Aluminum hulls have a built-in electric potential of -0.75 to -1.0 depending on the alloy, temperature, salinity and water movement. Conventional Galvanic Isolators provide only about 1.2 volts of isolation so protection is degraded by up to 80% to negative stray AC or DC voltages when used for Aluminum hulls.
The Yandina Galvanic Isolator PLUS for Aluminum boats provides approximately 2.5 volts isolation in the ground lead of your shore power supply to isolate DC and AC electrolytic voltages coming from the dock but yet pass safety currents to ground in the event of a short circuit, wiring error or power leakage on your boat.
The purpose of a Galvanic Isolator is to disconnect your underwater metal parts from the dock supply in order to prevent electrolytic corrosion. The trouble is, however, you need them connected together so that if there is an electrical short on the boat, it doesn't make the boat alive at 120 volts or worse which can give you quite a kick when you step off an aluminum dock or electrocute your friends in the water!!
The galvanic isolator relies on the fact that electrolytic voltages are quite low - usually less than one volt - whereas electrical failure voltages are quite high. Silicon diodes, which are used to conduct electricity in one direction but block it in the reverse direction, have a built in forward voltage drop of about 0.6 volts. It is not like a resistor voltage drop - no current has to flow to create the drop - so below 0.6 volts it is disconnected, above this it conducts with very little resistance to current flow.
Since we don't know the polarity of the fault voltage, and if it is an AC fault, current will be flowing both ways so the Galvinic Isolotor PLUS has 2 sets FOUR diodes in series placed in parallel pointing opposite directions so there is always one available path to conduct, but at low voltages they are switched off and no electrolytic current can flow. This gives about 2.5 volts of isolation.
Galvanic Isolators include a capacitor to increase the ability to conduct AC current, however this has a downside as it does allow low level AC currents to flow and cause electrolytic type activity, such as creating corrosive Sodium Hypochlorite on exposed metal and under paint. This activity can remove paint from the fitting and generate chlorine bubbles that damage surrounding antifouling paint.
The diodes need enough capacity to pop a shore power circuit breaker if there is a short on your boat or if it is mis-wired and using the ground connection for neutral currents. This require a capacity of more than 100 amps. Galvanic isolator diodes are designed to carry this current for a very short time - long enough to blow the circuit breaker plus a safety margin - but they cannot carry it for very long without overheating. The Yandina Galvanic Isolator PLUS is rated for 50 amps AC continuous current and each isolator is individually tested to 135% of this rating per ABYC standards except that the Yandina Galvanic Isoloator PLUS doubles the isolation voltage specified for normal isolators.
TESTING
Remove the shore power cable from the dock end only and bring the loose end aboard. The following tests not only the isolator but also tests the integrigy of your shore power connections. Make up a simple test circuit using a battery (6 volts or more) and lamp that lights when the circuit is closed. Do not use a battery already installed on the boat wiring.
Connect the test circuit to the ground pin of the shore power cable and any on-board ground and check the lamp lights. Measure the voltage between the test leads is around 2.4 volts. A lower or higher reading indicates the isolator has failed. You should then reverse the connection to the isolator and repeat the test with the current flowing the opposite direction.
Link to an easy to read introduction to electrolysis and how to troubleshoot your shore connection if you are having problems.