Screening the joint: semi-conductive tube, GDLT and GVOT

Why anything sits across the cutback at all

Cut the screen back on an MV core and the earthed surface stops at a hard edge. The manufacturer names the place in its own words: its stress control tube exists to control the high electrical stress present at insulation-screen terminals. Three of the four parts below are extruded to lie across that edge. The fourth deals with what leaks out of the paper behind it.

One edge, two jobs. At a termination the core goes out into air and the field has to be graded down over a length. At a straight-through joint the screen has to be rebuilt so it stays continuous past the connector. This page is the second job. The first — and the discrete stress control tube that does it, Insultube-Stress, radiation cross-linked, standard black, -40 °C to 100 °C, fully recovered at 130 °C — belongs to the termination reference.

Size these the way you size any tube here. D min. must clear the diameter over the built-up insulation; d max. must close below the cable's own screen where the tube lands. On GDLT the two ends are closer together than the codes suggest: 65/27 works out at 2.4:1 and 100/35 at 2.9:1, so nothing in the range is a 3:1 part.

One code or two, and what a reversed GDLT costs you

The manufacturer's own transition-joint sheet leaves the choice open. Item 10 on the GTJ/2436 bill of material is a screened dual-wall tubing, three per joint, with a parenthesis offering separate insulation and conducting tubings instead. So the fork is a stocking decision before it is an electrical one: one code and one recovery to make, or two codes and an overlap to get right.

Reverse a GDLT and the property table says precisely what you gave up. The 20 kV/mm figure belongs to the inner layer alone; against the outer layer the dielectric strength column prints N/A. Inside out, a 1 × 10³ Ohm.cm surface lies straight on the built-up insulation, and the face left looking at the joint case has no insulating figure published against it at all.

The sheet gives the standard colour as black/red and never says which face is which. The manufacturer's own product photograph settles it: black outside, red bore. Colour alone will not identify the part on the bench, though. The stress control tubing on the same bill of material is standard black, and the MV build-up grade is sold as INSULTUBE Red. Check the kit drawing before the gun is lit.

GOBT: keeping PILC compound out of the joint

Paper insulation in a PILC cable is impregnated, and a cut end gives the compound somewhere to go. It creeps along the paper, into the joint, onto surfaces meant to stay clean. GOBT is the stop: six sizes from GOBT 30/10 to GOBT 115/34, wall toleranced to ±10%, cross-linked polyolefin at 12 kV/mm minimum dielectric strength, continuous -40 °C to +110 °C, shrink temperature above 125 °C. Fungus and salt-spray resistant, and no cracking after four hours at -40 °C.

It also counts as insulation to 36 kV, which is why it is not simply a seal. You will rarely order one loose. The PILC termination bills of material list three oil barrier sleeves plus one belting oil barrier sleeve, and the GTJ/2436 transition joint between XLPE and paper lists three more. On those jobs the barrier is picked for you by kit code and cable size — the kit itself is cablejointkit.com's subject, and the part reference is the manufacturer's oil barrier sleeve page.