Consumers installations in the low-voltage range are very frequently connected to the power supply networks by underground cables. In this, the traditional paper-insulated compound-impregnated cable is more and more replaced by plastic-insulated dry cable.
At almost the same current-carrying capacity several advantages are offered by plastic-insulated underground cables as compared to compound-impregnated cables:
- omission of the expensive lead sheathing,
- the insulation value will not be reduced due to loss of compound.
- considerably lower weight.
- simpler preparation,
- no problems in overcoming differences in height.
Fig. 3.1. Plastic sheathed cable with copper conductor
1 copper conductor, 2 plastic insulation, 3 common conductor covering, 4 armouring of steel strip, plastic sheath
The concentric conductor of the plastic cable in fig. 3.2. may be used as centre conductor or protective conductor.
Fig. 3.2. Plastic cable with solid aluminium conductors
1 aluminium conductors, 2 plastic insulation. 3 common conductor cover, 4 concentric conductor of aluminium flat wires (armouring), 5 protection against corrosion, 6 plastic sheath
In lieu of the conductor material copper, becoming ever more expensive and scarcer, more and more underground cable the conductor material of which consists of aluminium is being applied.
Power cables have standardized designations from which the construction and type of cable may be seen!
The first thing to be done before starting to lay the cable is to fix the cable route under consideration the tracts of land and rights of ownership and utilization. On principle the same is valid here as when fixing the route of an overhead line.
Fig. 3.3. Fixing the cable route prior to running out the cable
1 building to be connected, 2 improper cable route since damage will be caused to ways, ornamental shrubs and banks of waters, 3 proper cable route along a way without mentionable damages to the property
Underground cables are laid in cable ditches. Depending on the properties of the soil and the degree of danger to damaging the cables these cable ditches are from 70 to 120 cm deep. The base of the cable ditch must be so wide as to offer sufficient space for the cables to be laid. The distance between cables is to be approx. 20 cm. In order to prevent damage to the cables they are placed on sand, fine gravel or screened soil. The excavated soil stays beside the cable ditch. After completion of cable laying it will serve to back-fill the remaining ditch.
Fig. 3.4. Cross section of a cable ditch
Prior to laying the cable the required cable and auxiliaries for this activity will have to be provided. During this, special attention will have to be paid to the transport of the cable drums. Unloading the cable drums is preferably to be done by mobile crane or from a special trailer. Under no circumstances must cable drums be dropped from a transport vehicle.
In difficult terrain a provisional ramp may serve as an auxiliary. By allowing the cable drums to drop down damages to the cables and cable drums may be caused.
Cable drums are to be moved only in rolling direction. The rolling direction is marked on the cable drum.
Prior to processing, the cable is to be tested for current flow!
In case of the cables showing damages to the insulation, in particular the external insulation, such cables must not be used. If necessary a sleeve will have to be fited to the faulty point.
The cable drum is firmly placed on cable jacks or a cable car. Care has to be taken to position the cable drum horizontally only a few centimeters above the ground. This will prevent the cable drum from slipping off laterally during the cable is run out. A braking device is to be provided at the cable drum.
Fig. 3.5. Jacked-up cable drum with braking arrangement
During reeling off the cable should continuously run out under tension from above
The cable has to be pulled off the drum opposite to the rolling direction indicated on the cable drum. The cable must neither be kinked, nor must there be formed loops or curls!
When laying cable, preferably devices for mechanical cable laying will have to be employed. For that purpose winches and cable handling machinery are to be taken.
Cable handling machinery is to be employed in case of long and winding routes.
When laying cable by means of a winch the cable will be joined to a pulling rope by means of a cable stocking of metal.
The pulling speed is to be slowly and flexibly adapted to the tensile force of the cable, in which case
the permissible tensile forces must not be exceeded!
In distances of 5 m idlers are to be positioned on which the cable runs. In curves angular idlers will have to be arranged. If application of machinery for cable laying is not feasable the cable will have to be pulled off manually. The cable is carried with the arms spread downwards.
Fig. 3.6. Support by idlers of cable being run out
Fig. 3.7. Position of workmen during running out the cable
1 workmen, 2 supervisor
Cables must not be carried on the shoulder because the permissible minimum bending radius may be exceeded resulting possibly in ruptures of cable or insulation. Cables must not be dragged over the ground or other obstacles!
During cable laying the permissible minimum bending radius will have to be observed.
The minimum bending radius is to be related to the external diameter of the cable!
Table 3.1. Permissible bending radii (da) of cables
Type of cable
during laying or bending
smooth aluminium sheath
da: external diameter of cable
At temperatures below 4°C cable is not to be laid any longer. Cables will become brittle at low temperatures and will be easily broken. In urgent cases also at low temperatures cable may be run out, if the cable is warmed up beforehand, e.g. by means of a heater fan.
Cables will be covered in cable ditches by bricks or concrete ducts resp. cable covering hoods. Cables may also be laid into cable ditches embedded in sand without cover. However, in this instance the route of the cable must be marked by an approx. 10 cm wide yellow plastic tape, placed 30 cm above the cable. This exemption holds, however, for cables with a concentric conductor only.
Cables being laid into the ground without cover must be dug in at least 1 m deep!
If cable covering hoods are being employed the cable will have to be surrounded by sand. Existing cavities between cable and covering hood will obstruct heat dissipation from the cable to the soil.
Beneath streets resp. roads cables are being passed, laid in cement ducts. There are cement ducts for 1 to 4 draughts. They are made of concrete and are to be put together without a gap between them.
Beneath railway tracks cables are to be laid in tubes 1.50 m deep. Alongside gas pipelines cables are being laid in a distance of approx. 25 cm.
Fig. 3.8. Cement duct with 3 draughts
Fig. 3.9. Cable covers
1 bricks, 2 cable covering hoods
Also in the instance of crossing other cables, e.g. telecommunication cables, both cables are to be protected mechanically. Each 3 to 5 m the laid cables will be marked. For that purpose plastic cable marking tape is being used.
Colour and printing on the cable marking tape will inform about
- cable type and number of cable as well as
- cross section.
Before cutting sheathed cable, the sheath of the cable will be tied on both sides of the cut. For that purpose endless bindings are made of firmly wound twine. The principle of such a binding may be seen from fig. 3.10. For cutting hand hack saws or strength-saving cable shears are being employed.
Within buildings the outer jute cover of cables is to be completely removed for reasons of fire protection!
Fig. 3.10. Cutting of a lead-sheathed cable with jute envelope and representation of an endless binding at the cable
1 pull twine by flat pliers until the twines upper end is in the binding, thereafter cut end at the binding
For drawing or correcting the cable layout plan, cables, cable joints, underpasses as well as particular crossings and exposures will have to be surveyed prior to back-filling the cable ditch. The cable ditch will be back-filled by 30 cm thick layers of soil. Each layer will have to be compacted.
Fig. 3.11. Simplified segment of a cable layout plan and cable marker with information on longitudinal extension
1 switchgear, 2 consumers installation, 3 cable, 4 site of cable laying machine, 5 sites for placing cable drums along the route, 6 direction of earth excavation, 7 crossing with pipeline, 8 street underpasses, 9 streets, 10 cable marker
The position of cables in ground without fixed reference points is marked by cable markers.
They consist of ashlar or concrete and bear an abbreviation.
Prior to making conductive connections at the cable, the individual insulation layers will have to be removed.
Processing plastic insulated cable is quite simple. For a cable according to fig. 3.2. the following working steps will be taken:
1. Removal of plastic sheath
Take measurements; by dismantling device make longitudinal incision in plastic sheath; make the circular cut; pull off sheath.
2. Bringing out the concentric conductor
Deflect the individual armouring wires and bunch the wires
3. Cutting of sheet
Nick the sheet by triangular file just above the deflected armouring and tear off
4. Removal of the common conductor cover
Make longitudinal incision by dismantling device; make circular cut; fan out conductor; tear off common conductor cover; clean conductors plastic insulation from adhering residues of plastic material.
Fig. 3.12. Removal of insulation layers from a cable with concentric conductor
1 plastic sheath, 2 cable fixing clamp, 3 concentric conductor, 4 common conductor cover, 5 plastic insulation, 6 conductor, 7 clip 10 x 1 mm in a distance of approx. 120 mm, 8 cable terminal
For removal of the insulation layers from paper-insulated compound-impregnated lead-sheathed cable the following working steps are to be taken: (see fig. 3.13.)
1. Make tie and remove the external jute cover.
2. Fit collar out of a piece of armouring sheet, cut the armouring by filing and remove same.
3. Remove cushioning layer.
4. Clean lead sheathing by a non-inflammable dissolvent.
5. Solder on copper strand to lead sheath.
6. Make circular incision on lead sheath; make parallel incision in lead sheath; roll-back the loose strip by flat pliers; tear off lead sheath along circular cut; slightly widen the upper edge of the remaining lead sheath.
7. Tie off the belt insulation; unwind and tear off belt insulation.
8. Spread conductors and cutt off fillers.
9. Tie off conductor insulation; remove conductor insulation; clean conductors; finally deflect conductors.
Fig. 3.13. Working steps on removal of insulation layers at paper-insulated lead-sheathed cable
Special care has to be taken in soldering on the earthing strand. For this purpose propane gas soldering devices or a blow lamp for soldering are being employed. The lead sheath and the copper strand to be soldered on are to be cleaned from all oxides. The lead sheath may be cleaned by scraping with a stripper. The copper strand is to be tautly placed around the lead sheath and the soldering surface to be wetted by soldering flux. Solder tin stick is to be laid on in pieces and to be shaped like a bulb by means of a dampened rag after thoroughly warming the soldering surface.
Caution! In case of too much warming of the lead sheath at one point, perforations of the lead sheath may result!
Fig. 3.14. Copper strand soldered onto lead sheath
1 lead sheath, 2 copper strand, 3 soldering surface
For protecting cables from ingress of humidity into the insulation, for mechanical protection as well as for preventing cable impregnating compound to emerge from compound-impregnated cables, conductor connections at cables are being made by means of sleeves and cable terminations.
Sleeves serve for connecting cables to each other. Connecting sleeves and tee joints are discerned.
Cable terminations are being installed at points of the cable where transition from cables to open-ended line systems, e.g. bus bars takes place.
There are differences in respect to the construction of sleeves and terminations. Practically sleeves and cable terminations of cast iron, plastic and cast resin are being used.
Fitting of cable sleeves takes place in the cable ditch. For easing the work of the cable fitter providing a work pit is recommended.
For making the sleeve joint both ends of the cable must overlap each other for approx. 50 cm and run towards the sleeves end as an s-shaped load reducing curve.
Sleeve bodies of grey cast iron are split. The upper and lower parts of the sleeve will be bolted together after accommodating the cable.
Fig. 3.15. Placement of cable in front of a connecting sleeve
1 cable ditch, 2 working pit, 3 ends of cable, 4 cable sleeve, 5 flatly placed brick
The outer cover of the cable extends into the inner end of the sleeves housing. Each opening of the sleeve for entering the cable will be sealed by a bituminous felt batting wrapped around the cable.
The lengths of the lead sheath and of the belt insulation in case of paper-insulated lead-sheathed cables to be stripped are dependent on the cable cross section. The connection of the conductor ends to each other can be achieved by stamping, soldering, welding or screwing. Stamped connections can be made in a very simple and firm way by sliding a stamping tube on to the prepared conductor ends which will - similar as in case of stamped terminals - conductively connect both conductor ends after stamping. Within the sleeve the conductors are insulated and spaced by means of stems.
Fig. 3.16. Sectional view of a connecting sleeve at a plastic cable with concentric conductor
1 cable, 2 lower portion of sleeve, 3 upper portion of sleeve, 4 lid, 5 compression clamps, 6 earthing socket, 7 bituminous felt batting, 8 flexible earthing strand, 9 stems, 10 sealing compound
The concentric conductors and lead sheath of the cables are being connected to each other.
It is also very important to include the sleeve body into the earthing system.
When installing service tap boxes the main cable needs not be cut!
Cable sealing compound by which the sleeve is completely sealed is easily melted and highly insulating. It will have reached its processing temperature if bubbles do not raise any more in the melting container. Then further heating must be stopped. If during heating the sealing compound foam is created on the surface it will have to be stirred until the foam subsides. Foam means humidity in the sealing compound reducing the insulating capacity of the sealing compound considerably.
After approx. 10 minutes the sealing compound is to be poured carefully through the lid of the sleeve. Since the sealing compound will shrink during the cooling process, refilling will have to be done.
In case of cable terminations to be sealed by sealing compound the same process will be applied as when preparing a cable sleeve.
In the instance of plastic-insulated cable sealing by cable termination is required only if ingress of humidity may take place!
Making cast-resin terminations is simpler than fitting cable terminations sealed by sealing compound.
Also in this case it will be important to seal the cable when working with paper-insulated lead-sheathed cable. This will be achieved if the conductor insulation, the belt insulation and approx. 10 mm of the metal sheathing are being wrapped in two layers of PVC-tape. Each layer of PVC-tape with an overlap of 50% is provided with a coat of PVC-adhesive. For this termination no box body, but a mould only will be required. Into this mould, being arranged around the cable and sealed, a mixture of synthetic resin and silica sand will be poured.
The moulds concentric position around the cable must be paid attention to!
If strictly observing the mixing ratio and the curing time the casting will be permanently hardened after approx. 24 hours. Then the mould may be removed and the casting be cleaned (de-burring).
Fig. 3.17. Cable termination of cast resin
1 cast resin, 2 cast resin and silica sand, 3 conductor insulation covered by PVC-tape, 4 belt insulation with binding (4 windings), 5 lower PVC-banding, 6 copper strand, 7 stamped cable terminal, 8 metal sheath, 9 external protective cover
Synthetical resins are chemically aggressive. For protecting the skin and the clothes, protective apparel will have to be worn when making terminations of cast-resin. During this work the eyes are particularly endangered. They are to be protected by safety goggles.
Fitting of Cable sleeves and cable terminations is a responsible work. Vocational experience and special knowledge will be required. Due to this, cable sleeves and terminations should in principle be mounted by experts having received special training only! The completed cables will be fixed by a double-tongued mounting clip (see fig, 3.12.) or other mounting elements, e.g. an angle iron, in front of the connection point.
Conductor connections on cable terminations are quite frequently made via cable terminals.
In case of the traditional methods of connection like soldering, welding or clamping of cable terminals to conductors no absolute safety for an unobjectionable connection will be given.
Stamped connections have replaced these traditional types of connection. Stamping is distinguished by reliability of the procedure, durability of the connection also under unfavourable conditions as well as by uncomplicated handling of the stamping tools. For the stamping of cable terminals mechanical hand stampers, or in case of bigger cross sections, hydraulic stamping devices are being employed.
Fig. 3.18. Hydraulic stamping tool
1 stamping head, 2 safety valve, 3 pressure hose, 4 pressure reducing valve, 5 pump lever, 6 pressure tank, 7 pump handle, 8 bearing grip
Fig. 3.19. Stamping head of a hydraulic stamping tool
1 Pressure tank of the stamping head, 2 space for stamping inserts, 3 hose connection, 4 screw cap, 5 piston
Stamping inserts being matched to the type of cable and cross section of the conductor will be fitted into the stamping tools.
There are two different types of stamping inserts:
- stamping inserts for V-grooving
- stamping inserts for hexagonal stamping.
V-grooving is on principle applied to copper conductors only!
Cable terminals are marked with information about the size of the diameter of the connecting pin and the conductors diameter.
Fig. 3.20. Stamping inserts
1 V-grooving, 2 hexagonal stamping
The stamping inserts for the cable terminal are in any case to match the cable terminal.
The designations of suitable stamping inserts are to be taken from the tables in the valid regulations.
When stamping on cable terminals the following fitting process is to be adhered to:
1. Cutting conductor lengthwise
The conductor is to be matched to the length of the cable terminal up to the connecting pin. It is to fill out the cable terminal. The elongation of the cable terminals connection during hexagonal stamping has also to be considered. It has to be subtracted from the length of the conductor. Depending on the cross section in question this change of length may be between 3 and 5 mm.
Fig. 3.21. Cable terminal prior to and after stamping
2. Removal of the conductors insulation and cleaning of the contact surfaces.
The outside of the conductor and the inner contact surfaces of the terminals barrel must be metallic-bright. Contaminated conductors and barrels of terminals will have to be cleaned from adhering dirt and oxides by wire brush.
Every conductor has to be cleaned only just prior to the stamping process, max. 10 minutes beforehand!
3. Entering the conductor into the barrel of the terminal
Difficulties which may arise in that process are best overcome by chamfering the conductor laterally or by fixing a taut wire binding adjacent to the cut surface which will have to be removed after having entered the conductor into the cable terminals barrel. Segmental conductors are to stamped circularly by the corresponding stamping inserts prior to putting on the cable terminal.
Fig. 3.22. Entering the conductor into the cable terminals barrel by means of a wire binding
4. Stamping on the cable terminal
V-grooving is to be performed so as to have the local impression situated at the upper or lower side of the terminals barrel aproximately in the centre. In particular, maintenance of the required stamping depth has to be observed. This will be achieved when the stamping jaws of the stamping insert will have closed or corresponding markings have been reached.
Fig. 3.23. V-grooving of the upper or lower sides
In case of hexagonal stamping of aluminium conductors the stamping insert has to be applied in between the stamping marks on the cable terminal. If two stampings are to be made, stamping will be started at the point nearest to the terminal lug. The burrs resulting from hexagonal stamping at the cable terminal will have to be removed.
Fig. 3.24. Connection to cable terminal of aluminium conductors with one or two hexagonal stampings
If after closing of the stamp jaws further pressure is exerted on the stamping tool this will unconditionally lead to heavy damage at the stamp inserts or the stamping device itself (risk of accident!)!
Stamp jaws which are not completely closed do not warrant a durable electrical connection (restamping to be done!)!
Lapping the conductors, which has been dealt with in section 3.4. on processing of paper-insulated lead-sheathed cables, takes place after fitting the cable terminals.
For soldering of cable terminals there are some restrictions. Soldering of cable terminals should be turned to only if stamped terminals cannot be applied.
How to proceed when soldering cable terminal is to be taken from fig. 3.25. and 3.26.
Fig. 3.25. Soldering a cable terminal
a) prepared conductor, b) conductor with cable terminal slid on, c) soldering on the cable terminal: 1 flame-proof batting, 2 solder, 3 heating nozzle, 4 cable terminal
When soldering on cable terminals the connecting area around the lug of the cable terminal is continued to be heated until the soldering tin laid on starts to flow. So much solder is put on to the soldering surface that all parts of the conductor in the total area are covered by tin which must show a mirror reflection.
In no case so much solder is to be brought on the soldering surface resp. heating is continued so long that the solder flows through the cable terminal into the conductor!
Soldering cable terminals requires extensive special skills and is therefore to be carried out by especially qualified experts only.
Fig. 3.26. Cable terminal finished by soldering
1 plastic tube or tape
Except for the current flow test prior to commencing cable laying the insulation value will be measured after installation of the cable. This test is performed by experts by applying a high voltage for an exactly fixed time unit according to test standards. Within this time no breakdown of the cable insulation must happen. The results of this test are being recorded on a test certificate. The cable may be put into operation after successful testing only.
The cable marking informs about:
- type of cable,
- construction of cable
- material and shape of conductors
- cross section of cable.
Cables are being supplied on cable drums in case of larger quantities.
Laying of underground cables is done from the cable drum into cable ditches.
Underground cables are placed 0.70 m deep into the ground at least.
The distance between two underground cables is to be approx. 0.20 m.
Below temperatures of +4°C underground cable is to be laid in warmed up condition in urgent cases only.
When running out cable the following points of emphasis are to be observed:
- the minimum bending radius must not be undercut,
- cables must not be dragged on the ground when being laid,
- underground cables are to be covered. Cables without coverage will have to be dug-in 1.00 m if these cables have a concentric phase conductor,
- every 3 to 5 m underground cables are to be provided with marking tape.
When removing the insulation from underground cables, the individual, insulation layers of the cable are to be removed. Minimum insulation lengths which must remain after stripping are to be adhered to. Cable joints are being made by sleeves and by means of cable terminations. In practical use are sleeves resp. cable terminations of cast iron, plastic or cast-resin.
Conductor connections are mainly made via cable terminals. For this, stamped cable terminals are being preferred to screwed, soldered and welded cable terminals. Stamped cable terminals are stamped on by means of hydraulic or mechanical stamping tools. In this tools matching stamp inserts are to be used.
After laying, the underground cables are tested for adherence to the required insulation resistance. The fixed minimum insulation values are to be kept.
Preparation of the underground cable will be decisive for its life.