Galvanic isolator

Andina:
"Also, that the ground and neutral being bonded at the dock wiring makes a difference. Ultimately they are the same: Earth's potential. " No they are NOT the same. Mis-wired boats can return the AC current through the ground wire instead of the neutral. The ground wire in docks is often a light gauge since it is not designed to carry heavy and continuous currents - just sufficient to blow a circuit breaker under fault conditions. So if you are in a dock in the vicinity of the faulty wired boat, your ground lead will have AC voltage on it which represents the voltage drop between the boat injecting current into the ground wire and the eventual dock bonding location where ground and neutral are connected. I've measured as much as 5 volts AC between a dock outlet ground and "Earth's potential".
If the ground and neutral are bonded together the conductor is capable of higer current. Therefore, the voltage drop will be lower . Ground wire is one gage size smaller by code. It can carry a much larger current than "just sufficient to blow a circuit breaker under fault conditions".

 

You are correct - the ground wire is typically a smaller gauge than the current carrying conductors however the neutral and ground are not sharing the current when a boat or shore power cord is mis-wired - all the current is returning through the ground lead so the resistance and voltage drop are higher.

The electrical codes (which are not necessarily followed on marinas!) require the neutral and ground to be bonded at only ONE location and that location is the power source, usually the first transformer you encounter in tracing back to the source of power. They should not be bonded on a boat when on shore power. They should be bonded on a boat when on inverter or genset power. Most modern inverters and genset connections include a relay to automatically connect/disconnect this bonding.

As such it is usually a long distance from the outlet on the dock and the transformer so the current in the ground wire has to travel all that distance before getting to the neutral at the transformer.

So the AC voltage at the ground connection on the dock outlet can have measurable (volts) of AC on it if a boat in the vicinity is incorrectly wired and passing current back through the ground instead of the neutral. This voltage can also reduce the efficiency of galvanic isolators by up to 50%.

This situation is unfortunately very common in boat wiring and can be quite dangerous. An electrician working on the ground wire might think it is safe to disconnect it temporarily while he splices in another connection. As soon as that connection is broken, there will be considerable arcing as he disconnects the mis-wired boat and one side of the "ground" lead on which he is working will become hot at 120 volts.

I maintained much of the dock wiring at a yacht club marina and encountered this situation a number of times. My usual diagnostic was to put a meter between the ground lead and aluminum dock at each dock outlet and see if the voltage dropped when I temporarily disconnected that boat's breaker. If it did that meant they were injecting power into the ground lead and were asked to correct it.

 

You have some misconceptions. For example, that the distance a current travels makes a difference. Electrons don't loose or gain energy by travelling. The resistance of the wire increases with lenght, but that is a different subject. An electrician working on a boat with shore power should disconnect it from the power source, i.e. unplug it. Then there would be no possibility of electrocution. Also, if the electrician has any idea of what he is doing, he'll test for polarity and verifiy that the power is off first. A proximity voltage meter is about $14.00 at any hardware store.

 

Huh? Who said electrons loose energy by traveling? The word appears no where in the post.

The longer the distance the higher the resistance. Current times resistance of the wire = voltage drop. So a long run back to the transformer on your ground wire will produce voltage on the ground pin of your outlet. This voltage then flows through the ground wire to all the other boats on the dock, through their shore power cord and puts AC on underwater metal. AC won't cause electrolysis directly but it can form bubbles of chlorine under your barrier coat on metal boats.

Also the AC current flowing through your galvanic isolator reduces its DC blocking power by up to 50% which CAN cause underwater electrolysis. In a situation like this you need a capacitor across the galvanic isolator to pass the AC without carrying the DC with it. Some galvanic isolators have a built in capacitor to do this, but they are woefully inadequate and the manufacturers do not state the AC current capacity. See the article "DOES MY GALVANIC ISOLATOR NEED A CAPACITOR TO CONDUCT AC?" at http://www.yandina.com/GIsolCap.html.

I guess you misunderstood my example. I was talking about maintaining the wiring in the MARINA, not the boat. The electrician may be working on a dock outlet installation 200 yards from the mis-wired boat in a marina with hundreds of slips. He will have cut the breaker off for the section of dock on which he is working, but ground current can be flowing from another dock that still has power. How are going to "test for polarity" on a ground wire? A proximity meter is not going to respond to 3 volts AC on a ground lead. The only way to check the ground lead would be to use a clamp on ammeter around the wire before cutting it.

 

If you are talking about the dock wiring, then it makes sense.

 

Forget the galvanic isolator and go for an isolation transformer. Read Nigel Calder's "Boatowner's Mechanical and Electrical Manual", pages 99 thru' 103 to understand the dangers that G.Is can bring.