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I've read about this and thought about it too much. We have three electrical points all claiming to be "earth" and a device with two terminals. It's this thinking which is leading me to ask more questions... 🤔 I have read the often-instructed... "Gibbo's Smartgauge website here " and it agrees with the instructions on any (all?) GIs I've seen. I've also read a lot of old GI threads here, and I understand this- I understand back-to-back double diodes as working by Accepting voltages generated by dissimilar metals, in either direction, without significant conduction Conducting fault currents (such as Live touching accessible metalwork) with insignificant voltage drop, so that breakers can trip promptly. Being able to continue conducting fault currents indefinitely? Else the specs and heatsinks on some are ridiculous overkill. Sometimes, using any developed voltage to light diagnostic LEDs. I believe this isn't going to be triggered by dissimilar metals as the ones we use won't develop quite enough voltage. I understand that capacitor(s) across the GI are to conduct AC at significantly higher frequency than 50Hz, so those currents bypass the GI. I also know an isolating transformer with a shore-earthed screen is the gold-plated solution, but I don't have the budget for that. Money, weight or space. All instructions say to insert the GI between shore-earth and (consumer-earth = hull). Sorry, this scribble is the most convenient just now. This protects against ordinary galvanic corrosion in the absence of bad equipment, and it offsets you against leakage currents generated elsewhere on the shore-earth. It doesn't protect against in-boat leakage currents. Nor does it provide status lights to warn about that. So why do we not insert the GI between (shore-earth = consumer-earth) and hull? I think this must be because one could accidentally short the GI and not realise, an thereby lose the hull protection. Electrocution protection remains solid. After this I got into thoughts which surely are heading towards madness, but I can't really see why. 🤕 Is there some justification for a star or delta type GI, in order to have protection from and detection of onboard DC leaks? Yeah this would have been quicker with pencil, paper and camera... I give you the "wacky delta wired GI". I know you won't like it. I don't like it! But I cant actually see why it's wrong. If each basic GI has status lights then you know what's leaking where. Obviously it's overkill to do it with three ordinary GIs. If this schematic were actually sensible you would want three bridge rectifiers on the same heatsink, because only one will ever be conducting fault current.

Moored in a Marina but not using the mains power? Don't imagine for a minute that you are not at risk of galvanic reaction from other people's boats! I was horrified to discover that despite not using the mains supply for more than a few minutes every month, my hull had developed serious pitting causing the value of the boat to sink without trace. The buyer's surveyor said that the hull needed overplating urgently at a cost of some £8,000 to £10,000. The only cause of the reaction I can think of is being bracketed on the Marina by live-aboard boats plugged in permanently to the mains power and possibly not having a galvanic isolator fitted, thus causing the problem with my hull. When last examined (four years ago) the pitting was minimal and the hull was sound - now it is down to 2 to 3mm in places. Sadly I had to cut my losses and accept a seriously low price for the boat. There was no way I could afford to have the overplating done. I had hoped to raise sufficient to make the first few payments on a new Sailaway hull, but those plans have now gone on hold for the foreseeable future. Ah well, we learn from our mistakes and oversights. Maybe I should have gone for a GRP hull instead.

Hi - I recently moved marinas, and when I first plugged into the mains at the new place everything worked fine. The next day, however, the mains had stopped working - I checked the trip switches onboard and on shore and there didn't appear to be a problem. The mains finally switched on when I removed the galvanic isolator from the circuit. However, I have since noticed the warning light on my battery charger flashing to say there has either been a short circuit, the fuse has gone or the polarity has been reversed. (I have changed the fuse and the warning light is still flashing the same warning). Any ideas what I should do next, or what the problem is?