IS MY GALVANIC ISOLATOR WORKING, HOW DO I TEST IT?
Whenever different metals are placed in a conductive liquid you create a battery. If you connect these pieces of metal together, current will flow. The current will be removing metal from one of the metal pieces creating "electrolysis". If one of the pieces is your propeller you have a problem.
The zincs you use on a boat are called "Sacrificial Anodes". Zinc is used because it has a higher voltage in the water so the current will be more inclined to flow from it than from your propeller. To complete the electrical circuit, the zincs must be connected to the items they are intended to protect. Usually this is no problem because the zinc is bolted right to the shaft or underwater housing. Non metal boats will usually have a copper bonding wire inside that connects all the underwater metal items together so they all share the protection from zinc anodes.
If other currents are allowed to get into this bonding circuit they can easily overpower the small voltage available from your zincs and defeat the protection you need. This is usually the most destructive form of electrolysis and you notice it because your zincs get eaten up very quickly trying to keep up. Under normal circumstances, zincs should last at least a year if they are working normally, and much longer if you don't have any problems. If they are being "sacrificed" in a shorter period you need to find where the external current is getting in.
The first line of defense is a galvanic isolator in the shore power ground circuit. This is not necessarily the only path but by far the most likely path. The galvanic isolator is able to break the circuit for low voltages but maintain the ground connection for voltages higher than about 1.2 volts. Yandina now has a 2.5 volt Galvanic Isolator for problem locations and metal boats.
Other paths for electrolytic current flow can be from dissimilar metals connected together on the boat. Although it has been traditional and advised by experts to bond underwater metal if you are bonding dissimilar metals the bonding can actually provide the circuit and do more harm than good. An isolated piece of metal in the water on a fiberglass boat has no electric circuit so no current flows. Why provide it with an electrical connection and invite problems. My philosophy has been to only bond underwater metal if it shows evidence of electrolysis - if it ain't broke don't fix it.
IS MY GALVANIC ISOLATOR WORKING?
95% of the time the ground conductor in the shore power cord is the culprit completing the circuit to the rest of the world. The Galvanic Isolator provides a window into this activity so simple measurements can reveal how well it is working. First thing to do is measure the AC and DC voltages across the isolator without making any changes to the electrical status - whatever is on/off at your dock, leave it this way to make the measurements.
If the DC voltage is zero (less than 0.01 volts) something is bypassing the isolator (or it died). The bypass can occasionally be a second shore power connection, a shore cable TV connection or a metal boarding ramp to a metal dock but it is usually a wiring error and rarely a bad isolator. If you can't identify what is shorting out the isolator, it should be disconnected and the voltage across the disconnection measured. If you now get a reading the isolator is dead, if not you have to find what is bypassing the isolator to shore ground.
If the DC voltage is greater than 1 volt you have a potential problem that is not so easy to diagnose. I would try disconnecting the positive terminal of every 12 volt battery to see if you are creating your own problem and if that makes a difference then follow the DC chain from that battery to see which load is the culprit. For example a depth sounder with a DC leak from positive to the underwater transducer mount could be the cause.
If you can't get the DC voltage down Yandina recently introduced a "Galvanic Isolator PLUS" that has 2.5 volts of isolation instead of the conventional 1.2 volts.
If the AC voltage is greater than 0.25 volts you have an AC leakage problem that decreases the DC isolation level every time the AC is in the same direction as the DC you are blocking. Try and track down the AC source by totally disconnecting items from the AC distribution one at a time and looking for the one that is causing it, then repair that item. If you can't find the culprit you can bypass the isolator with a capacitor. Although most isolator manufacturers say they already have a capacitor in the isolator they will rarely reveal its size and I suspect they are often inadequate. Read this article DOES MY GALVANIC ISOLATOR NEED A CAPACITOR TO CONDUCT AC? for help.
HOW TO TEST A GALVANIC ISOLATOR
You will need a voltmeter and make a simple test circuit using a 6 or 12 volt battery and a light bulb that comes on when you close the circuit. You cannot use one of your boat batteries (unless you remove all positive and negative connections first).
1. Turn off dock power and unplug the shore power cord from the DOCK end.
2. Bring the dock end of the shore power cord on board to a location where you can access the negative terminal of your starting battery - a basic 12 volt negative for the boat.
3. Connect one test lead to the GROUND pin of the shore power cord and the other test lead to the boat DC negative such as the engine block or starting battery negative terminal. The light should turn on. If it doesn't turn on you have a problem somewhere in the boat ground circuit, most likely a bad connector or broken wire in the shore power cable but it can also be a faulty Galvanic Isolator.
4. If the light comes on, measure the voltage between the two test leads that are sending the current through the shore power loop. It should read about 1.2 volts (or 2.4 using a Yandina Galvanic Isolator Plus). If it is less than 1.2 you have leakage across the isolator, unlikely to be in the isolator itself. If it is greater than 1.2 volts you have a bad connection somewhere or a faulty isolator.
5. Repeat 3 and 4 with the polarity reversed so the current flows in the other direction.