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Section 5
Cable Installation
Introduction
The installation of a power cable can look deceptively simple it
isnt! There are many aspects to be considered, as usual too many to
be dealt in any detail within our time constraints. So in these
notes we will look at some points with regard to installing cables
in trenches (the commonest situation for power cables), leaving the
student to extrapolate to other situations as they arise.
Objectives
At the end of this section you will be able to
state the main items of legislation that need to be considered
when installing cables
list key points for route preparation
describe cable installation methods
calculate pulling loads for cables in straight trenches and
ducts
describe methods of minimising pulling loads
Time
You will need about 3 hours for this section.
Resources
PC with Windows Excel or some other spreadsheet package.
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5.1 Excavation, Blinding, Backfilling & Reinstatement
Some definitions first:
Excavation obvious really, the digging of the hole!
Backfilling the placing and compaction of material to fill the
remainder of the trench, up to the level where the road or footpath
sub-structure commences.
Blinding the placing and compaction of backfill material in the
immediate vicinity of the cable(s), typically 75 mm below the
cable, 75 mm above, and the full width of the trench. Blinding
material is almost invariably imported to site soft sand for
circuits that dont require specific material properties; thermally
stable or stabilised backfill where rating requirements demand
material of known thermal properties.
Reinstatement the placing and compaction of the sub-base, base
and surface materials to restore the surface to its original
condition and strength.
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5.2 Legislation
Although not appropriate to deal in detail with the various
items of legislation that affect our work when installing cables
and their accessories, the student should be aware of the following
items in particular. But please note: the following items are by no
means an exclusive list, and you should always be aware of any and
all legislative items that might affect your works.
5.2.1 New Roads & Street Works Act 1991
The New Roads & Street Works Act deals with virtually every
aspect of cable installation in public land with the exception of
actually installing the cable! It applies to all undertakings who
work in roads, footpaths, etc. It is legally binding, and there are
severe penalties for not complying with its requirements. It covers
such things as:
Liaison with local authorities, police forces, etc.
The rights and duties of the various parties involved it is
notable that the rights of local authorities etc. far outweigh
those of the undertakings, who now have few rights but lots of
duties!
Signing and guarding of the works, traffic control, pedestrian
access, etc.
Technical requirements for backfilling and reinstatement of the
excavations, to ensure that the finished works are to a
satisfactory standard and will perform as required without further
attention, e.g. support the intended traffic without subsiding.
In the case of the Highways, the Secretary of State is the
manager of the highways under the Highway Act 1980. Highway
Authority is here the Street Authority. For non highways, then
Street Managers are the authority body or person responsible to the
management and control of the street example Local Authorities.
5.2.2 Health & Safety at Work etc. Act, 1974
This major piece of health & safety legislation
(www.legislation.gov.uk) affects every aspect of our lives whilst
at work, visiting the supermarket,
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riding on fairground rides whatever. The Act itself contains
little of direct relevance to any specific activity. Its
significance being principally two-fold.
1. It places a duty of care on both the employer and employee to
work safely, identify hazards and take steps to reduce those
hazards to acceptable levels, and:
2. It is enabling legislation, under which the Secretary of
State can introduce specific regulations etc. applicable to
particular activities or circumstances. For example:
The Management of Health & Safety at Work (Amendment)
Regulations 2006
Construction (Health, Safety and Welfare) Regulations 1996
Construction (Design and Management) Regulations 2007
Electricity at Work Regulations 1989
The UK Health & Safety Executive (HSE) publishes a large
number of supporting documents such as (in decreasing order of
legal obligation) Codes of Practice, Guidance Notes and Information
sheets. One of particular interest in the present context is their
Guidance Note:
Avoiding danger from underground services
The list of these health & safety publications can seem
almost endless, and is too extensive to be covered here in detail.
A visit to http://www.hse.gov.uk is highly recommended for anyone
undertaking site works.
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5.3 Some Points Regarding Cable Installation
Having designed the installation, carried out the route survey,
got agreement to the route, the timings of the works, etc.,
complied with your legal requirements, specified, tendered, ordered
and taken delivery of the cable, you can actually start digging the
hole!
5.3.1 Trenches & route preparation
Always bear in mind that cable routes can be dangerous places,
and always think first and foremost of the safety aspects of the
works safety of site personnel, the public (including traffic),
your equipment and materials, and, of course, the safety of other
undertakings apparatus.
Trenches should be carefully set up bearing in mind the
following points (it goes without saying that, once again, this is
not an exclusive list!).
Trench sides stable, full or half timbered as necessary to
support the sides.
Spoil to be placed away from trench edge to avoid it falling
back in.
Walkway along at least one side of the trench workmen walking on
piles of spoil is asking for trouble.
Maximise bend radius and ensure that the bends are uniform.
Smooth bottom, with gradual transitions in level.
Soft material to be placed and compacted before pulling the
cable.
No stones, sharp objects or protrusions.
Ducts should be proved with mandrel and cleaned immediately
before cable pulling.
A bellmouth should be installed in the start of the duct,
irrespective of the duct material.
A sump should be dug beneath the start of the duct to prevent
soil, stones, etc. being carried in to the duct.
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Rollers placed every metre, or more frequently for heavy
cables.
Rollers should actually roll (!) and have no sharp
projections.
Skid plates installed at bends, NOT vertical rollers.
Rollers and skid plates to be adequately secured.
Rollers and skid plates to be properly greased.
5.3.2 Alternative excavation methods
Numerous alternatives to open cut excavation methods are
available, and can often be used to advantage none provide the
perfect solution. All should be evaluated for their suitability for
the particular job in hand:
impact moling
guided boring/directional drilling
rockwheel
chain trencher
mole ploughing
micro-tunnelling
jack heading
thrust boring
auger boring
deep tunnelling
5.3.3 Installation equipment
The installation of large cables involves the handling of large
weights, and the application of large forces so the equipment needs
to be:-
suitably proportioned
in full working order
properly set up and properly applied.
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Cable drum set up
The drum stand or drum jacks must obviously be of adequate
rating for the drum weight, and they must be firmly supported on
stable ground, with plating beneath the stands/jacks if the ground
is incapable of supporting the weight directly.
The jacks must be erected vertical before any attempt is made to
lift the drum as any deviation from vertical will result in
instability as the drum is raised.
The drum spindle must be capable of bearing the weight of the
drum.
Collars should be fitted to the spindle on either side of the
drum in order to prevent a) the drum from shifting along the
spindle and fouling the drum stand/jacks, and b) the spindle
shifting in the stand/jacks.
The spindle should sit in bearing blocks and be greased to allow
rotation of the drum on the spindle. Minimising friction here is
important to minimising the pulling loads required to install the
cable.
The drum should only be raised to a height just sufficient to
allow the drum to rotate, and the spindle must be set up level.
The drum should be positioned so that the cable is pulled off
the top of the drum so that in the event of an overrun a loop of
cable is thrown over the side of the drum. If the cable is pulled
off the bottom, any overrun will result in the cable becoming
trapped between the drum and the ground and quite possibly severely
damaged.
The drum must not be allowed to rotate freely a braking system
needs to be available to stop the drum continuing to rotate under
its own inertia in the event of the pulling operation having to be
interrupted.
Pulling equipment
Once again it goes without saying that the equipment used to
pull the cable must be of adequate strength for the job, properly
maintained and correctly applied.
The winch must be capable of exerting the required pulling load,
and firmly anchored so that the cable is moving towards the winch
rather than the winch moving towards the cable a condition that is
not unknown!
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The winch should be fitted with a dynamometer to monitor/measure
the pulling load, and ideally should be provided with a device that
can be set to limit the pulling tension to a value that will not
damage the cable.
The pulling wire (or bond) must be suitable for the task, and in
good condition. Broken strands, rust (which may not be visible on
the outer surface of the bond) and kinks can singularly or in
combination weaken the bond significantly. A bond that breaks under
tension is a very dangerous animal indeed.
NO ONE SHOULD BE IN THE TRENCH WITH THE BOND WHEN THE BOND IS
UNDER TENSION.
All pulling bonds should be of the type referred to as killed
bonds, where the natural tendency of the wire to untwist as tension
is applied has been reduced (or preferably removed) by virtue of
the construction of the bond and pre-conditioning.
Although rarely seen on site, a sheathed bond is to be highly
recommended. An extruded plastic covering is applied over the wire
bond, which serves to protect the wires against damage and
corrosion. It reduces the tendency to kink and significantly
reduces the coefficient of friction of the bond against the
rollers, skid plates, duct walls, etc. Sheathing also reduces
abrasion damage to plastic ducts.
A swivel must be installed between the bond and the attachment
to the cable so as to prevent any residual twist in the bond
transferring to the cable, and vice versa.
If the winch is not fitted with a dynamometer, a portable
dynamometer should be fitted between the swivel and the attachment
to the cable.
All shackles etc. used to connect the various components need to
be of adequate rating, in good condition and properly installed and
secured.
A length of rope should be tied to the nose of the cable and
used to lift the cable onto each roller failure to adopt this
simple precaution can result in rollers becoming displaced and
misaligned, risking damage to the cable itself.
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5.3.4 Backfill & reinstatement
The best advice here is refer to the New Roads & Street
Works Act, which goes in to these matters in some considerable
detail! Two points are worth stressing here.
Proper compaction is essential to satisfactory performance, both
thermally and mechanically. Compaction by mechanical means is
essential, the material being compacted in 75 mm layers with each
layer being tested before the next is placed and compacted.
The moisture content of the materials is critical to correct
compaction. If it is too wet or too dry, correct compaction cannot
be achieved. Materials must therefore not only be delivered to site
at the right moisture content, they must also be stored in such a
manner as to maintain it! If the moisture content moves outside
limits, the moisture content must be corrected, or the material
discarded.
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5.4 Cable Pulling Tensions
Reference has already been made to the high forces involved in
the installation of power cables, and the need for the equipment to
be up to the task we will now look at the magnitude of those
forces, and the factors that determine them.
Most engineers will be familiar with the simple arithmetic of
pulling a cable on a straight route:-
Figure 5.1 Calculating cable tension
Some typical coefficients of friction are given in the table
below:
Situation
Coeff. of friction
Rough surfaces
0.5 - 1.0
Cement ducts
0.4 - 0.5
Plastic ducts
0.3 - 0.4
Lubricated plastic ducts
0.25
Cable rollers
0.2
Ball bearing rollers
0.1
Table 5.1 - Coefficients of friction for different surfaces
Cable - W kg/m Pulling tension, T
L metres
= W.L. kg
Coefficient of friction =
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When a bend is involved, the situation becomes more
complex:-
is the angle of the bend in radians, and R its radius in metres.
T1, the load on the nose of the cable as it approaches the bend, is
given by
T1 W L1 :=
(5.1)
Actually, T1 = T0 + W.L1. , where T0 is the input tension to the
first part of the route the amount of pulling load required to pull
the cable off the drum. Even for a small drum T0 might amount to 50
kg, for large drums it can be several hundred kg.
When the nose has passed round the bend, the new nose load T2 is
given by
T2 T1 cosh ( )( ) T12 W R( )2+ 0.5
sinh ( )+:= (5.2)
Fortunately for most purposes this horrendous expression can be
replaced by the slightly more memorable:-
T2 T1 e := (5.3)
There is a small error involved when using the simpler
expression, about 3% when T1 = 100 kg, but the error decreases to
zero at T1 = 750 kg. Hence:-
T2 W L1 ( ) e := (5.4)
R
T1
T2
T3
L1
L2
Figure 5.2 Cable pulling tensions on a cable with a single
bend
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By the time the cable has reached position 3 the nose load T3
has increased to:-
T3 W L1 ( ) e W L2 +:= (5.5) Multiple bends further complicate
matters:-
and the number of s required escalates:-
T1 W L1 :=
T2 W L1 ( ) e 1:=
T3 W L1 ( ) e 1 W L2 +:=
T4 W L1 ( ) e 1 W L2 + e 2:=
T5 W L3 ( ) W L1 ( ) e 1 W L2 + e 2+:= (5.6)
Note: The above equations have a further simplification. They
assume that the coefficient of friction is uniform throughout the
route. If this is not the case, different values have to be used
for each section.
Even so, it is apparent that multiple bends can result in very
high pulling tensions being required at the cable nose, and that as
the bend angle increases, so does the resulting pulling tension.
And since the equations involve powers of powers, the increase is,
to put it mildly, not exactly linear!
The equations are for cables pulled horizontally, and have to be
modified for uphill and downhill pulls. And strictly speaking, they
only apply to a
R1
1
T1
T2
T3
L1
L2
R2
2
L3
T4
T5
Figure 5.3 Cable pulling tensions on a cable with multiple
bends
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single circular cable and have to be modified for twisted
trefoil assemblies; the equations for three single core cables
pulled into the pipe simultaneously (cradle formation) are
different again.
Exercise 5.1
It is suggested that you set up a spreadsheet program in order
to more conveniently undertake the following exercises.
Imagine that the cable drum is to be positioned to the left of
the above route, and the cable pulled towards the right. Ignoring
the load needed to pull the cable from the drum (i.e. T0 = 0),
evaluate the above equations for T1 T5 for the following
conditions:
W = 30 kg/m 1 = 30 degrees
= 0.25 2 = 90 degrees
L1 = 100 m R1 = 5 m
L2 = 100 m R2 = 3 m
L3 = 50 m
Your answer to Exercise 5.1
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Your answer to Exercise 5.1 continued
Turn to the end of the book for suggested answers to the
exercise
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Exercise 5.2
Repeat the above exercise, but now let T0 = 100 kg.
Your answer to Exercise 5.2
Turn to the end of the book for suggested answers to the
exercise
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5.4.1 Limits on pulling tensions
The pulling bond is commonly attached to the cable via a pulling
stocking, slipped over the end of the cable. Such arrangements are
only suitable for very light work, as the pulling load is
concentrated on a short length of cable, and the loads are applied
to the mechanically weak outer components of the cable.
These difficulties are overcome by pulling on the conductors,
using a specially designed pulling eye. This may be connected to
the conductors by soldering, by compression techniques, or simply
by mechanical clamping to the conductors. Of course, the pulling
eye must also incorporate suitable means of sealing the cable
end.
The conductors are the strongest components in the cable, but
they are not infinitely strong. Pulling loads must be limited,
depending upon the conductor material.
Conductor
material
Allowable load in N/sq.mm of
conductor area
Copper 50.
Aluminium 35
Table 5.2 Pulling loads for conductor metals
For 3-core cables, the maximum allowable load is calculated on
the basis of two conductors only. For a 3-core 185 sq.mm. cable,
copper conductors can withstand 1.85 tonnes, whilst an aluminium
conductored cable would be limited to 1.3 tonnes
5.4.2 Bond pulling
Where very heavy cables have to be installed, even conductor
pulling may be insufficient to accommodate the tensions needed for
a nose pull, particularly on difficult routes. The answer is to
adopt the bond pulling technique that distributes the pulling load
along the cable. And since the cable goes round the bends without
Figure 5.3 Bond pulling
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contact with skid plates, there are no bend effects to increase
the pulling tension so effectively the pulling load is simply:
weight/metre x length x coeff. of friction
5.4.3 Side loading limits
But the pulling tension is not the only important parameter.
When the cable travels round a bend it experiences a side load, and
the amount of side loading that a cable can withstand depends upon
its construction. Excessive side loading can flatten metallic
sheaths and apply continuous pressure to the dielectric, insulating
papers can be fractured, voids produced in solid dielectric cables,
screen wires indented into XLPE cores, and strippable screens
separated from the XLPE insulation. Not to be recommended!
The side loadings on the bends (sidewall pressure Pn) are given
by the equation.
P nT nR n
:=
where (5.7)
Tn = Pulling tension at the bend (kg)
Rn = Radius of the bend (m)
Taking the second bend of the earlier example.
P2W L1 ( ) e 1 W L2 + e 2
R2:=
(5.8)
The following are maximum side load limits with the cable
against skid plates, and wherever possible pulling should be
arranged to provide lower loads.
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Cable construction
Maximum side load
skid plates (kg/m)
Polymeric cables, no metallic sheath 400
Cables with corrugated aluminium sheaths 2000
Cables with lead sheaths 500
Table 5.3 Maximum side loads using skid plates for three cable
types
If vertical rollers are used on bends, the small diameter of the
rollers results in the side loading increasing dramatically.
Consequently, vertical rollers should NEVER be used on bends
well-greased skid plates are essential when heavy and/or arduous
pulls are required. But if you must use vertical rollers, the
following side load limits must be applied. See Table 5.4.
Cable construction
Maximum side load
rollers, kg/m
Polymeric cables, no metallic sheath 100
Cables with corrugated aluminium sheaths 200
Cables with lead sheaths 50
Table 5.4 Maximum side loads using rollers for three cable
types
This means that the pulling tension must be greatly reduced, and
hence the length of cable that can be pulled is reduced.
Obviously, the tension plays a major role in determining the
sidewall pressure.
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Exercise 5.3
Using the same parameters as for the previous exercises,
calculate the sidewall pressure on the cable at the second bend (2,
R2) of the above route, when pulling from left to right, with T0 =
0 kg and T0 = 100 kg.
Your answer to Exercise 5.3
Turn to the end of the book for suggested answers to the
exercise
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Exercise 5.4
Now imagine that the drum is to be positioned at the right hand
end of the above cable route, and the cable pulled from right to
left you will have to re-write the equations accordingly. Using the
same parameters as for the previous exercises, and again ignoring
the drum load (i.e. T5 = 0 kg) calculate the tensions on the cable
nose at the positions corresponding to T0T5, and the sidewall
pressure on the cable at the (2, R2) bend.
Your answer to Exercise 5.4
Turn to the end of the book for suggested answers to the
exercise
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Exercise 5.5
Repeat the calculation, but this time set the drum load to 100
kg.
Your answer to Exercise 5.5
Turn to the end of the book for suggested answers to the
exercise
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5.4.4 Minimising pulling loads
It should now be apparent that careless setting up of a route
can result in pulling loads being much higher than necessary,
thereby restricting the amount of cable that can be installed in a
single pull, and increasing the risk of damage being inflicted upon
the cable. Consideration of the equations, together the examples
that you have worked through, will show how the tensile and
sidewall loadings on the cable can be minimised. In summary:
1. Maximise the radii of all bends and wherever possible
minimise their included angle.
2. Calculate the pulling loads for both directions of pulling
before deciding which way the cable should be pulled.
3. Reduce the coefficients of friction by:
lubricating ducts and skid plates
ensuring that adequate number of rollers are used
ensuring that the rollers are well lubricated.
4. Pay particular attention to the start of the route, as high
loads here have the most significant effect on the pulling load. In
particular:
minimise the load needed to pull the cable off the drum.
Powered rollers or caterpillars can also be used, either at the
start of the route or part way along it, to further reduce the
loads. As an alternative, multiple winching positions can be set up
along the route the cable output from one section of the route is
then the input to the next section, but the input tension is
reduced to zero, effectively splitting the route in several smaller
pulls.
5.4.5 Other factors affecting maximum pulling lengths
It should be obvious from the foregoing that, so far as nose
pulling is concerned, the route itself can determine the maximum
length of cable that can be pulled in one go when the pulling and
side loads are at the
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maximum values that the cable can withstand, then thats it, you
cant go any further without damaging the cable.
Thats not the case with a full bond pull of course, which
distributes the pulling load along the whole cable length and
eliminates side wall pressure limitations for a bond pull the
length that can be installed in one pull is, in theory, almost
unlimited.
However there are other, perhaps less obvious, factors that,
individually or in combination, may impose other limits on the
cable length. These might include:
The maximum length of cable that can be manufactured as a single
length
The maximum weight of cable that can be handled by the
factory
The maximum weight of cable that can be transported to site
The maximum diameter of drum that can be transported to site
Space available on site for accommodating the transport vehicle,
crane and the drum stands
The ability of the ground to support the drum stand and cable
drum
The power of the available winching equipment
The length of pulling wire available
The strength of the available pulling wire
The time required to set up the route and pulling equipment, and
actually pull the cable into position
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References and Further Reading
1. Bartnikas, R. and Srivastava, K. D (2000) Power and
Communication Cables: Theory and application, , McGraw-Hill, New
York, , ISBN 0-07-135385-2
2. Heinholdt, L. et al, (1990) Power Cables and their
Application (3rd Edition), Vol. I, Siemens Aktiengesellschaft,
Berlin, ISBN 3-8008-1535-9
3. Moore, G. F. et al (1997) Electric Cables Handbook (Third
Edition), , BICC Cables, London, ISBN 0-632-04075-0
4. Peschke, E. F. and von Olshausen, R. (1999) Cable Systems for
High and Extra-High Voltages Development, Manufacture, Testing,
Installation and Operation of Cables and their Accessories:
Publicis MCD Verlag, Munich, , ISBN 3-89578-118-5
For safety related information, visit the HSE website at:
http://www.hse.gov.uk
When the nose has passed round the bend, the new nose load T2 is
given byand the number of s required escalates:-where (5.7)Taking
the second bend of the earlier example.