Basic Math for Cable Termination

8/12/2019 Basic Math for Cable Termination

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Cables



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1. General

Cables f or m an impor tant par t of any installation but, because they ar e static,

and in nor mal ser vice ar e ver y r eliable, they do not always r eceive the attentionthat they deser ve.

Ther e ar e thr ee categor ies of cables associated with industr ial installations -power cables, contr ol cables, and special cables, f or example,communications and data tr ansmission cir cuits. It is the f ir st two categor ieswhich ar e descr ibed in this chapter . A power cable contains one, two, thr ee orf our cor es each consisting of a copper conductor sur r ounded by insulatingmater ial; a contr ol cable usually has many cor es and is known as a 'multicor e'cable. Aluminium is sometimes used as a conductor mater ial; although itsconductivity is less than that of copper , it is somewhat cheaper . lessCorr osion pr oblems, however , pr eclude its use on Shell installations, par ticular ly off shor e.

2 Power Cables

Cables are designed for both high voltage and low-voltage transmission of

power , the general constructions is similar in both cases, high-voltage cables have

thicker insulation and usually have smaller conductors than the low-voltage cables

which handle the heavier Currents.

one, two, three or four insulated conductors or mechanical protection, wire armor,

and a colored outer protective sheath, over the armoring, as shown in Figure 1,

2.1 General Constr uction

A power cable is made up of one, tow, three or four insulated conductors enclosed

in a bedding, for mechanical protection wire armoring is wrapped around the

bedding and a colored outer protective sheath usually of PVC is extruded over the

armoring as shown in Fig. 1 , each insulated conductor is known as a core .



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Fig. 1 Three Phase Power Cable

2.2 Conductors

The size of the copper conductor f or ming one of the cor es of a cable is

expr essed in squar e millimeter s (mm2), and the cur r ent r ating of the

cable is dependent upon the cr oss-sectional ar ea of each cor e. The ver y

smallest cables have conductor s consisting of only one str and of copper ;

lar ger cables however have str anded conductor s consisting of many

individual str ands or wir es laid up together ; this gives f lexibility, allowing

the cable to be bent mor e r eadily during installation. To achieve a cir cular

conductor , the number of str ands f ollows a par ticular pr ogr ession: 3, 7,

19, 37, 61, 127 etc, the diameter of each str and being chosen to achieve

the desir ed cr oss-sectional ar ea of whole conductor .

As seen in Figure 2, 3-cor e and 4-cor e cables in the lar ger sizes have

conductor s with the str ands laid up in a segmental f or mation; this



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achieves a better space f actor and r educes the over all diameter of the

cable. It also r educes the inductance of the cable due to decr eased

spacing between phases.

Fig.2

Segmental Cor es

Standar d conductor sizes r ange f r om I.5mm2 to 400mm2 f or 2-cor e, 3-

cor e and 4-cor e cables, and f r om 50mm2 to I000mm

2 f or single-cor e

cables.

2.3 Insulation, Covering and Stress Relief

Natur al r ubber or o i l -impr egnated paper is no longer used f or the

insulation of cables up to 3810/6600V; synthetic mater ials ar e now

used. For high-vo1tage cables the insulation is ethylene pr opylene r ubber

(EPR) and f or low-voltage cables it is polyvinyl chlor ide (PVC). EPR has

good electr ical properties and is r esistant to heat and chemicals; it is

suitable f or a conductor temper atur e up to 85oC. PVC is a ther moplastic

mater ial, ther ef or e car e must be taken not to over heat it; it is suitable

f or conductor temper atur es up to 70°C. PVC insulated cables should not

be laid when the temper atur e is less than 0o

C because it becomes br ittleand is liable to cr ack.



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High-voltage cables have an ear thed metallic scr een over the insulation of

each cor e. This scr een consists of a lapped copper tape or metallic f oil,

and its purpose is to contr ol the electr ic f ield within the insulation and

thus the voltage gr adient acr oss it, as shown in Figure 3. Also, it avoids

any inter action of the electr ic str esses due to the voltages on diff er ent

phase conductor s within the same cable.

Fig.3

Voltage Gr adient Acr oss High Voltage Cable Insulation

Cor e insulation may be colored r ed, yellow, blue and black to identif y the

thr ee phases and neutr al. Twin cor es ar e colored r ed and black. Single-

cor e cables ar e identif ied by colored PVC tape applied to the outer

sheath.

2.4 Cable Str ess Relief

The copper scr een is of ten ter minated in a 'str ess cone', which may be

seen in Figure 7. This is to spr ead the electr ic str ess which would

other wise tend to concentr ate wher e the scr een is cut off at a cable end

and could lead to br eakdown. This is f ur ther discussed in par a. 6.4



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2.5 Bedding

The bedding consists of a layer of PVC extr uded over the cor e insulation

as a base f or the ar mour ing.

2.6 Ar mour ing

Mechanical pr otection of the cable is pr ovided by a single layer of wir e

str ands laid over the bedding. Steel wir e is used f or 3-cor e or 4-cor e

cables, but single-cor e cables have aluminium wir e ar mour ing. With 3-cor e

or 4-cor e cables the vector sum of the cur r ents in the conductor s is

zer o, and ther e is vir tually no r esultant magnetic f lux. This is not so

however f or a single-cor e cable, wher e eddy-cur r ent heating would occur

if a magnetic mater ial wer e used f or the ar mour ing. Ar mour ing is

descr ibed as Steel Wir e Ar mour ed (SWA) or Aluminium Wir e Ar mour ed

(AW A).

2.7 Outer Sheath

The outer sheath of extr uded PVC pr otects the ar mour ing and the cable

against moistur e and gener ally pr ovides an over all pr otective cover ing.

High-voltage cables ar e identif ied by outer sheaths coloured r ed; a black

sheath indicates a low-voltage cable (see also par a. 7)

2.8 Selection of Power Cables

The f ollowing consider ations ar e taken into account when selecting a

power cable f or a par ticular application:

(a) The System Voltage and Method of Ear thing

A low-voltage system usually has a solidly ear thed neutr al so

that the line-to-ear th voltage cannot r ise higher than (line

volts) ÷ √ 3. However , cables f or low-voltage use ar e

insulated f or 600V r ms scor e to ear th and 1000V r ms cor e

to cor e.

High-voltage cables used in Shell installations ar e r ated

19000/3300V or 3810/6600V or 6600/11000V, phase/line. Inselecting the voltage gr ade of cable, the highest voltage to



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ear th must be allowed f or . For example, on a nor mal 6.6kV

unear thed system, a line conductor can achieve almost 6.6kV

to ear th under ear th-f ault conditions, To withstand this, a

cable insulated f or 6600/11000V must ther ef or e be used.

(b) The Nor mal Cur r ent of the Cable

The conductor s within a cable have r esistance, and

ther ef or e R2 heating occurs when curr ents pass thr ough

them, The maximum per missible temper atur e of the cable

depends upon the mater ial of the insulation, and a conductor

size must be chosen so that this temper atur e is not

exceeded. Tables giving the continuous cur r ent-carr ying

capacities of diff er ent cables ar e given in manuf actur er s'

liter atur e and in the Regulations f or the Electr ical

Equipment of Buildings published by the Institution of

Electr ical Engineer s.

The temper atur e of a cable depends not only on the r ate of

heat input due to the passage of load cur r ent but also on the

r ate at which the heat can be carr ied away. When using the

tables of current ratings it is impor tant to note whether they r ef er to cables laid in the gr ound laid in ducts or laid in air .

De-r ating may be necessar y if a number of cables ar e r un in

close pr oximity to each other .

Another consider ation in selecting a cable is the voltage Drop

(IR) f r om the source of supply to the load. A dr op of 1V in a

440V circuit is of little consequence, but it is a signif icant

per centage when the cir cuit oper ates at 24V,

(c) Abnor mal Cur r ents in the Cable

One abnor mal condition is a sustained over load; a cable must

be pr otected so that an over load cannot per sist long enough

to cause damage to the insulation by over heating. For

example, f or PVC cables laid in air , the over load must not be

gr eater than 1.5 times the continuous maximum r ated

cur r ent and must not per sist f or longer than f our hour s.

Another abnor mal condition is when a cable has to carr y a



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thr ough shor t-cir cuit cur r ent. In this case the temper atur e

of the conductor may be allowed to r ise to a higher value,

say 150°C, f or the shor t inter val between the onset of the

f ault and its disconnection. The shor t-cir cuit cur r ent that a

given cable can withstand depends upon the speed with which

the pr otection oper ates. For example, a PVC cab le having

conductor s of 185mm2

has the f ollowing shor t-cir cuit

r atings:

46kA f or 0.2 s

20.3kA f or 1.0 s

11.7kA f or 3.0 s

The 0.2s r ating would be suitable f or use with f use

pr otection, but wher e r elay-oper ated cir cuit br eaker s ar e

concer ned, a longer time r ating would be necessar y. Again,

tables of shor t-cir cuit r atings ar e available in

manuf actur er s' liter atur e.

3 Contr ol Cables

Contr ol cables usually have conductor s either 1.50mm2 or 2.50mm2

in cr oss-

section. The insulation, bedding and outer sheath ar e of PVC, and they ar e steel

wir e ar mour ed. Multicor e cables ar e available having 2, 3, 4, 7, 12, 19 and 27

cor es, each cor e being identif ied by a number on the insulation. The outer

sheath of contr ol cables is coloured gr een.

4 Miner al Insulated Cables

Miner al-insulated (MI) cables ar e used wher e the integr ity of a cir cuit is of

gr eat impor tance. They ar e par ticular ly r esistant to f ir e and ar e used in

cir cuits, such as communications or emer gency lighting, which must continue

oper ational as long as possible af ter f ir e has br oken out. They ar e also ver y

r obust and r esistant to mechanical damage.



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Fig.4

Miner al Insulated Two Cor e Cable

MI cables are constructed by assembling the single –strand conductor or conductors

inside a seamless copper tube.

After threading a number of 'tablets' of magnesium oxide insulating mater ial onto

the conductor s, the whole assembly - conductor s, insulation and copper tube -

is dr awn down thr ough a ser ies of dies until the magnesium oxide is cr ushed to a

powder and the whole cable is solid. The f inal appear ance is as in Figure 4.

Af ter annealing to make the cable mor e f lexible, an outer sheath of PVC is

applied.

MI cables ar e available in single-cor e f r om 1 mm2 to l50mm

2, in 2-cor e, 3-cor e

and 4-cor e f r om 1mm2 to 25mm2, and in 7-cor e f r om 1 mm

2 to 4mm2.

Special jointing techniques and mater ials must be used f or ter minating MI

cables, and gr eat car e must be taken to seal. the cable ends against the entr y

of moistur e.



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5 Method of Specif ying Cables

Ther e is a 'shor thand' method used to descr ibe the constr uction of any cable,

using abbr eviations to indicate the natur e of the va

r ious mate

r ials. Fo

r example,

a low-voltage cable might be descr ibed as:

(Ref er ence) (1) (2) (3) (4) (5) (6) (7) (8)

Abbr eviation 0.6/1kV STR CU/PVC/PVC/SW A/PVC

3-cor e, l50mm2

Inter pr eted this means:

1 0.6kV line to ear th

2 1kV line to line

3 Str anded conductor

4 Copper conductor

5 PVC conductor insulation

6 PVC bedding

7 Steel wir e ar mored

8 PVC outer sheath

Another example is:

6.6/11kV STR CU/EPR/SCR/PVC/ AW A/PVC/HO2/HCL

1-cor e, 630mm2

Wher e EPR indicates ethylene pr opylene conductor insulation

SCR indicates scr eened

AWA indicates aluminum wir e ar mored.

In par ticular PVC, when bur nt, r eleases lar ge quantities of HCI and also

pr oduces dense black smoke; f or example, a 1m Length of cable containing,

say,6kg of PVC can completely black out a r oom 1000m3

in site within f ive

minutes of the f ir e star ting.



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6 Installation

6.1 Cable Runs

Cables may be laid discr etely in the gr ound, r un in ducts or clamped to

cable tr ays; the thir d method is the most common in off shor e

installations. Each cable must be identif ied at each end, using a mar ker

bear ing the cable number .

Ther e is a pr actical limit to the conductor size which can be r un as a 3-

cor e or 4-cor e cable - it becomes too stiff and heavy to handle. A 3-

phase cir cuit is then r un as thr ee (or f our ) single-cor e cables. To minimise

the electr omechanical f or ces between the cables under shor t-cir cuit

conditions, and to avoid eddy-cur r ent heating in near by steelwor k due to

magnetic f ields set up by load cur r ents, the thr ee single-cor e cables

compr ising the thr ee phases of a 3-phase cir cuit ar e always r un clamped

in 'Tr ef oil' f or mation, as shown in Figure 5.

Fig.5

Single Cor e Cables Laid in Tr ef oil

At any instant in time the net magnetic f lux outside the gr oup of cables

due to the thr ee line cur r ents in them appr oximates to zer o because of

the symmetr ical cable Iayout.

Heavy cur r ent cable r uns, such as the low-voltage connections f r om a

tr ansf or mer , may consist of up to f our single-cor e cables in par allel per

phase; all 12 cables ar e r un bunched into f our 3-phase sets, each set laidin tr ef oil. In the case of 4-wir e systems the neutr al conductor needs a



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smaller cr oss-sectional ar ea than that of the phase conductor s and may be

met by one or mor e smaller single-cor e cables in par allel.

6.2 Cable Ter minations

A power cable is ter minated in an air -insulated cable box in off shor e

installations; it enter s the box thr ough a compr ession gland which gr ips

the wir e ar mour ing and seals the entr y of the cable. The outer sheath,

ar mour ing and bedding of the cable ar e str ipped back, enabling the cor es

to be spr ead to match up with the f ixed bushing ter minals, and the

insulation is r emoved to expose the conductor s.

In some high-voltage onshor e installations, especially outdoor ones, the

cable box may be f illed with compound, a tar -like substance which is

poured in hot and then sets har d to exclude moistur e. It can only be

r emoved by heating.

Fig.6

Low Voltage Cable Ter mination

Conductor s ar e ter minated either with lugs bolted to the f ixed bushing stems

as shown in Figure 6 or , f or heavier cur r ents, with cylindr ical f err ules which ar e

clamped into ter minal blocks. In either case the ter minations ar e cr imped onto

the conductors using either hand or hydraulic crimping tools. To make a good connection

it is vital that the lug or f err ule is the corr ect size f or the par ticular

conductor and that the corr ect die is used in the cr imping tool.



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Fig.7

High Voltage Cable Ter mination

Special measures must be adopted, when scr eened high-voltage cables ar e

ter minated, to pr event a concentr ated electr ic f ield being developed wher e the

copper scr eening tape is cut back; this str ong electr ic f ield could lead to the

insulation at that point being so over str essed that a br eakdown occurs. Special

str ess cones ar e f itted which ar e bonded to the scr eening tape; they contr ol

the electr ic str ess and r educe the r esulting voltage gr adients to a saf e value.

This arr angement is shown in Figure -7.

6.3 Single-cor e Cables

The conductor of a single-cor e cable and its sur r ounding metallic ar mour ing act

as a cur r ent tr ansf or mer having a 1:1 tur ns r atio; load cur r ent passing thr ough

the conductor pr oduces a magnetic f lux which, linking with the wir es of the

ar mour ing, induces an emf in them. If a cir cuit is pr ovided between the

ar mour ing at one end of the cable and at the other , a cur r ent f lows in the

ar mour ing which, if suf f iciently la

r ge, causes heating. This is shown in Figur e

8(a).

To contr ol these cir culating cur r ents insulated cable gland adapter s ar e used

wher eby the body of the gland, and consequently the wir e ar mour ing of the

cable, is electr ically isolated f r om the ear thed gland plate of the cable box by a

layer of insulation. Figure 8(b) shows a 3-phase cir cuit r un with single-cor e

cables using insulated cable glands. To contr ol the voltage of the ar mour ing it

must be bonded to ear th; this is done by deliber ately br idging the gland

insulation using bonding links. The ar mour ing can be bonded in one of two ways.In Figure 2.8{a) it is bonded at both ends of the cable r un (shown in r ed); the



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emf induced in the ar mour ing causes cur r ents (I A) to cir culate in the ar mour ing

which in heavy cur r ent cir cuits may lead to an undesir able temper atur e r ise in

the ar mour ing. Alter natively, the ar mour ing may be bonded at one end only as in

Figure 2.8(b); ther e is no cir cuit f or cur r ent to f low, but a voltage (EA) is

developed acr oss the gland insulation at the unbonded end. Wher e one end of

the cir cuit is in a hazar dous ar ea, it is customar y to bond this end so that any

ar cing that may occur due to emf s induced in the ar mour ing can only take place

in the non-hazar dous ar ea.

Ther e is one other magnetic pr oblem associated with single-cor e cables: wher e

such cables enter a cable box or pass thr ough par titions the conductor s must

pass thr ough holes in the gland plate. If these plates ar e made of a magnetic

mater ial such as steel, the magnetic f ields due to the load cur r ents in the

conductor s induce eddy cur r ents in the gland plate which may cause it to

become ver y hot. For ter minating or passing a.c. cir cuits using single-cor e

cables, gland plates of non-magnetic mater ial must be used.

Fig. 8

Insulated Cable Glands



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6.4 Contr ol Cables

Contr ol cables ar e also ter minated using compr ession glands. The sheathing and

ar mour ing ar e str ipped back to leave tails of the r equir ed length. Each cor e is

identif ied using plastic f err ules bear ing the wir e number , and ter minated using a

cr imped connector . The cor es ar e either laced up into suitable r uns using plastic

cable ties, or secured to cable

r acks within a cont

r ol panel.

7 Outer Sheath Colours

Standar d colours ar e used in Shell installations to identif y the system to which

the var ious cables belong; they ar e:

Red : High-voltage system

Black : Low-voltage system

Gr een : Contr ol and instr umentation system,

Or ange : Fir e and gas detection and telecommunications systems.

Blue : Intr insically saf e systems.

Yellow : Ther mocouple cir cuits,



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Types of Cables

PILC Paper Insulated, Lead Alloy E sheathed

PILCPVC Paper insulated, Lead Alloy E sheathed,

PVC Over sheath

PILCSW AS Paper insulated, lead alloy E sheathed,

single wir e ar mour ed, bitumen

PILCSW APVC Paper insulated, lead alloy E sheathed,

double steel tape ar mour ed, PVC

PILCDT APVC Paper insulated, lead alloy E sheathed,

double steel tape ar mour ed, PVC

PVCSW AS PVC Insulated, single wir e ar mour ed,

bitumen impr egnated hessian/ jute

PVCSWAPVC PVC insulated, single wir e ar mour ed,

PVC Over sheath

PVCLCSW AS PVC Insulated, lead alloy E sheathed,

single wir e ar mour ed, bitumen

PVCLCDT AS PVC insulated, lead alloy E sheathed,

double steel tape ar mour ed, bitumen

PVCLCSWAPVC PVC insulated, lead alloy E sheathed,

single wir e ar mour ed, PVC Over sheath

MICC Miner al insulated, copper cover ed

MICCPVC Miner al insulated, copper cover ed, PVC

Over sheath

EPRCSPGWBCSP EPR insulated, CSP sheathed, galv.

Steel wir e br aids ar mour ed, CSP

XLPESW APVC XLPE insulated, single wir e ar mour ed,

PVC Over sheath.



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Notes

1. The lead sheath is r equir ed f or elastomeric and PVC cables to

pr event r adial ingr ess of petr oleum based solvents which might per meate

thr ough PVC and cause deter ior ation of insulating mater ials and might

enter and

pass along str anded conductor s into equipment ter minations.

2. The PVC Over sheath is r equir ed as a pr otection against

corr osive conditions which could attack the steel wir e ar mor or lead

sheath. Cer tain conditions may r equir e other types of compounds.

Abbr eviations

PVC POLYVINYL CHLORIDE

SWA SINGLE WIRE ARMOUR

CPE CROSS LINKED CHLORIN ATED POLYETHYLENE

CSP CHLOROSULPHON ATED POLYETHYLENE

EPR ETHYLENE PROPYLENE RUBBER

HCI HYDROGEN CHLORIDE

HOFR CHLOROSULPHON ATED POLYETHYLENE OR

CHLORIN ATED POLYETHYLENE COMPOUND (HEAT, OIL,

RESIST ANT AND FLAME RET ARD ANT)

Si SILICON RUBBER

FR SPECI ALLY FIRE RESIST ANT

M M ARINE

XLPE CROSS LINKED POLYETHYLENE

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Basic Math for Cable Termination