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When grounding a tall tower in a rocky island, can we just throw a couple naked cables in the sea? If not, why not?

Please provide real-world examples if known.

Emilio M Bumachar
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  • Found this question in Hacker News. Couldn't find by searching the web either. https://news.ycombinator.com/reply?id=9378196&goto=item%3Fid%3D9376086 – Emilio M Bumachar Apr 15 '15 at 08:21
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    Why are you wishing to ground a tall tower? – Andy aka Apr 15 '15 at 08:34
  • First problem that comes to my mind: What happens if spume throws the cable back on land? You have to make sure, that the water-endian keeps in water. If you would tie the cable around some rocks should be enaugh. Additional, as Nick Johnson mentioned, corrision is a big problem over time, saltwater could corrupt the cable within months. – jawo Apr 15 '15 at 10:44
  • @Andyaka I'm just a curious bystander, but the original asker on HN has a wind turbine for power generation in the island, and wants to ground it to avoid lightning strikes. – Emilio M Bumachar Apr 15 '15 at 18:31
  • Grounding the tower will ensure that it does get lightning strikes. – Transistor Sep 01 '17 at 16:20
  • @Transistor Not correct: the tower is a tip around a flat area (and "attracts" lightning strikes), it's not insulated as you might think (foundations are grounding it to the island, but not as good as standards require and common sense wishes), so you need a good grounding that let the lightning current discharge (e.g. 100 kA) with a limited overvoltage and overheating. Otherwise the lightning discharge will break or worse blow up foundation reinforcement, depending on the local resistance between interfaces. – andrea Sep 02 '17 at 08:35
  • I'm aware of all those points, @andrea. I was responding to Emilio who, I think, meant that the lightning strikes should be diverted from the equipment and directed without damage to the ground. Emilio seemed to be saying that grounding would prevent or reduce lightning strikes and my point is that it doesn't. Have I missed something? – Transistor Sep 02 '17 at 10:07
  • @Transistor. I understand your last point above: Emilio said "to avoid lightning strikes", that is of course not correct, as you say. He meant for sure "diverting" into LPS, keeping coupled overvoltage to equipment low. My comment was on your "Grounding the tower will ensure that it does get lightning strikes": it is not true that "ungrounding" the tower will divert lightning strikes away or there will be a substantial reduction of exposure. The field gradient will be such that lightning discharge right there; the tower is only poorly grounded, it is not insulated for hundreds kV. – andrea Sep 02 '17 at 11:48
  • @transistor For the record, I did mean to avoid the consequences of lightning striking. I do know that grounding cannot make lightning go strike somewhere else. Apologies for my poor expression. – Emilio M Bumachar Sep 02 '17 at 22:36

2 Answers2

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In principle, yes - all you need is a sufficiently good connection to ground, and saltwater will certainly provide that. Single Wire Earth Return power transmission systems can use this for return power; they usually use titanium grid electrodes to prevent corrosion.

Nick Johnson
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Rocky soil does not offer a good resistivity, so that typical methods of driving electrodes into the soil will not reach a satisfactorily low earthing resistance.

It is assumed that the supply earthing is coming from the distribution network (at some extent utility), the purpose of local earthing is protection against lightning. It is a tall tower, on an island, so the risk assessment will come up with a large equivalent capture area, and correspondingly the need for an efficient Lightning Protection System. Required earthing resistance is 10 ohm or less (IEC 62305). If expecting very large lightning current (near or above cat. I of 62305; e.g. it happened in Malaysia ... >300 kA recorded!), resistance shall be correspondingly reduced.

Earthing with electrodes in the sea is possible and acceptable, provided that:

  • the earthing resistance can be measured (inspection pits and suitable method ... remember that you do not have good soil for auxiliary measurement electrodes, so you will need to adapt a method like Fall-of-Potential method to the sea : floating auxiliary electrodes with a buoy?)
  • calculation by design maybe done assuming the sea water with conveniently good, but conservative, electrical conductivity: typically, it is said some S/m
  • in case of lightning a large current will discharge through the earthing system to the sea: are there phenomena of gasification, electrolysis and chemical decomposition, suddenly increasing the effective resistance, exactly when you need it? don't know, interesting
  • corrosion of conductor surface: copper will be prone to heavy oxidation increasing the resistance; probably when discharging the overvoltage will perforate any oxide barrier, but during measurements results may be "false". Moreover, corrosion will bring to a reduction of cross section. Be careful also of ethero-junctions and combinations of different metals, that might corrode quickly the joints. Probably a steel cable directly connected to an earth bar in a suitable dry protected cabinet is the best solution.
andrea
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