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bri2014
bri2014

Posted: Thu Apr 24, 2014 8:14am

Post Subject: Galvanic isolator gauge reading

I have an aquafax galvanic isolator on my Narrowboat that has a Gauge on it with a needle (like an ammeter). Right hand side of the gauge is labelled "Shore Power" and the left side "Earth" Needle in centre means pass and then each side has a green area that goes to 1.1 and after that the gauge is red and says fail. Mine is showing the needle slightly towards shore power and still in the green. Does this mean it's OK but only just OK so I should be looking for a possible fault/weakness in the earth somewhere? I'm right at the end of a very long pontoon in a Marina off the non tidal Thames that has a large number of boats, all connected to shore lines. My narrowboat is new so I'm particularly keen to ensure there's no stray currents affecting the boat.

Tony-B
Tony-B

Posted: Thu Apr 24, 2014 9:46am

Post Subject: Galvanic isolator gauge reading

If you are keen to ensure no stray currents are affecting the boat the it is arguable that you have chosen the wrong device. Not only do galvanic isolators breach the regulation (if strictly interpreted) that says nothing should be placed in the protection (earth) wiring but they also do no totally isolate the mains supply from the boat. A properly in stalled and used isolation transformer totally isolates the mains supply from your boat so the only earth involved is the one local to your boat and that is isolated from all the other boats so not electrical path exists between your boat, the bank, the mains earth and other boats. The mains electricity is transmitted to your boat via a 1:1 transformer with your boat's earth only connected to the secondary coil. Now to answer the question. Galvanic isolators use two sets of two or more diodes in series with each other with each set in parallel with each other but pointing on opposite directions. These are inserted into the mains earth wire as it enters your boat. A diode creates about 0.5 to 0.7V of voltdrop so two would produce (say) 1.2 volts while 3 would produce 1.8 volts. As long as any potential voltage flow in the earth line is below this figure the diodes will not conduct so isolating your boat from the mains earth. However if a fault occurs then the full mains voltage is applied to the diodes so they open and conduct, tripping your protection device (RCD, Fuse, MCB etc.) Being AC electricity the diodes sets conduct alternatively as the AC flows back and forth. What your meter is allegedly showing is that the diodes have not gone conductive because the voltage present on the earth line is less than 1.1 volts which is when the diodes would presumably conduct and stop protecting your boat. Assuming that you can rely upon that meter before, during and after a fault condition then your boat is protected. Move to a different part of the marina or a different place and the reading may well go away or be different. Please be aware that even a pure 12V DC boat can suffer corrosion if it is moored in close proximity to two boats with shore power connected, one of which has a fault. What close proximity means depends upon a number of factors including what is dissolved in the water. However this is a rare occurrence. Tony Brooks

bri2014
bri2014

Posted: Fri Apr 25, 2014 5:45pm

Post Subject: Galvanic isolator gauge reading

Thank you so much for this Tony. Taking your amazingly detailed explanation into consideration I'll look into the isolating transformer route.

Tony-B
Tony-B

Posted: Sat Apr 26, 2014 8:23am

Post Subject: Galvanic isolator gauge reading

Was it you who posted an almost identical question on the CWF an hour or so after I answered? If so it seems a battery charger is involved in the meter reading. This can be explained by the fact that modern chargers use similar circuits to switched mode power supplies and those produce horrendous radio interference. This is suppressed by conducting the interference to "earth" and that in turn puts a small voltage on the earth line. It would be instructive to see what the meter reads with a charger, computer and TV all connected. Tony Brooks

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