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With Rose & David
Galvanic Isolator Explanation (For Australia)The purpose is to disconnect your boat's 240volt supply earth from the main's supply on shore 240volt earth in order to prevent electrolysis. The trouble is, however, you need them connected together so that if there is an electrical problem on the boat, it doesn't give you an electric shock!
The galvanic isolator relies on the fact that electrolysis 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, it will be flowing both ways, two diodes are placed in parallel pointing opposite directions so there is always one available to conduct, but at low voltages, both are switched off and no electrolytic current can flow.
Since some electrolytic voltages are higher than 0.6 volts, a good galvanic isolator should have two diodes in series in each direction to give a 1.2 volt isolation. Some also add a capacitor to increase the ability to conduct AC current, however I personally think this is a mistake as it does allow low level AC currents to flow and cause electrolytic type activity, even if not true electrolysis. This activity can remove paint from metal fittings and generate chlorine bubbles that damage surrounding antifouling paint.
The diodes have to have enough capacity to pop a shore power circuit breaker if there is a fault on your boat. This can require a capacity of more than 100 amps. Some commercial 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. They should be able to stand the shore power current rating indefinitely.
There are two ways for a galvanic isolator to fail. Either the diodes are shorted, or they are blown open. You can test them with a digital volt meter. With the voltmeter on the DC range, put it across the shore power side to the boat side of the isolator. There should always be some residual electrolytic voltage (unless you are hauled out) so the meter should read something less than one volt. If it always reads zero, the diodes are shorted out. If it reads greater than about 1.2 volts then the diodes are open circuit. Switch to AC volts and check again since if AC is flowing, the DC meter setting may not show any activity.
What Does This Voltage Indicate?
Different metals exposed to seawater in the whole system (boat, dock, neighbor etc) will cause a multitude of voltage potentials. If no current flows, the voltage will be higher. If you allow more current to flow, the voltage will be lower just like a flashlight battery will read a lower voltage when the lamp is on - so you could argue that higher voltages are better than lower ones as they indicate that electrolytic currents are not flowing.
On a typical steel boat, with no current flowing from the zincs to the hull, the boat is about 0.8 volts above water voltage. As the zincs are used up, the voltage will fall.
In a typical system, the actual voltage you would read across a galvanic isolator gives very little indication of what is happening with regard to electrolysis.
How To Make Your Own Galvanic Isolator
WARNING: This project involves a modification to the safety earth circuit of your shore power connection. The purpose of this ground connection is to conduct fault currents to ground and prevent you from getting an electric shock. Do not attempt this project if you are not confident in your ability to construct and test this device as errors could be life threatening. The author takes no responsibility for any consequences of, or for the content of this construction article.
Safety rules require that the diodes have a continuous duty rating equal to 135% of the service rating. This is because it there is an error in the wiring and the ground wire is being used to carry the current instead of the neutral, the diodes have to carry the full load without rupturing. Were they to fail under this load, the whole ground system on your boat could be at 240 volts AC which would be very dangerous.
So for a 10 amp shore power connection you will need 4 diodes (or two bridge rectifier packs, recommended) rated for at least 15 amps and for a 15 amp service they will need to be rated for at least 20 amps, as most shore power services in Australia are rated at 15 amps, it is recommended to use 20 amp diodes. The voltage rating is unimportant because they never experience more than 1.2 volts so any voltage rating will do.
Diode packs usually have a hole in the centre to make mounting on the heat sink simple, the heat sink is required to dissapate any heat generated from a fault condition. Keep in mind, they will only get hot if there is a fault conition, so, if you ever notice heat from the isolator, there is something very wrong.
For a heat sink, you need an aluminum bar or plate, preferably at least 1/8" thick and at least 30 square inches of surface per side. I used some 2" x 2" aluminum angle about 8" long. Drill the two mounting holes spaced to spread the heat. Make sure there are no burrs and attach the diodes. A little silicon heat sink grease is good but not too necessary.
The incoming and exiting wires should be at least as heavy as the ground wire in your shore power cable. Typically 10 gauge for 15 amp. Strip plenty of bare wire and solder one lead to each pair of diodes. The circuit is symmetrical so it doesn't matter which is the shore power and which the boat side. Connect one lead to the green ground terminal of the incoming shore power connector. The other lead goes to the boat grounding circuit. When you mount the final assembly, the heat sink that the diodes are mounted on MUST be insulated from any metal boat fixtures unless you use bridge rectifiers which have internal isolation. The voltages are all very low so no electrical protection is needed from the exposed connections or the heatsink.
Note - The following information supplied by Bob Fantozzi - Yacht Gratis (Spray 42)
There are standards in place that regulate galvanic isolators the main standards are European (IEC), American (AYBC) and Australian AS/NZ (3004.1 & .2)