Water-Blocking in 400 mm2 Stranded Conductors: Grams per Metre and the Swell Test

Water travelled 62 metres down a 400 mm2 stranded aluminium conductor before a utility opened a joint and found it. The cable had passed a sheath integrity test, the joints had been made to procedure, and the fault only surfaced when a 33 kV circuit failed twice in one winter. The strands were dry at the top of the run and full at the bottom. There had been no blocking powder in the interstices at all.

Stranded conductor blocking is a small line item that decides whether a cable survives a flooded duct. This article covers where the powder has to sit, how many grams per metre the annulus actually needs, which swell test a utility will accept, and how the material behaves after ageing in hot water.

Blocking the Interstice, Not the Conductor

A 400 mm2 compressed stranded conductor has 37 or 61 wires depending on construction, and the gaps between them form a continuous capillary path. Blocking powder is applied during stranding, either by spraying a slurry onto the wires or by dusting dry powder as the layer closes, so that the voids carry 0.6 to 1.8 g per metre of powder.

The powder must not sit on the outer surface, where it will be rubbed off by the insulation extrusion die. It has to be inside the layer, held by the wires. That is why spraying a slurry at the closing point works better than dusting at the pay-off: the slurry rides the wire into the gap before the layer closes and traps the particles in place.

Three Swell Tests and Which One a Utility Accepts

The first test is a simple beaker swell: 1 g of powder in 200 ml of deionised water, read the gel volume at 1 and 10 minutes. It tells you the material is alive but nothing about performance in a conductor. The second is a trough test: a 1 m sample of finished conductor, one end immersed in water at 0.3 bar, measure travel after 24 hours. Under 1.5 m is the usual pass.

The third, and the one most utility specifications actually cite, is a long-duration immersion at 90 C for 14 days followed by the trough test again. Powders built on lightly crosslinked chains pass the first test and fail the third, because soluble fractions wash out and the remaining network cannot swell enough to seal. Ask for the 14-day figure before you order a tonne, and ask what water was used, because tap water flatters every grade.

Grams per Metre at a 0.8 mm Annulus

The void fraction of a compressed 400 mm2 conductor is about 8 percent of the circumscribed area, which works out to roughly 22 mm2 of void per metre, or 22 ml. To seal that volume with a gel at 25 times swell, you need the equivalent of about 0.9 g of dry powder per metre, distributed along the length rather than concentrated.

In practice lines run 1.2 to 1.8 g/m to allow for losses at the die and at joints. A 1 km drum therefore carries 1.2 to 1.8 kg of powder, which at 2,400 dollars a tonne is under 5 dollars of material. Compare that with the cost of one failed joint at 4,000 dollars and the argument for over-dosing slightly is easy to make.

Ageing: 7 Days at 90 C in Water

Heat and water together attack the network in two ways. Residual soluble polymer leaches out, and the crosslink junctions hydrolyse slowly at 90 C, particularly in the presence of the copper ions that migrate from a copper screen. A powder that is fine for an aluminium conductor can lose 30 percent of its swell after a week at 90 C next to bare copper.

If the construction includes copper, specify a grade with a lower soluble fraction and check the swell after immersion rather than before. The test is cheap: 2 g in 500 ml of water at 90 C for 7 days, then measure. Anything that holds 70 percent of its original swell is acceptable for a 40-year asset; anything under 50 percent will leave you with the failure described at the top of this page.

For conductor and tape blocking, the powder needs a fine, tight cut and a documented swell rate rather than a headline capacity. The polyacrylate powder grades for cable blocking cover the 100 to 200 mesh range these lines use.