installation of oil filled bnc

S5: Cable Installation

137 EEE8050 Power Cables

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.



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.



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,



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.



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.



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.



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!



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.



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.



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 =



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



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



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



Your answer to Exercise 5.1 continued

Turn to the end of the book for suggested answers to the exercise



Exercise 5.2

Repeat the above exercise, but now let T0 = 100 kg.





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



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.



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.



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.





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.





Exercise 5.5

Repeat the calculation, but this time set the drum load to 100 kg.





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



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



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.

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