Splicing techniques of HV cable

Why splicing cannot be avoided

Splicing (jointing) means joining two lengths of cable so that the joint is electrically and mechanically as good as the cable. It cannot be avoided because:

• Cables are made and transported on drums of limited length (a few hundred metres for HV cables), while routes are often longer.
• A damaged section after a fault or digging accident must be cut out and a new piece joined in.
• Tee/branch joints are needed to tap supply to new loads.
• Cable routes may need changes when roads and buildings are extended.

Splicing techniques of HV cable (e.g. 11 kV XLPE)

Common types of joints:

1. Heat-shrink joint: tubes of cross-linked polymer that shrink tightly when heated with a gas torch. Most common in Nepal and India.
2. Cold-shrink joint: pre-expanded silicone rubber tubes held on a removable core; they shrink when the core is pulled out. No flame needed.
3. Pre-moulded (push-on) joint: factory-made rubber body pushed over the prepared cable.
4. Resin (cast) joint: a mould is filled with epoxy or polyurethane resin.
5. Taped joint: layers of insulating and semiconducting tapes applied by hand (older method).

Steps for an 11 kV XLPE heat-shrink straight-through joint

Layers built over each core joint (inside out):
 conductor + crimped ferrule
 stress-relief mastic over ferrule
 stress-control tube (over screen cut-back)
 insulating tube(s)
 semiconducting tube
 tinned copper mesh (screen continuity)
Over all three cores:
 armour bond (braid) + outer sheath tube

1. Prepare cables: check for moisture, overlap the ends, cut to length. Slide on the outer sheath tube and other tubes beforehand.
2. Remove outer sheath, armour and inner sheath to the dimensions given by the kit. Clamp the armour ends.
3. Remove the copper tape screen and the semiconducting layer back to the marked length, leaving a clean, smooth cut on the insulation.
4. Remove insulation from the conductor ends equal to half the ferrule length.
5. Join conductors with a mechanical or crimped (compression) connector/ferrule; file off any sharp edges.
6. Fill voids over the connector with stress-relief mastic.
7. Stress control: shrink a stress-control tube over the semicon cut-back to spread the electric field evenly; otherwise the field concentrates at the screen edge and causes breakdown.
8. Insulation: shrink one or more insulating tubes over the joint, then a semiconducting tube (or a dual-wall tube) over them.
9. Screen continuity: wrap tinned copper mesh over the joint and solder/clamp it to the copper screens of both cables.
10. Repeat for all three cores, then bind the cores together.
11. Armour continuity and earth: bridge the armour across the joint with a braided earth connector.
12. Outer protection: shrink the outer heavy-wall sheath tube (or a joint box) over the whole joint to keep out water and give mechanical strength.
13. Test: insulation resistance test and, for HV, an HV/VLF withstand test before energising.