|Electrical Installation - Basic Vocational Knowledge|
|3. Laying of underground cables|
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.
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.
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).
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.