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Not sure if it is exactly what you are asking, the code does permit Non Neutral Earthed Systems (IT System) under certain circumstances when continuity of service is required. It has strict requirements for fault protection and "touch" voltage.

Not likely to be a typical situation for our size of boats 

If by " the code does permit Non Neutral Earthed Systems (IT System) under certain circumstances when continuity of service is required"  you mean non MEN system,

no, that is not what I mean.

Rather MEN supply where earth has neutral polarity, an an isolating transformer on board which separates neutral and earth.

So unless you touch both live an neutral you will not form a circuit. Then AC onboard has no reference to terrestrial earth.

Maybe I have this wrong, but it was how I was taught it. (I think.  :think: )

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So , in fact, an isolating transformer is NOT recommended?

No point in using it if you are going to give earth  neutral polarity post the TF.

ie, grounding, bonding, whatever. (Nothing to do with lightning protection)

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I was staying out of this because I though it was all being covered well, but it is getting complicated. Listen to what IT is saying. He understands the rules really well.

When it comes to Earthing, we use the MEN system and do so for several reasons. One being that it eliminates stray currents, while at the same time, allowing the potential of a fault current to flow and thus strip an RCD if they are used.

The use of either an Isolating Transformer or Ground/Galvanic Isolator is about breaking the Earth between Shore power and he Boat. NOTHING ELSE. The Earth MUST REMAIN connected as per normal on the boat. An Isolating TX is just as effective as a Galvanic isolator. The only difference is expense and weight/size.

The Neutral and Earth are once again connected at the circuit board with a cable running to the Engine Block where they meet the Battery Negative. All metals that are in contact with the battery DC system must also be at the same Earth potential. This is so anyone that may come in contact with a mains live device will trip an RCD and maintain electrical protection.

The Earthing plate is an interesting one. It's good to see that a little common sense has occurred in the Industry, but "on paper" there is no clause in the rules that state this is for Commercial only. The idea is that the water surrounding the Boat is at the same Earth potential as the metal components of the Boat. If someone is dangling their feet in the water and touching a SST rail for instance, they should not be able to become the conductor. The chances of a Rail being live on a GRP boat is unlikely. The chances of one being live on a larger commercial vessel is perhaps much higher.

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Wheels I don’t understand.
If you reconnect transformers secondary neutral to boat earth and boat earth has plate to water, you are reestablishing polarity to secondary live.
So negating the purpose of the isolating transformer?
Sorry, I think you have explained this to me before but it didn’t sink in.

 

Umm, think I have it. AC Neutral (dock) is tied to boat DC ground,. Not Neutral output from Tx  secondary.

Duh!

 

I don't have an AC panel just your vanilla box and must be one of the only ones using an isolating tranny.

Everything AC is double insulated.

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Good question lateral. But no, the Earth is still separate. Think of it this way. If you generated your own power onboard, your generator and the shore power are two distinctly different systems. The isolator does the same thing. The isolator does the same thing. The power system on the boat can now be thought of as a different system. Even though the Galvanic isolator is using the Neutral and Phase from the shore, the Earth is isolated.

The danger takes place when there is a fault and the fault current is wanting to get back to the shore power Earth. The water then becomes live. But if the Earth and Neutral are connected on the boat, the fault current is kept to within the Circuit Board. The plate is ONLY there to keep the watewr at the circuit board potential and thus metal on the Boat is then the same potential, thus no current flows.

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Thanks Wheels.

So, in my case, where the AC onboard has no terrestrial polarity because of isolating transformer,  I still need a further leg neutral---> earth---> DC ground-----> water plate. For  vanilla box ---> to transformer? Which is the only terrestrial AC on boat.

Is there a neutral---> earth to water connection on every dock submain or just one on end of dock??

Just curious.

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just for clarification, the Neutral and Earth are only connected on board the vessel when you are generating power on board, by way of a generator or inverter, and this is done within the equipment, not at the switchboard. This connection is open if this equipment is not in service. 

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