia)eter0 A scre&ed connection bet&een t&o lengths of conduit is sho&n in +igure 3010

A )ale electrical thread is cut on the ends of both lengths of conduit to be oined and a

standard cou'ler &ith a fe)ale electrical thread is scre&ed over the)0 A loc/ nut. &hich

has been 'reviousl- threaded &ell u' out of the &a- on one of the )ale threads. is then

&ound do&n and tightened against the cou'ler0 he scre&ed connection is relied on for

continuit- of the earth 'ath and the loc/ nut is essential to 'revent the soc/et &or/ing its

&a- along the threads until it engages )ore on one conduit than on the other0 he reason

for &anting an earth 'ath is discussed in Cha'ter 90 ethods of oining conduit to bo(es

of the /ind described in Cha'ter 1 are sho&n in +igure 3020 A bush of so)e sort )ust

al&a-s be used to 'rovide a s)ooth entr- into the bo(. to avoid shar' corners &hich

could da)age the cable insulation. and in certain cases to )aintain earth continuit-0

+igure 301 Conduit cou'ling


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+igure 302 Conduit entries into bo(es

Connections to distribution boards and s&itchgear are )ade in a si)ilar )anner0

*n addition to the bo(es described in Cha'ter 1. other fittings are )ade for use &ith

conduit0 hese include the soc/ets and bushes needed to )a/e connections. and alsobends and ins'ection covers. so)e of &hich are illustrated in +igure 3030 he use of

bends and ins'ection covers is not. ho&ever. regarded as good 'ractice. because the-'rovide inadeuate roo) for dra&ing in cable and because the- loo/ unsightl- &hen theinstallation is co)'leted0 +or long lengths of run. ins'ection sleeves are available0

Conduit is thic/ enough for the cross


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Wiring 4!

+igure 303 Conduit ins'ection fittings

of the installation de'ends on good electrical contact at all the oints0 Even though it )a-

be decided not to use the steel conduit as the circuit 'rotective conductor. in 'reference

for a se'arate 'rotective conductor. usuall- co''er. the conduit )ust be erected 'ro'erl-

&ith tight oints0 Since it is classed as an e('osed conductive 'art of the installation. and

therefore could beco)e live in the event of a fault. it reuires earthing 'ro'erl-0

Conduit is )ade in t&o standard finishesG blac/ ena)el and galvanied0 *t is al)ost

universal 'ractice to use galvanied conduit &here it is e('osed or &here it )a- besubect to da)'0

he final connection to )achines and )echanical eui')ent such as 'u)'s. boilers.

fans. fan heaters. &or/sho' eui')ent and so on is usuall- )ade in fle(ible conduit0 he

fi(ed &iring ter)inates in a bo( either in the &all near the eui')ent to be connected or

on the surface of the &all. and fro) this bo( a short length of fle(ible conduit is ta/en to

the eui')ent0 =er- often the )achine is delivered to the site after the electrician has

done the bul/ of the &or/0 At the ti)e of installing the &iring. the 'osition of theter)inals on the )achine is not /no&n and so the outlet bo( can onl- be 'laced to &ithin

a foot or so of its e(act 'osition0 Solid conduit fro) this to the )achine could involve a

large nu)ber of bends in a short distance &hich &ould be difficult to )a/e andi)'ossible to 'ull cable through0 +le(ible conduit can ta/e u' a gentle curve and also

serves to isolate the fi(ed &iring fro) an- )echanical vibrations on the connected

)achine. and allo&s for belt tension adust)ent of the )otor0

here are several t-'es of fle(ible conduit0 etallic fle(ible conduit is sho&n in

+igure 3040 *t is )ade fro) a ste''ed stri' &hich is &ound in a


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+igure 304 etallic fle(ible conduit

continuous s'iral so as to 'roduce a long c-linder &ith s'iral corrugations0 he )aterial

used is nor)all- galvanied steel0 +le(ible conduit is also )ade in a nu)ber of 'lastic

)aterials0 *n so)e of these the fle(ibilit- is conferred b- a corrugated structure. as in the

case of )etallic fle(ible conduit. and in others b- the fle(ible 'ro'erties of the )aterial

itself0

+le(ible conduit cannot be used as a 'rotective conductor0 his is obvious in the case of

'lastic fle(ible conduit &hich is )ade of non


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Wiring 4"

+igure 30! Earth cla)'

in casting need not. therefore. cause a 'roble) during asse)bl-. although a ver- large)isalign)ent &ill 'ull the fle(ible into such a shar' S that the site electrician &ill not beable to 'ull cables through it0 +le(ible conduit is also used to bridge electrical services

fro) one to the other side of the e('ansion ga' in structures0

A conduit s-ste) )ust be co)'letel- installed before an- cables are 'ulled into it0 *t

is. therefore. essential that it is set out so that an electrician can 'ull cables into it &ithout

difficult-0 Conduit s-ste)s are intended to be re&irable that is to sa- the intention is that

2% or 3% -ears after the building has been erected. it should still be 'ossible to 'ull all the

cables out of the conduit and 'ull ne& ones into it0 *f this is 'ossible. then uiteregardless of &hat ha''ens &hen the building is first constructed. the la-out of the

conduit )ust be such that cables can be dra&n into it &hen it is co)'lete and finished0

he original reason for &anting to have electrical s-ste)s &hich could be recabled

during the life of the building &as that =;* cable deteriorates in about 2% -ears to the

stage at &hich it should be re)oved0 P=C cable a''ears to last indefinitel- so that all

)odern installations &hich use this cable should not need re&iring0 he use of electrical

a''liances has increased greatl- in the last !% -ears. and &hen old buildings &hich had =;*

cable are re&ired the o''ortunit- is invariabl- ta/en of )oderniing the installation b-

adding e(tra outlets and circuits0 5e& cables then have to be run &here there &ere no

cables 'reviousl- and the original conduit has at best to be added to and at &orst

abandoned altogether0 ;e&ireabilit- is then no hel' and in fact the need for a re&ireables-ste) is not as great as is often su''osed0

?n the other hand. there is al&a-s the 'ossibilit- that a cable )a- beco)e da)aged

during the construction of a building. and it is obviousl- an advantage if it can bere'laced &ithout difficult- after the building has been finished0 *f the conduit is installed

so that the s-ste) is re&ireable. re'airs &ill al&a-s be 'ossible0 he reuire)ents for

re&ireabilit- should. therefore. be /e't to as far as 'ossible. but the engineer in charge

should have discretion to rela( the) if e(ce'tionall- difficult circu)stances are

encountered0

o achieve re&ireabilit-. dra&


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conduit bet&een successive dra&


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Wiring 49

5ote that the s'ace factor relates to the s'ace ta/en u' b- the cable and not theunoccu'ied s'ace0

*t is harder to 'ull several s)all cables together than one large cable. and &hen anu)ber of cables have to go in the sa)e conduit. it is advisable to

+igure 30" Conduit bending )achine

/ee' the s'ace factor &ell belo& 4% 'er cent0 S'ace factors of less than 2% 'er cent need

not be considered at all e(travagant0 +or the sa)e reason. it is often better to use t&o sie2!)) conduits side b- side than a single 32)) or !%)) even &hen in theor- the latteris adeuate0

an- t-'es of insulation deteriorate if the- beco)e da)'0 *t is. therefore. i)'ortant that

)oisture should not collect in the conduit s-ste)0 oisture can occur through &ater

entering during building o'erations and also later on through condensation of )oisture in the

at)os'here0 A conduit s-ste) )ust be laid out so that it is &ell ventilated. &hich &ill

'revent condensation. and so that &ater &hich does enter &ill drain to one or )ore lo&

'oints at &hich it can be e)'tied0

*t is good 'ractice to s&ab through the conduit after it is erected and before cables are

dra&n in to re)ove an- )oisture and dirt &hich have collected0 his is done si)'l- b-

t-ing a suitable sie of s&ab on the end of dra& cable and 'ulling it through the conduit

fro) one dra&


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Design of electrical services for buildings !%

o avoid da)age to cables as the- are dra&n in. burrs on cut ends of conduit )ust bere)oved &ith a rea)er before the lengths of conduit are oined0

here are a nu)ber of 'ositions in a building in &hich the conduit can be fi(ed0 *t canobviousl- be run on the surface of &alls and ceilings. and &hen a building is constructed offair


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Wiring !1

in the design as the decision &ill affect the t-'e of &iring &hich the engineer has todesign0

Conduit can also be buried &ithin concrete slabs0 an- )odern buildings have floors

and even &alls of concrete &ith ver- little or no finish on to' of it this is 'articularl- true

of industrialied )ethods of building0 *n such buildings. the onl- 'racticable alternativeto 'utting &iring on the surface is to bur- conduit &ithin the structural concrete0 his

needs considerable care0 he e(act 'osition of the conduit &ithin the de'th of the slab

)ust be agreed &ith the structural engineer and close su'ervision is reuired of the &or/

on site to ensure that the conduit is correctl- 'laced0 *t has to be fi(ed in 'ositioni))ediatel- after the steel reinforce)ent has been laid in the shuttering and before the

concrete is 'oured0 *f it is not &ell tied either to the reinforce)ent or to the shuttering. it

)a- be dislodged as the concrete is 'oured and vibrated0 ?'en ends of conduit &hich

)a- have to be left at the end of the section of concrete being cast. read- for connection

to the ne(t 'iece of conduit. )ust be covered &ith )etal or 'lastic ca's to 'revent ce)ent

or stones getting into the conduit0 Ever- electrician of an- e('erience can tell a horror

stor- of a bloc/ed conduit0 he conduit bo(es )ust also be filled &ith a )aterial &hich

&ill 'revent ce)ent and stones entering but can itself be easil- re)oved once theconcrete has set and the shuttering has been struc/0 he )ost co))onl- used )aterial for

this 'ur'ose is e('anded 'ol-st-rene0

?nce conduit has been cast inside a concrete slab. it is totall- inaccessible for re'air or

re'lace)ent0 he rules for installing it in such a &a- that the dra&ing in of cables is eas-

are. therefore. of e(ce'tional i)'ortance0 *t is advisable for the conduit to have 'lent- of

s'are ca'acit- for the nu)ber of cables to be dra&n into it. for bends to be ver- eas- and for

there to be 'lent- of dra&


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Design of electrical services for buildings !2

+igure 309d. but it )ust be re)e)bered that this introduces a fairl- shar' bend in the

conduit &hich could )a/e it harder to 'ull in the cable0 Another 'ossibilit- is to 'ut the bo(in line &ith the conduit and fit an e(tension ring to the bo( to bring the cover for&ard to thesurface0 his is sho&n in +igure 309e0

When buried conduit has to feed surface distribution boards or s&itches. the conduit

)ust be brought into a flush recessed bo( so that the cables enter the surface board or

s&itch through the bac/0 *f necessar- an e(tension ring has to be 'laced bet&een the bo(and the surface0 +igure 309f sho&s an e(a)'le of buried conduit feeding a fuseboard on

the surface0

ost buildings larger than a single d&elling have a three


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Wiring !3

'rovided that the lo&er voltage circuits are insulated for the highest voltage 'resent. it is

better not to do so0 BS "%1G1994 reco))ends that there be a )ini)u) distance bet&een

)ains 7Band **8 and teleco))unications circuits 7Band *80

he ne(t )atter to receive our attention is ho& to fi( conduit0 +igure 301% sho&s

various devices for fi(ing conduits0 he 'i'ehoo/ or cra)'et. +igure 301%a. is asatisfactor- and si)'le fi(ing. but is too unsightl- to be used on surface &or/0 *t can be

driven into ti)ber. bric/ or )asonr-. but is )ore li/el- to be dislodged than a scre&ed

fi(ing &here the conduit is to be buried in 'laster after it has been fi(ed. this does not

)atter because the 'laster &ill hold the conduit in 'lace. but &here the conduit is to

re)ain e('osed a fir)er fi(ing is desirable0 he saddle hoo/ sho&n in +igure 301%b is b-

far the co))onest fi(ing0 *t 'asses round the conduit and is secured to the &all b- t&o

scre&s0 he onl- advantage of the cli' sho&n in +igure 301%c is that it saves one scre&0 *t

is not as secure as the saddle and the cost saving is not sufficient for a good engineer to

use it0Soc/ets and other conduit fittings necessaril- have a larger outside dia)eter than the

conduit itself0 *f these co)'onents are tight to a &all. the conduit )ust be slightl- 'roud

of the &all0 Because of this. &hen an ordinar- saddle is tightened. it &ill tend to distort

the conduit0 his can be 'revented b- the use of a s'acer saddle. +igure 301%d. &hich has

the sa)e thic/ness as the soc/ets0 he s'acer saddle has the further advantage that it

'revents the conduit fro) touching da)' 'laster and ce)ent &hich could corrode and

discolour decoration0

When conduit is fi(ed to concrete. the ti)e ta/en to drill and 'lug holes in the

concrete is a ver- large 'ro'ortion of the installation ti)e0 A s'acer bar saddle has onl- one

scre& to be fi(ed to the &all and the saving in ti)e can be greater than the e(tra cost of the)aterial0

he distance saddle sho&n in +igure 301%e holds the conduit about 1%)) fro) the &all0

his s'acing eli)inates the ledge bet&een the conduit and the &all &here dust can collect

and )a/es it 'ossible to decorate the &all behind the conduit0 *t also )a/es it i)'ossible

for )inute dro's of )oisture to collect in the crac/ bet&een conduit and &all and thus

reduces the 'ossibilities of corrosion0 +or these reasons. distance saddles are al)ost

invariabl- s'ecified b- hos'ital boards and local authorities for surface conduit0

Conduit often runs across or along steel girders or oists. either e('osed or &ithin afalse ceiling0 *t is not desirable to drill and ta' structural steel&or/ and it is better to use

girder cli's of the t-'e illustrated in +igure 301%f0 Whilst standard girder cli's can be

bought fro) conduit )anufacturers. it is usuall- si)'ler to )a/e s'ecial cli's to suit

individual conditions on each ob0 +or )ulti'le run and runs &ith other services. uni


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Design of electrical services for buildings !4

+igure 301% Conduit fi(ings

Electrical conduit is not thic/ enough to su''ort its o&n &eight over long distances

&ithout sagging0 he su''orts )ust. therefore. be at uite close intervals. and the

)a(i)u) distances &hich should be allo&ed bet&een su''orts are as follo&sG

6oriontal =ertical

2%)) conduit 10"!) 20%

2! and 32)) conduit 20%) 20!

4%)) and over 202!) 20!

he *EE >uidance 5ote 1 and the *EE ?nuide both give guidance on the

)a(i)u) s'acing of conduit fastenings0

he cables are dra&n into the conduit &ith the hel' of a steel ta'e and a dra& cable0

he steel ta'e has a he)is'herical brass ca' on the end &hich 'revents its stic/ing on

irregularities at oints of the conduit and also hel's guide it round bends0 he ta'e alsohas a loo' at its other end and a steel dra& cable is attached to this0 he cablesthe)selves are then attached to the other end of the dra& cable0 he electrician attaches


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Wiring !!

the cables b- threading the) through a series of loo's in the dra& cable0 he- should notall be attached to the sa)e 'oint other&ise there is a significant. sudden enlarge)ent inthe bunch of cables and this 'resents an edge &hich can catch in the bore of the conduitand &hich &ill be difficult to negotiate bends0 When each cable is loo'ed through the

dra& cable. it is folded bac/ on itself and the end is ta'ed0 his gives a s)ooth surface tobe 'ulled through the conduit and 'revents shar' ends of s)all strands of cable fro)

stic/ing out and catching the inside of the conduit0 he )ethod of connection is sho&n in+igure 30110

Pulling cable through conduit is a ob for electrician and )ate0 ?ne 'ushes the steel

ta'e &ith the dra& cable attached to it fro) one dra&


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Design of electrical services for buildings !

Plastic conduit

P=C conduit is being increasingl- used in 'lace of heav- gauge steel conduit0 *tsadvantages are that it is chea'er and )ore easil- installed than steel conduit and that it is

non


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Wiring !"

bending 'rocess0 he s)aller sies can generall- be bent cold. but 32)) conduit and

larger )ust be gentl- heated for a distance of about 3%%)) on either side of the intended

bend0

?ne has to re)e)ber the susce'tibilit- of P=C to high te)'eratures if one 'ro'oses to

sus'end lu)inaires fro) P=C conduit bo(es0 he heat fro) the la)' is conductedthrough the fle(ible cable and through the fi(ing scre&s. and it could ha''en that it

softens the P=C bo(0 o overco)e this 'roble). it is 'ossible to 'rovide the bo(es &ith

)etal inserts0

P=C conduit is )ade not onl- in the nor)al circular cross section but also &ith an

oval section0 he reduced de'th of an oval section enables it to be acco))odated &ithin

the thic/ness of 'laster in 'laces &here the use of round conduit &ould )a/e it necessar-

to chase the bric/&or/ behind the 'laster0 his )a/es the oval conduit ver- useful for

s&itch dro's and for s)all do)estic installations0 *n the latter case. it )a/es it ver- eas-to add ne& &iring in an old house0 he electrician can cut a&a- and re'air 'laster

&hereas hel' )ight &ell be &anted fro) another trades)an if bric/&or/ needed cutting0

he sa)e P=C )aterial is )ade as rectangular and se)icircular channelling0 his is

intended 'ri)aril- as a 'rotection over P=C insulated P=C sheathed cable &here the

latter is installed on the surface of &alls0 *t can also be used as a 'rotection to P=CP=C

cable &hen the latter is buried in 'laster. the ustification for this use being that it saves

de'th and that the side of the cable ne(t to the structural 'art of the &all does not need

'rotection0

+le(ible P=C conduit is available in t&o t-'es0 *n the one. fle(ibilit- is conferred b- a

corrugated construction0 *n the other. the P=C itself is a 'lasticied grade so that the

fle(ibilit- is a 'ro'ert- of the )aterial itself0 +le(ible P=C conduit can be used tonegotiate a&/&ard bends and in situations &here rigid conduit &ould be difficult to

install. and it is so)eti)es resorted to for the solution of unforeseen 'roble)s &hich so

often see) to arise in the course of building &or/0 here is. ho&ever. a danger to using it

in this &a-0 *t is 'ossible to ta/e such advantage of the fle(ibilit- that the conduit curves

so shar'l- that it is i)'ossible to 'ull cables through it0 *f this ha''ens the 'roble) of

installing the conduit has been solved onl- b- the creation of a )ore difficult 'roble) for

the ne(t stage of the erection 'rocess0 +le(ible conduit should. therefore. be used &ith

caution0

P=C sheathed cable

here are )an- cases in &hich &iring can be installed in P=CP=C &ithout further

'rotection0 +or e(a)'le. this )a- be done in an- voids in a building such as false ceilingsand &ooden floors0 When the cable runs 'arallel to oists in a &ooden floor. it can be

cli''ed to the sides of the oists0 When it has to run across the) it is better to thread it

through holes drilled in the neutral a(is of the oists than to notch the to' of the oists0

6oles drilled on the neutral a(is &ea/en the oist less than notches cut in the to'. and

because the cable is further fro) the floorboards on to' of the oists. it is safer fro) nails

driven into the floor0 BS ""1 states that a cable 'assing through a ti)ber oist )ust be

!%)) fro) the to' or botto) of the oist. or be )echanicall- 'rotected0


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Design of electrical services for buildings !#

+igure 3012 Dr- 'artition

5e& buildings have internal 'artitions constructed of ti)ber studding &ith light

'lasterboard facing. as illustrated in +igure 3012. or 'reconstructed 'artitioning0 his is

'articularl- the case &ith 'ro'rietar- industrialied s-ste)s of building0 Such a 'artition

has a void &ithin it &hich is used for engineering services. and this is also a situation in

&hich P=C sheathed cable &ithout further 'rotection is the )ost suitable s-ste) of

&iring. 'rovided that the cable is at least !%)) fro) the surface0 =oids of this sort. in

&hich P=C sheathed cable is run. are usuall- sufficientl- accessible to )a/e re&iring. ifnot eas-. at least 'ossible0 +i(ing of accessories is )ade eas- b- the use of dr- liner

bo(es as illustrated in Cha'ter 1 7+igure 10180

*t )a- ha''en that the building structure is such that P=C sheathed cable on its o&n isthe )ost suitable s-ste) to use. but that there are a fe& 'laces &here cable has to dro' in

'lastered &alls or run across floors0 *t is then desirable to give the cable additional'rotection at these 'laces b- running it inside conduit at these 'laces onl-0 his has theadditional advantage of )a/ing re&iring easier0 As the conduit is used onl- for short

lengths for local 'rotection. both light


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Wiring !9

+igure 3013 Cable buried in 'laster

+igure 3014 Cli'

1 and the *EE ?nuide0 he basic rule is that cables can be run under 'laster at a

de'th of less that !%)) &ithout 'rotection. in straight lines bet&een accessories. also

&ithin 1!%)) of a change of direction of the &all0

here are situations &here the a''earance of an installation is of secondar-

i)'ortance. and &here at the sa)e ti)e a surface s-ste) &ill not receive rough usage0

Such a case )ight occur in an old building used for co))ercial 'ur'oses or in si)'le

huts at a holida- ca)'0 P=C sheathed cable )a- then be run on e('osed surfaces &ithout

further 'rotection0 Since it is visible it &ill not be da)aged accidentall- b- 'eo'le tr-ing to

fi( things to the &alls0

P=C sheathed cable is fi(ed &ith )oulded 'lastic cli's0 An e(a)'le is illustrated in+igure 30140 he cli's should be s'aced a''ro'riate to the sie of cables0 *EE >uidance5ote 1 Selection and Erection of Eui')ent and the *EE ?nuide give guidance onthe s'acing of cable cli's0

Cable trun/ing

Where a large nu)ber of cables has to be run together. it is often convenient to 'ut the)

in trun/ing0 run/ing for electrical 'ur'oses is )ade of 1#


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Design of electrical services for buildings %

!%))@1%%)) and 1%%))@1%%)) are also available0 *t is usuall- su''lied in 2)lengths and one co)'lete side is re)ovable. as sho&n in +igure 301!0 he re)ovableside. or lid. either scre&s on or cli's on &ith a sna' action0 he latter arrange)ent is

chea'er but a little )ore a&/&ard to handle0

A variet- of bends. tees and unctions is available fro) all )anufacturers of such

trun/ing0 So)e of these are sho&n in +igure 301!0 he- enable the trun/ing to be ta/en

round corners. to reduce in sie as the nu)ber of cables is reduced and to allo& a )ainrun to serve a nu)ber of branches0

+igure 301! Cable trun/ing

o 'ut cables in such trun/ing one nor)all- ta/es the lid off. la-s the cables in and

re'laces the lid. but it is 'ossible over short distances or straight length to 'ull the cables inas one does &ith conduit0 Whichever )ethod is ado'ted. the nu)ber of cables and sie oftrun/ing )ust be such that no da)age is caused during installation0 he sa)econsiderations a''l- as in the case of conduit0

Being so )uch larger than conduit. trun/ing can uite clearl- not be buried in the

&alls of a building0 *t has to be run on a surface0 here are occasions &hen there are

)an- circuits running together inside a builder,s &or/ vertical duct &hich also contains

other services such as heating 'i'es or gas0 his is one situation in &hich cable trun/ing is an

ideal &a- of installing the &iring0 *t can be si)ilarl- used in false ceilings0 *n both these

cases. there )ust be sufficient doors or access tra's to enable electricians to reach the

trun/ing for re&iring0Buildings such as asse)bl- halls and g-)s often have e('osed steel lattice fra)e&or/

su''orting the roof0 *t is then 'ossible to run cable trun/ing neatl- through the s'aces of


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Wiring 1

the lattice0 So)eti)es the architect &ill 'er)it the cable trun/ing to be fi(ed under the

bea)s and along the)0 Either )ethod is si)'ler and neater than fi(ing several conduits'arallel to each other on the surface of the ceiling. and has the further advantage that

during the life of the building. the &iring can be altered ver- easil-0 $ighting trun/ing has

additional folds in the cross section &hich )a/es it )ore rigid than conventional trun/ing andis able to s'an greater distances0

*n &or/sho's and laboratories there is usuall- a large nu)ber of )achines and othereui')ent &hich have to be served &ith electricit-0 A conduit s-ste) can then beco)eco)'le( and. therefore. e('ensive0 A si)'le and neat )ethod of &iring these areas is torun trun/ing round the &alls and to install all the circuits inside the trun/ing0 *n roo)s ofthis class. this is uite acce'table and no one obects to the a''earance of trun/ing visibleon &alls0 Again there is the advantage that &hen )achines are re'laced or &hen ne&

)achines are installed. the conseuent changes to the electrical service are easil- )ade0he sa)e consideration a''lies. but &ith added force. to factories0

When )achines are 'laced in the centre of a roo) a good )ethod of serving the) is to

run trun/ing at high level under the ceiling and dro' to each )achine &ith a length of

conduit0 *t is. of course. 'ossible to install conduit &ithin the floor &ith an outlet near

each )achine. but there then has to be either rigid or fle(ible conduit at floor level. and if

)achines are )oved or additional ones brought in the floor has to be dug u' before the

conduit can be e(tended to the ne& 'ositions0 +or the initial installation the electrician

&ould have to /no& the e(act 'ositions of the ter)inals of each )achine before the floor

is laid. and it is ver- seldo) that either the builder or the final occu'ier of the factor- can

'rovide this infor)ation so earl-0 he overhead s-ste) avoids the difficult- of locating

e(act 'ositions of )achines too earl- in the construction 'rocess and )a/es futurechanges )ore eas-0

*n so)e cases. 'articularl- &ood&or/ roo)s in schools and colleges. long 'ieces of

)aterial such as ti)ber have to be carried fro) stores across the roo) to various

)achines0 *t can then ha''en that vertical dro's of conduit fro) the ceiling to the

)achines obstruct the )aterial and cause difficult- in handling it0 *f this is li/el- to

ha''en there )a- be no alternative to installing conduit &ithin the floor. but the

custo)er,s attention should be dra&n to the infle(ibilit- of this arrange)ent0

*t is also 'ossible to install cable trun/ing neatl- in the corner bet&een a &all and

ceiling0 his is so)eti)es acce'table to an architect. and the author /no&s of at least onecase &here it has been done along the length of a 'ublic corridor in a bloc/ of flats0

Si)ilar situations could arise in offices and hos'itals0 *t )a- be necessar- to run )an-

circuits the length of a corridor in such buildings and a conduit s-ste) &ould reuire

)an- conduits cast into the ceiling slab of the corridor0 *f the slab is thin and heavil-

loaded structurall-. as is a't to ha''en in )odern building design. it beco)es uite a

serious 'roble) to get the conduit into the slab &ithin the restricted &idth of a corridor0

he difficult- is avoided if the architect can be 'ersuaded to acce't a neat 'iece of

surface trun/ing0 here are no& several 'ro'rietar- s-ste)s of trun/ing s'ecificall-

designed for such an a''lication and giving a reasonabl- neat finished a''earance0he sides of cable trun/ing can be drilled to )a/e holes at &hich conduit can enter the

trun/ing0 oining conduit to trun/ing )a- be convenient &here one circuit leaves a linefollo&ed b- several circuits0 All such holes )ust be fitted &ith s)ooth bore bushes toavoid shar' edges. &hich could da)age insulation0 *f P=C sheathed cables run for 'art of


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their length in trun/ing. that is to sa- continue free &ithout further 'rotection. the edge of the

hole in the trun/ing )ust be 'rotected b- a rubber gro))et. or a suitable 'ac/ing gland.

so)eti)es referred to as a stuffing gland,0 Should the )etallic trun/ing not br reuired to

serve as a circuit 'rotective conductor. it )ust be erected so as to give good earth continuit-.

since it is an e('osed conductive 'art0

Plastic trun/ing

Cable trun/ing is also )ade out of rigid high


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Wiring 3

+igure 301" Si)'le( s-ste)

grade of P=C and the entire asse)bl- of insulated cable &ithin conduit is fle(ible0 +ro)

the installer,s dra&ings. the )anufacturers )a/e a /it consisting of cable alread- &ithin

the conduit. cut and for)ed to the lengths needed on the building0 he /it is delivered to

site and si)'l- laid in 'osition. a 'rocess &hich eli)inates )ost of the ordinar- site &or/

reuired for a conduit s-ste)0 Although it is su''lied &ith the cable alread- in the

conduit. the s-ste) is re&irable because each cable is loose in its o&n 'assage0 Suitable

conduit bo(es for unctions and accessories are su''lied as 'art of the s-ste)0

here are 'roble)s &hich can arise &ith such a s-ste)0 he conduit and cable are cut tosie at the factor- fro) dra&ings0 A slight error in setting out of the building can result in the/its not fitting at site. &ith )an- un'leasant recri)inations bet&een designer. su''lier

and installer0 Discre'ancies can occur even if di)ensions are ta/en on site usuall- at-'ical house or roo) is )easured and all of that t-'e are )ade the sa)e0 *f the buildingsetting out varies fro) one to another. so)e of the /its )a- not fit e(actl- and the siteelectrician )a- have so)e adust)ents to do0 *f ever- roo) has to be individuall- )easured.a great deal of the ti)e


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*t can be cli''ed to &alls and ceilings in a si)ilar &a- to P=C sheathed cable0 Shar'

bends should. ho&ever. be avoided. and a safe rule is to /ee' the radius of each bend to)ore than si( ti)es the cable outside dia)eter0 Cli's or saddles should be fi(ed at a

s'acing eual to "! ti)es the cable outside dia)eter0 *f the cable is to be buried &ithin

the structure. it should be fi(ed do&n fir)l- before concrete or ce)ent is 'oured over it0

Accessories are contained in standard bo(es and the *CC cable is brought into the

bo(es0 Where it enters a conduit bo( or the ter)inal bo( of a )achine. it ends in a sealand a gland0 he seal for)s the end of the cable and 'revents )oisture getting into the

)ineral insulation it seals the cable0 he gland oins the cable to the cable entr- on thebo(.

s&itch or other eui')ent and 'rovides earth continuit- bet&een the cable sheath and the

bo( or eui')ent case0 An asse)bled seal is sho&n in +igure 301# and the 'rocedure

for )a/ing it is as follo&s0

he end of the cable is cut to length and a notch is )ade round the sheath &ith a

ringing tool0 he sheath is then stri''ed off the cable. fro) the cable end to the notch.

and the )ineral insulation is bro/en a&a- to e('ose the conductors0 Alternativel- a

'ro'rietar- stri''ingringing tool )a- be used0 At this stage. the gland should be sli''edover the cable and 'ushed u' out of the &a- &hile the seal is being finished0 he gland&ill then be in 'lace to be brought for&ard over the seal after the latter is co)'leted0

he insulating sleeves are no& asse)bled0 he sleeves are cut to the length reuired.&hich )a- need e(tension sleeving for ver- long tails0 When the sleeve is 'ushedthrough the hole in the ca'. the &edge 'revents it fro) being 'ulled right through and

/ee's it in 'lace after the seal is asse)bled0

+igure 301# *CC cable seal


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Wiring !

+igure 3019 *CC gland

he ne(t ste' is to 'ush the 'ot over the end of the cable0 *t is )ade to suit the cable and is

a fairl- tight fit. so that it has to be scre&ed on0 *t is scre&ed on until the cable sheath is level

&ith the shoulder at the base of the 'ot0 he 'ot is then 'ac/ed &ith a 'lastic

co)'ound. &hich is 'ressed fir)l- in to fill the &hole of the 'ot0 he ca' and sleeve

subasse)bl- is no& 'ushed over the ends of the cables and forced into the end of the 'ot.

&hich is cri)'ed over the ca'0 his final o'eration can convenientl- be done &ith a

co)bined co)'ression and cri)'ing tool s'eciall- )ade for this 'articular ob0 *t is good'ractice to test each end for insulation resistance after the end has been )ade off if the test

is left until both ends are co)'lete. there is no &a- of /no&ing &hich end is fault- should

one of the ends be )ade off incorrectl-0

he gland is sho&n in +igure 30190 *t consists of a gland bod-. a co)'ression ring and

a gland nut0 he co)'ression ring is co)'arativel- soft and as the nut is tightened onto

the bod- the ring is co)'ressed bet&een the) and defor)s0 As it does so. the &ea/ened

ridge on its inside dia)eter bites into the outside of the cable sheath and )a/es a fir)

electrical and )echanical bond bet&een the sheath and the gland0 he gland is thus fi(ed

to the cable over the seal and has a 'roecting )ale thread &hich can be inserted into a

lu)inaire or conduit bo( and secured b- a ring bush and bac/ nut0 +or haardous areas a

s'ecial gland is reuired

E(ternal &iring

*t is so)eti)es necessar- to run cables fro) one building to another0 his can be done

either &ith cables running under the ground or &ith overhead cables0 Cables that are to berun underground can be ar)oured 'a'er. ar)oured P=C or *CC cables0 All these are

strong enough to be laid directl- in the ground and buried. but in order that the- shouldhave reasonable 'rotection against da)age the- should be laid at least %%)) belo& theground level0 Since the- all have )etal ar)ouring or sheathing. no 'rotection in the


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Design of electrical services for buildings

ground is reall- necessar-. but it is ver- usual to 'rovide a bed of sand. and cover the

cable loosel- &ith tiles before the trench in &hich it has been laid is bac/filled0 *f

so)eone later has occasion to dig the ground near the cable the- &ill hit the tiles first and

be &arned that there is so)ething underneath the)0 iles are available for this use &hich

have lettering on the) sa-ing Danger


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Wiring "

bet&een the t&o end su''orts and a sheathed cable can be sus'ended fro) the catenar- atregular intervals of about 2 or 3)0

Alternativel- it is 'ossible to obtain cable &hich incor'orates a catenar- cable &ithin the

sheath0 Such cable is s'eciall- )ade for overhead use. and the )anufacturer,s

reco))endations on s'acing should be noted and adhered to0Since air is a good insulator. overhead cables &hich are out of reach of 'eo'le or

ani)als do not need further insulation0 he cables &e have ust described are sheathed. but

overhead line cable is also )ade consisting of bare co''er or alu)iniu) conductors of a sie

to have enough )echanical strength to su''ort the)selves0 hese cables can s'an long

distances and are used b- electricit- co)'anies for their distribution s-ste)s outside to&ns0

he- are used )uch )ore in 'ublic su''l- s-ste)s than in building services0 Whatever

)ethod of running overhead cables is used. at the end of the overhead section the cables )ust

be connected

design of electrical services for buildings 4th edition 1

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