Installation of Electric Cables

8/18/2019 Installation of Electric Cables

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Procedure for Transport, Handling and Installation of Electric Cables

Issue 02, October 2009

Please note. This is not a treatise on cable installation. It is supplementary information intended

to enhance engineers, technicians, electricians and cable installation personnel’s knowledge of

cable installation.

Cognisance must also be taken of Installation practices specified in BS 7671, IEC 60364 and

other International Standards for Electrical Installations.

We hope users find this information useful and invite and constructive (or corrective) criticism.

Cable Drums

a) Cable drums should be clearly marked or labeled with the following information:

1. Manufacturer’s name or trade mark.

2. Rated voltage, rated area, number of cores and specification.

3. Length of the cable in meters.

4. Year of manufacture.

5. Gross mass in kilograms.

6. The instruction “NOT TO BE LAID FLAT”.

7. Serial number or other identification.

8. On each flange an arrow with the words “ROLL THIS WAY”.

9. Mark of the Standards authority (if applicable).

b) Both ends of the cable on the drum should be sealed and the inner end fixed to the

flange of the cable drum to prevent loose coiling. The outer end is fixed to the flange as

well, for the same reason.

c) Drum protective closure should be maintained until cable is utilised.

d)

Cable drums should stand on firm, well drained surfaces.

e)

It is recommended that periodically the drum is rotated.


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Cable transportation

a) Preparation

i. The truck must be suitable for the drum size and weight.

ii. Do not overload trucks

iii.

Cable ends must be sealed, secured and protected.

iv. Use special cable trailers for depot to site transportation where possible. (See

below). Cables should not be removed from the cable drum and transported

independently to the installation site.

b) Loading

i. Check drums for correct cable and size, serial number, mass and any

form of possible damage.

ii. Select correct forklift or crane.

iii. Select correct slings and spindle and check their condition.

iv. If a crane is to be used, ensure that a spreader is incorporated to prevent damage to

drum flanges.

v.

If the drum is to be rolled, observe correct rolling direction by referring to arrows onthe drum flanges.

vi. Ensure the drum bolts are tight.

vii. Ensure that the truck surface is clear of obstructions, nails, etc.

viii. Do Not drop drums onto truck loading bed.

c) Securing

i. Secure drums to the truck bed to prevent sliding and rolling, using adequate steel

chains and chocks.

ii. Always try and pack drums flange to flange.

iii.

Do not lay cable flat.

iv. Stop the vehicle at periods during transportation and check that the load is secure.

d) Off Loading

i. Check the drums for any trace of damage. Damaged drums to be reported before off

loading.


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ii. Select correct spindle slings for the drum size and mass and ensure that they are in

good order. Ensure that a spreader is used.

iii. Do not drop the drums. Lower gently onto a firm and relatively level surface.

iv. Off load drums in such a way that they are easily accessible.

v.

If using a forklift, ensure that it is of adequate size relative to the task at hand.vi. Ensure the forklift forks lateral spacing is correct.

vii. Take care that the protruding forks do not damage other equipment or drums.

viii. There are two methods of rolling drums off loading beds if cranes are not available.

(see below).

Cable Storage

a.

Indoors

i. Stack flange to flange and preferably not one on top of the other. Do not lay the

cable drum flat.

ii. Stack so that drums are easily accessible.

iii. Observe fire precaution rules.

iv.

Cable ends must be sealed at all times.

v. If drums are expected to be stored for a long time they should be specially treated,

or, if applicable, use pesticides at regular intervals in the storage area to avoid

termite and rodent attack on wooden drums.

vi. Despatch on a “first in – first out” basis.

b. Outdoors

i. Drums should be stored on a hard surface at a slight angle and the area should have a

drainage system.

ii. Drums should be released on a “first in – first out” basis.

iii.

Cable ends must be sealed at all times.

iv. Stack flange to flange, but if this is not possible, limit vertical stacking practice to

smaller drums only. Do not store the cable drum flat.

v. Stack in such a way that drums are easily accessible.

vi. In areas having a “hot climate”, drums should be stored under a shade and should be

protected from direct sunlight.

Method 1: Hole excavated (Maximum slope 1 in 10) to receive truck.

Method 2: Ramp construction (Maximum slope 1 in 4).

Drum stop


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vii. Observe fire protection rules.

viii. Cables must be identifiable at all times.

ix. Cable racks are ideal for storage but take care not to overload.

x. If drums are expected to be stored for a long time they should be specially treated,

or, if applicable, use pesticides at regular intervals in the storage area to avoidtermite and rodent attack on wooden drums.

Mechanical Forces on Cables during Installation

All cables have a maximum pulling force which should not be exceeded during installation.

The cable construction imposes the limitation on the pull-in force (F). When a cable stocking

(Pulling Sock) is used the maximum force can be related to the overall cable diameter in mm

as follows :-

Steel Wire Armoured cables F = 0.94 * D4

* 10-6

kNSteel Tape Armoured or Unarmoured Cables F = 0.39 * D

4 * 10

-6 kN

Control and Communication Cables F = 0.26 * D4 * 10

-6 kN

Attempts should be made to limit the pulling force required to a minimum to avoid

stretching in the outer layers of the cable. This is particularly relevant when installing cables

at high ambient temperatures when thermoplastic bedding and sheathing materials may be

softer than usual, and thus having reduced tensile properties.

An increase in the pulling force is permissible when the cable is laid by means of a pulling

eye attachment to the conductors. As a rule of thumb, the following forces may be appliedto a conductor :-

Copper conductor 4.9 * 10-2

kN/mm2

Aluminum conductor 2.94 * 10-2

kN/mm2


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Then, for example, the maximum force that should be applied via a pulling eye to a 70mm2

3 core copper cable is :-

70 * 3 * 4.9 * 10-2

= 10.29kN

Generally when cables are installed using well-oiled rollers and jacks, the following forcescan be expected :-

Straight route 15 – 20% of cable weight

2 - 90° bends 20 – 40% of cable weight

Cables laid in open trenches should be left slightly “snaked” so that any longitudinal

expansion or contraction can be accommodated. Similarly when cables are installed in

cleats or on hangers a slight sag between fixing points is recommended.

Pulling cables Through Pipes or Ducts

When a cable is pulled through a pipe, friction between the cable serving and the pipe

material increases the longitudinal force requirements. Representative values for the co-

efficient of friction (µ) between the more common cable servings and pipe materials are

given in the table below.

Serving Material Pipe Material Co-efficient of friction (µ)

PVC Asbestos 0.65

PVC Metal (Steel) 0.48

PVC Pitch Fibre 0.55

PVC PVC 0.35

Bitumenised

Hessian

or Jute

Asbestos 0.97

Metal (Steel) 0.76

Pitch Fibre 0.86

PVC 0.55For Cables with servings of PE or LSF material, use the same Co-efficient of friction as PVC

This information can readily be used to determine the maximum length of cable that can be

pulled through a given pipe without exceeding the maximum permissible pulling force. Take

for example the 70mm2 3 core cable previously quoted, is this is a low voltage cable with a

PVC sheath and it is desired to know the maximum length of PVC pipe it can be pulled

through then :-

Maximum pulling force = 10.29kN (by pulling eye)

For PVC to PVC µ = 0.35

But force = µ * Reactive force

= µ * Cable weight


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= µ * Cable mass/102

Mass of cable that can be pulled = 10.29/0.35 * 102 = 2998 kg

(1kN = 102kg)

The mass of a 70mm2 3 core copper PVC insulated and wire Armoured cable is 3.6kg/m.

Thus the maximum length of this cable that can be pulled through a PVC pipe is :-

2998/3.6 = 832 metres

If there are any bends in the route then these will create additional loading and reduce the

length of cable that can be installed.

In certain instances when long runs in pipes or ducts are encountered it may be beneficial

to grease the cable with petroleum jelly or some other non aggressive compound to

facilitate pulling-in.

Considerable damage can be done to cable servings at the mouth of a pipe and precautions

should be exercised at such points. (The potential for damage is increased when a cable is

installed in hot conditions, as materials may soften due to the heat and the tensile strength

of the serving material is thus reduced). Fitting of a rubber grommet, or a Bellmouth*, at

the mouth to the pipe will reduce the potential for serving damage.

*A Bellmouth (in separable hinged halves) will guide the cable into the duct, avoiding

potential damage at the ducts entrance edges.

Preparation for cable laying

a. Planning

When planning a cable route there are several factors to be considered, amongst the

most important are :-

i. Ground Thermal resistivity (TR) tests.

ii. Position of joint bays


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iii. Provision to indicate on the “as laid” drawings, the serial or drum number of the

cable installed.

iv. Suitability of the cable type to the installation. e.g. LSF cables are not

recommended for use in installations in direct sunlight

*

(Cables with LSF propertiesare usually used in buildings or confined spaces where smoke and fumes in a fire

situation could endanger human life), or, buried direct in soil ( LSF properties serve

no purpose in the ground, the additional cost is therefore wastage).*

LSF materials

have lower mechanical strength than conventional materials, and therefore extreme caution must

be exercised when installing this type of cable. Twisting or over bending the cable during

installation could result in increased forces being exerted within the cable that could cause cracking

or splitting of the outer sheath. This is particularly important when installing this type of cable in

areas with hot climates.

v. Correct derating factors for cable current ratings for multiple circuits, differing soil

or ambient temperatures, depth of burial, etc. ( Variation of installation conditions along aroute - When the heat dissipation differs in one part of a route to another, the current carrying

capacity shall be determined so as to be appropriate for the part of the route having the most

adverse conditions).

b. Drum Handling

i. Always use the best hoisting equipment available.

ii. Do not drop drums of cable onto the ground as this can damage the drum as well

as the cable.

iii. It is most important that a minimum of rolling the drums on the ground be allowed

and then only in the direction of the arrows indicated on the drum.

iv. When rolling a drum of cable, to change direction use 2 steel plates with grease

between them, and by standing one flange on these plates the cable drum may

then be swiveled in the desired direction.

v. Position the drum prior to cable-pulling so that the cable is pulled from the top of

the drum.

vi. Note that a drum of power cable can weigh in excess of 10 tons, so make sure that

adequate cable drum jacks are used, that the spindle is strong enough to hold the

drum and that the jacks stand on firm ground and that they hold the spindle

horizontal.

vii. Site the drum at the most convenient place for cable-pulling, usually at the start of

a reasonably straight section near the commencement of the trench work.

viii. Allow for drum braking.


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c. Ground Thermal resistivity

i. This often governs the rating of a power cable buried directly, as does the

temperature of the soil. Losses for cables running at the maximum temperature at

which the dielectric system can faithfully operate for a maximum life of, say 30

years, are considerable, ranging from 15 W/m for normal distribution cables. Cableconductor temperature and the soil surrounding the cable must be able to

dissipate this heat effectively or thermal instability (runaway) will result. For

example an XLPE insulated 11kV cable with the conductors running at 90°C could

end up with a surface temperature close to 80°C resulting in drying-out of the soil.

Depth of burial plays an important factor here and has been set at 800mm. Most

MV cable current ratings are calculated with ground temperatures at 25°C at

depths of burial of 800mm.

LV cables are normally buried at 500mm. Soil thermal resistivity (the ability of the

soil to conduct or dissipate heat) is usually 1.2K.m/W. For variances in these

temperatures or thermal resistivity of the soil, contact the cable supplier for the

relative derating factors.

ii. The actual soil thermal resistivity along with the proposed route should be

measured as different soil compositions along the route will have different rates of

heat dissipation and could result in “hot spots”.

iii. To overcome this, bedding and backfill soils may have to be “imported”. Bedding

should be a layer of well compacted sifted soil. If the soil taken from the trench is

suitable, break it up and sift it through a sieve of maximum mesh size 12mm, then

ram it well down to provide a stone free bed on which to lay the cable. Where the

route lies in an area of made-up or aggressive ground containing rock, sharp

stones, clinker, slag, ash, bricks, etc., do not use this soil even if the particles are

small enough to pass through a 12mm sieve. For such sections, import loamy soil

for use as bedding and covering. Sand that consists of spherical particles, obtained

from a river bed or sea shore, is not suitable as its thermal resistivity is very high.

d. Positioning of Joint Bays

Ensure that there is sufficient working space, consider passing traffic and other

obstructions. If it is not possible to position the joint bay at standard cable length

distances, remember that the cable can be ordered in specific lengths. Consider

drainage for large bays and try to construct the bays prior to cable pulling to prevent

any damage to the cable at a later stage.


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e. Recording Cable drum Serial Numbers on “As Laid” Drawings

In the unlikely event of a cable failure in the future, quoting the drum serial number will

assist the cable manufacturer in his quality control, as this serial number is related tothe manufacturing and raw material management in the factory.

f. Preparing for cable Laying

The following “VITAL ACTIONS” must be observed prior to a cable pull.

i. Cable rollers must be placed between 2 and 4m apart in the

trench (Depending on cable size).

ii. Ensure that rollers can rotate freely. Grease or apply graphite

lubricants if required.

iii. Check that skid plates, or corner rollers, are secure and in

position.

iv. Ensure that each member of the pulling gang knows exactly what he is to do and

that communication signals between members are clear.

v. The trench floor must be clear of stones and other obstructions and the cable

bedding is correctly dispersed.

vi. Cable covers are available at convenient points.

vii. Any objects that may fall into the trench and damage the cable during the pull

and prior to backfilling have been removed. Similarly for cables installed on cable

ladders care should be taken that no sharp edges or protrusions are present that

can damage the cable.

viii. If the ambient temperature is below 10°C, or has been for 24 hours, the cable on

the drum will have to be covered with a tarpaulin and heated with suitable

lamps or heaters for at least 24 hours under close supervision. Ensure that

sufficient ventilation exists, and pay the cable off the drum slowly and carefully.

Similarly, if the ambient temperature is above 45°C, the cable drum should be

shielded from the sun prior to and during paying the cable off the drum.

ix.

Place the drum at a convenient point, prior to the pull, on strong jacks and on

sound footing (as mentioned earlier) with the arrow on the drum flanges

POINTING IN THE OPPOSITE DIRECTION to the rotation when the cable is being

pulled. The cable, on the jacks, should be at an angle of about 10 - 15° to the line

of the trench.

http://img.alibaba.com/photo/241732212/Cable_Pulley_steel_buried_cable_roller_cable_sheaves_cable_jacks_cable_drum_roller.summ.jpghttp://www.cablejoints.co.uk/images/Corner-Roller-CR41237475718.jpg


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x. Before pulling, cut the inner end of the cable free. Failure to do this will cause

buckling of the cable.

xi. The cable must be payed off from the top of the drum, but take care not to bend

it too sharply.

xii.

Cable pulling stockings should be examined and placed over the nose of thecable with care. The pulling rope or wire must be attached to the stocking in

such a way that the cable cap will not be damaged during the pull. The use of

swivels is recommended to prevent twisting of the stocking. The use of rope or

slings attached directly to the cable for the pull-in is not recommended.

Permissible mechanical forces are indicate in the section titled “Mechanical

Forces on Cables during Installation”. A dynamometer should be attached in line

with the pulling rope to indicate the pulling force exerted on the cable.

xiii. Bending radius of cables should not be less than what the cable manufacturer

recommends. Usually a minimum of 15 * the cable overall diameter is used.

xiv. One man should remain at the drum and “brake’ the drum in order to maintain

the correct tension on the cable during the pull. If the drum is allowed to

continue turning when cable pulling has ceased, there is a danger of the cable

kinking or twisting.

xv. Avoid twisting the cable, as it can result in ”bird caging” of the armour wires and

subsequent damage to the outer serving of the cable. This is particularly

important if the cable is to be installed by the flaking (Figure of 8) method where

twisting within the cable is more prevalent, especially in areas with hot climates,

as the heat can soften the sheathing material resulting in lower tear resistance of

the material. If the flaking method is used, the radius of bend should be as large

as the available space allows, but never less than the supplier specified radius,

and the number of layers at cross-over points must not be excessive.

xvi. Cables should be pulled to their final position in a continuous manner.


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xvii. Cables should be “snaked” on either side of each joint position to relieve the

joints of any thermo mechanical stresses.

xviii. If a winch is being used to pull the cables and unavoidable sharp bends are

encountered, a snatch block could be used to assist the pulling tension at the

bend.

g. Sealing of cable Ends

Once the cable pull is completed, the nose end of the cable is carefully lifted off the

rollers and placed on the bottom of the trench, leaving enough slack to terminate the

cable and observing the minimum bending radius. Immediately after cutting, the cable

must be suitably sealed on both ends of the cut to prevent the ingress of moisture.

Examine the nose cap, and if any damage has occurred to it during pulling, the seal

should be replaced.

h. Bond Pulling

These techniques are applied when heavy cables are to be laid, or the trench undergoes

many changes in direction, or very long lengths of cable have to be laid, or a

combination of these.

As in the previously mentioned methods of cable pulling, the trench would have been

prepared with cable rollers, corner rollers and skid plates. Snatch blocks would have

been anchored to the sides of the trench at bends and a winch placed at the far end ofthe section. At the near end a mobile bond carrier is placed conveniently adjacent to the

cable drum, ensuring that its braking system is adequate and that it has rewinding

facilities. A steel rope, more than twice the length of the cable to be pulled, is wound

onto the bond carrier drum and it’s end fed over the rollers and through the snatch

blocks and then secured to the drum of the winch.


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The cable end is manhandled onto the first rollers and tied to the steel rope (see below)

at intervals of about 2 meters. Start the winch and as the nose arrives at the snatch

block untie it from the steel cable, take it around the corner roller and retie on the

straight.

Once the nose has reached the winch end, and allowing the necessary slack, the cable

can be untied, the steel rope rewound onto the bond carrier. Further preparation for

backfilling may then be commenced.

i. Backfilling and Reinstatement

Once the cables have been laid, and before commencing with backfilling, carry out a

visual inspection of the installation to ensure that :-

a) The cables are properly bedded.

b)

Spacing between cables is correct, if there is more than one in the trench.c) Cable entrances at ducts are suitably protected against possibility of vermin gaining

entrance.

d) Laying and pulling equipment has been removed.

e) That there is no obvious damage to the cable sheaths. (Up to 90% of the service

failures experienced in any cable system can be avoided if appropriate action is

taken at this stage).

Before jointing is commenced, most of the cable trench should be backfilled. Tip sifted

soil, or specially selected sand, into the trench and rake it smooth to cover

the cable to a depth of 150mm, then ram it well down by means of hand

compacting tools. DO NOT use mechanical compactors at this stage.

Backfill and consolidate all of the trench with the exception of the joint bays

in this way. Where necessary, lay suitable cover tiles to cover the whole

area in which the cables are buried.


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Above this backfill layer, the trench soil, if suitable, can be replaced and

consolidated, in layers of 150mm. A Mechanical compactor can be used at

this stage.

If a marker tape is required, install it at a depth of about 200mm below

the ground surface.

j. Clamping or Strapping of cables on Racks or cable ladders

Similarly to cables installed for burial, once the cables have been laid on the racks or

cable ladders, carry out an inspection of the installation to ensure that :-

a) Spacing between cables and configuration is correct, if there is more than one

cable on the rack.

b)

Laying and pulling equipment has been removed.c) That there is no obvious damage to the cable sheaths. (Up to 90% of the service

failures experienced in any cable system can be avoided if appropriate action is

taken at this stage).

d) Where cable clamps are fitted, they have adequate cushioning material between

the clamp and the cable surface.

e) Where cable straps are fitted, that they are not excessively tight and do not have

sharp edges that may stress the cable serving material.

Available with Neoprene inserts

http://www.indianexporters.com/uploadedimages/202500.jpg


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We trust this basic procedure will assist you with your cable

installations.

References :-

“Power Cables and their applications” by Lothar Heinhold

“Electric Cables handbook” by Mc Alistair

BS 7671:2008 – Requirements for Electrical Installations.

IEC 60364 – Electrical Installations of buildings.

BS 8512:2008 – Code of practice for the storage, handling, installation and disposal of

cables on wooden drums

SANS 0198 – “Code of practice for the selection, handling and installation of power

cables”

Installation of Electric Cables

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