Copper-clad steel strand: we supply it, we do not recommend it for buried electrodes

Appendix to the Copper-Clad Steel Earthing Selection Guide · Supply position 2 / 3

Again the conclusion first: we supply copper-clad steel stranded conductor, and we do not recommend it as a buried horizontal earthing conductor. Where it belongs is the earthing lead and any connection that has to flex. There are three reasons: poor value for the copper, a corrosion allowance that cannot be designed cleanly, and the highest quality-control risk of any copper-clad steel product. The last one is the one we most want buyers to know about.

1. Poor value: the same material, stranded, carries far less copper per wire

A strand is made by twisting together a number of thin copper-clad steel wires. For the same total section and the same copper content, the copper layer on each wire falls with the square root of the number of wires: at equal copper content, the per-wire layer of a 7-wire strand is about 38 % of that of a solid round wire of the same section, and of a 19-wire strand about 23 %. Corrosion life is roughly proportional to copper thickness, so the same copper, stranded, starts with 40–80 % less life. More wires mean more flexibility, thinner copper and worse corrosion resistance – that is geometry, not workmanship.

Turn it round: to give every wire of a strand the same 0.6 / 0.8 mm as a solid conductor, the wires have to be thick, the copper content and the price go up, and the point of saving copper is gone. That is what "poor value" means here – not that it cannot be made well, but that made well it is not cheap.

2. The corrosion allowance cannot be designed cleanly

For solid round wire the copper layer is sized directly as corrosion rate × design life + layer remaining at end of life (see precise selection). A strand does not allow that. Corrosion acts on every wire, the gaps between wires add crevice corrosion, the outer wires go before the inner ones, and there is no ready formula for how the effective section of the whole conductor changes with time. The designer can only set a conservative per-wire minimum and accept the uncertainty. For a component that will spend thirty years underground and is never seen again after backfill, that uncertainty is a real risk.

3. The biggest quality-control risk: the copper layer cannot be measured

This one is unique to strand. The copper layer on copper-clad steel is normally gauged with a magnetic thickness gauge (GB/T 4956), which needs a surface of sufficient radius; solid wire and rods are fine, but the single wires of a strand are too small in diameter for the gauges on the market. That means three things:

• The manufacturer's own inspection cannot gauge the copper. Routine testing is supposed to check thickness on every reel; with strand it falls back to sampling the single wire before stranding, and nothing can be re-checked afterwards.
• The buyer cannot gauge it on receipt either. On site the gauge gives no reading; the only option is to cut a sample and send it to a laboratory for a metallographic section, wait days for the result – and the result covers only the piece that was cut.
• It ships on reels, and nobody inspects the middle. On receipt only the two ends can be seen. To our knowledge there have been strands on the market whose copper was within specification at both ends and below it in the middle – not an accusation against anyone, but a statement that this product form invites it, and the buyer has almost no way to catch it at delivery.

Solid round wire has none of this: any reel, any length, a gauge gives a reading on the spot, and anything that disagrees with the factory report can be rejected on the spot. That is what lets us write on the Testing & quality page that a sample may go to any laboratory and we pay for the second test if it disagrees – a line we cannot write for strand, because the customer cannot even make the first measurement.

4. What we do about it

We supply 70–240 mm² continuous-cast copper-clad steel strand (Products), continuous-cast only, because a plated layer does not survive stranding. Specify it by per-wire minimum copper thickness, not by conductivity grade alone. Its place is the earthing lead and the connection between equipment and grid that has to bend repeatedly – above ground, inspectable, replaceable. For the buried horizontal electrode, use solid round wire; in the free assessment, wherever a design uses strand as horizontal conductor we give a solid-wire alternative and the copper comparison between the two.