Nigel Calder gets to the bottom of this tricky subject
NIGEL CALDER RESPONDS TO A READER’S QUERY ABOUT GALVANIC ISOLATORS IN THE DECEMBER 2006 ISSUE OF YACHTING MONTHLY. HERE IS THE FULL TEXT OF THE QUESTION AND ANSWER.
Q: Having recently installed a galvanic isolator in accordance with commonly published recommendations, I decided to test its effect with a voltmeter. To my surprise it seemed to achieve the opposite effect to that intended.
First, the isolator appears to have no effect upon DC leakages into the sea, which I understood to be its primary function. Worse, it seems that my mains power system, for whatever reason, introduces a 1 volt AC leak into the earth wire which previously had escaped harmlessly back to the sub-station.
Now that the isolator is preventing this escape, this stray AC voltage is instead using the bonding wire from AC earth to DC negative to leak into the sea via my propshaft. I feel inclined to remove the isolator but all these people cannot be wrong so where is the flaw in my reasoning please?
NIGEL CALDER REPLIES:
A major source of corrosion on boats is galvanic corrosion caused by immersing dissimilar metals in an electrolyte (e.g. sea water) and in some way electrically connecting the metals (i.e. wiring them together). This results in a galvanically-generated DC current with a negative voltage that may reach -1.00 volts DC.
When a boat plugs into shorepower, the shorepower cord will often make an electrical connection between the underwater metals on all the boats that are plugged in, creating a risk of galvanic corrosion. A galvanic isolator is designed to prevent this by blocking DC currents with voltages that reach as much as -1.2 volts DC. This is achieved by installing two sets of devices known as ‘diodes’, with one set installed in the opposite direction to the other.
There are two types of galvanic isolator, one with a device known as a ‘capacitor’ wired around the diodes, and one without. Without a capacitor, if there is AC leakage on the shorepower ground circuit that has a voltage above 1.2 volts AC, this AC leakage will ‘bias’ the diodes into a conductive state, in which case the galvanic isolator may as well not be in the circuit – in other words, the circuit will act as if it is not there. The 1.9 volts AC that is being recorded is high enough to do this if the galvanic isolator in question does not have a capacitor.
So far as I know all galvanic isolators currently sold do have a capacitor, so I have to assume the one in question has a cpacitor. The function of the capacitor is to allow AC leakage current to bypass the diodes and not bias them into conduction, so that now you have conduction for AC leakage currents (which is what you want to get them safely down the ground wire) with blocking for galvanically-generated DC currents (which is what you want for corrosion prevention). Note, however, the capacitors have limited current-carrying capability, generally less than 8 amps. If AC leakage current exceeds the carrying capacity, the AC leakage will now bias the diodes into conduction, voiding the galvanic isolation.
It is possible there is an AC leakage current that exceeds the current carrying capacity of the capacitor in this galvanic isolator, in which case the situation is potentially lethal. But in this case, the onboard RCD should trip (on all European boats they are rated at 30 milliamps), and/or the shoreside RCD should trip. It could be the RCDs have failed in the conductive position, but this seems unlikely as both would have to have failed.
There is another possibility here. RCDs measure the differential current running in the hot and neutral wires. If this varies by more than the set amount (e.g. 30 mA) it signifies that current from one or other side of the circuit is leaking to ground by some other (potentially lethal) path, and so the RCD trips. But in a situation of a short circuit, in which both hot and neutral wires become the leakage path (e.g. if you or I accidentally bridge the wires) the current flow to ground can be equal in both wires and as a result the RCD measures no imbalance and does not trip. It is possible this boat has this kind of a fault, which again could be potentially lethal, although I think it unlikely that this is the problem.
One way or another, the galvanic isolator should not block stray AC currents, and it should block galvanically-generated DC currents. It seems most likely to me to me that the galvanic isolator is either improperly wired and/or has a fault of some kind. I have a series of tests for galvanic isolators in my Boatowner’s Mechanical and Electrical Manual which you might want to try before going any further. It would also be helpful to measure the magnitude of the leakage currents as well as their voltage.
I would recommend persevering rather than removing the galvanic isolator and giving up. There may be a fault condition that needs to be tracked down and cleaned up.