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Galvanic Isolator Inneffective

I had my hull surveyed a few months ago which showed quite a lot of corrosion pitting. I was concerned that this will continue to happen, so I fitted a new galvanic isolator because I was getting strange readings on my multimeter across the old galvanic isolator. I still get strange readings, and after much checking of my boat's system, eventually I connected a cable directly to the shore power socket (with live & neutral wires made safe) and measured the resistance between the shore power electrical earth and the boats hull. This was 160 ohms. As I was expecting a reading of 'infinity', or close to that, 160 ohms is a surprise. There are no other connections to the shore, other than the (dry) nylon ropes and the (rubber) fenders.
As there is clearly a bypass round the galvanic isolator, it will do nothing. Do you have any clue as to how I can electrically isolate my boat from the earth?

Asked by: FlatBattery  | 6.32pm, Sunday 6 October

WW says:

The electrical earth connection is exactly that; a connection to the ground, so a resistance reading between the boat hull and the electrical earth is not surprising. This is because the hull is also connected to the ground through the water; it is not zero because the electrical earth connection to the ground is some distance away. This leads to induced currents which will flow to ground via the boat if the mains earth is connected directly to the hull, which is why galvanic isolation is required to prevent corrosion.
Deciding whether a galvanic isolator is working or not is a difficult task as it is not simply proved by measuring the apparent resistance. If there is any doubt it would be worth considering fitting an isolation transformer. This will provide absolute isolation as the only connection to the shore power is magnetic. The onboard earth connection is to the hull, with no connection to the shore power earth.
If your boat is moored close to other boats also connected to mains power, but without working galvanic isolation; the current flows causing corrosion to their hull, can also leach metal from nearby boats or piling. The only real effective prevention against corrosion of steel immersed in water is a good protective paint film.

Rupert Smedley  | 12.10PM, Monday 7 October

An isolating transformer will stop any corrosion from any stray earth currents from your mains supply, however it will not stop all corrosion.

Rupert Smedley  | 10.20AM, Tuesday 8 October

Readers say:

Thanks for explaining why the hull is 'earthed' without the cable. I hadn't realised that canal water is that conductive! Will fitting an isolating transformer (ie disconnecting the electrical earth) stop corrosion, or will the corrosion simply slow down because there is still a path to earth with a high resistance?

FlatBattery  | 9.37PM, Monday 7 October

I am currently designing my own narrow boat from scratch and this was one of the issues concerning me. I've done a lot of research and found a very interesting and informative link that explains it all which I highly recommend you read. http://www.smartgauge.co.uk/galv2.html

richardw66  | 11.31AM, Tuesday 26 November

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