Electrolysis earth breaker

[Trevor]

I have a relatively news SO36i that is kept at a marina berth. I leave the mains power lead plugged in to keep the batteries topped up. I know that earth blockers (back to back diodes) can be installed to reduce the possibility of small circulatory currents being set up with nearby vessels, hence reducing any electrolysis issues to local ones within the one yacht.

Does anyone have any definite opinions on the installation of these devices? While I don't want to suffer from large scale electrolysis, I figure if they were that essential, Jeanneau may have installed it.

Thanks,

Trevor

[Don Reaves]

I was under the impression that isolation was done using transformers, not diodes. These transformers are quite expensive, and are not usually installed by builders for that reason.

I would keep an eye on the zincs and see if they show signs of problems before investing in a blocker.

Don Reaves

[Trevor]

Hello Don,

The isolation takes place between the earth on the boat and the earth at the marina. Diodes are used as they has a forward bias voltage of 0.7 volt. That means that the voltage must exceed 0.7volts before any conduction takes place so for very small electrical potential differences, no current flows. In other words for the sort of potential differences that is set up through dissimilar metals immersed in a salt water solution, no current can flow. In the event of a serious earth leakage problem, the potential well and truly exceeds the diode's forward bias voltage and the earth conductor simply conducts the earth leakage to marina earth.

The cost of the diode bridge that could do this job is about A$10 but somehow when packaged up on a nice heat-sink the cost increases to circa A$400.

The transformer isolation you mention probably refers to isolation transformers that remove the reference of the Neutral conductor to earth. That is so accidental contact with the active conductor will not pass a current through the body to earth as the active has now lost any reference to earth.

I am just trying to guage how many yachties actually fit these diode earth isolators and if strong opinion exists about their use .

Many thanks for your reply,

Regards,

Trevor

[Don Reaves]

Hi Trevor,

You're right. Isolation transformers are used in the AC system to remove any offset that may be present. That's what I thought you were referring to.

Thanks for setting me straight on the use of diodes. Your explanation makes a lot of sense.

Don

[gemini]

Trevor,

Your right about the .7V.... however on and AC system either 110 or 220, .7 is miniscule... another words if you have a problem it will most likely be grater than .7 all the time... besides remember we are dealing with AC not DC and while you can block DC it is almost impossible to block AC... you in effect are creating a rectifier and trying to draw the AC to ground which in most cases will just blow the circuit breaker on the dock.... and then you have no charging any way.

Gemini

[Trevor]

Hello Gemini,

I guess that in the event no problem exists within the electrical devices on the yacht, no earth current should flow. In the event a fault does exist, your right, the 0.7V is irrelevant so the earthing system works as it would if no diodes were present.

In the event no electrical fault is present, in the normally hard wired earth configuration, a very slight earth current can flow because of the dissimilar metals sitting in an electrolytic solution like salt water. This causes the electrolysis which the molecular transfer from one metal object to another which causes pitting.

Many thanks for your response.

Trevor