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Shore power

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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.

 

This is the bit I don't get, if the AC shore earth is only connected to the engine (along with the DC earth) then is the small amount of water that might be left in the impeller pump sufficient to earth the boat to the water?

 

My impeller pump just happens to be below the water line, I guess others might not be, thus no connection to the water at all?  I suppose if there is enough vacuum that the heat exchanger might stay connected giving more surface area, but my maintenance has shown that the rubber pipe from the heat exchanger to the impellor is empty at the water line.

 

If this connectivity is tested or not by an eWOF inspector is kind of irrelevant, it's obviously required.

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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. 

But, Earth  (which is neutral polarity) from the AC box onboard should be tied to DC ground?

Then if I use my isolating transformer I should not need this. (and other downstream safety devices)

All AC used on board is from isolating transformer. ( Also onboard)

Dock to Isolating transformer is protected by dock RCD.

Isolating transformer does away with the complexity. But I suspect iso Tx is not catered for in rules.

 

Thanks for tip WP'er.

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This is the bit I don't get, if the AC shore earth is only connected to the engine (along with the DC earth) then is the small amount of water that might be left in the impeller pump sufficient to earth the boat to the water?

 

No absolutely not. The actual earthing plate is about a sq m in area. 

 

 

But, Earth  (which is neutral polarity) from the AC box onboard should be tied to DC ground?

Then if I use my isolating transformer I should not need this. (and other downstream safety devices)

Just to be a little more technically correct here, the earth is "Bonded" to neutral.

Only DC has a polarity of positive and negative. AC does not have a Polarity as such, because it is AC. The same voltage flows in both conductors, but is 180deg phase opposing and alternating 50 times per second.

 

Absolutely, the Earth is still used when using an Iso Tx. The Earth still remains that life line. The only difference is that the Phase/ Neutral/ Earth are isolated from the Shore power.

By the way, not all Isolating Tx's are wired the same when it comes to Earth. In some cases, the Earth in and out can be bonded together. Not the best for a Marine install when the entire point is to isolate from the Shore power Earth. (A reason why I like Galvanic Isolator) Some Iso tx's and best for a marine install, can have a lifted Earth. The Mains Earth is connected to the core of the Tx. That keeps the Tx safe should a short occur. The Secondary earth is made by bonding to the Neutral. Thus all 2 conductors are isolated from shore power supply.

 

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This is the bit I don't get, if the AC shore earth is only connected to the engine (along with the DC earth) then is the small amount of water that might be left in the impeller pump sufficient to earth the boat to the water?

 

No absolutely not. The actual earthing plate is about a sq m in area. 
 

 

But, Earth  (which is neutral polarity) from the AC box onboard should be tied to DC ground?

Then if I use my isolating transformer I should not need this. (and other downstream safety devices)

Just to be a little more technically correct here, the earth is "Bonded" to neutral.
Only DC has a polarity of positive and negative. AC does not have a Polarity as such, because it is AC. The same voltage flows in both conductors, but is 180deg phase opposing and alternating 50 times per second.

Absolutely, the Earth is still used when using an Iso Tx. The Earth still remains that life line. The only difference is that the Phase/ Neutral/ Earth are isolated from the Shore power.
By the way, not all Isolating Tx's are wired the same when it comes to Earth. In some cases, the Earth in and out can be bonded together. Not the best for a Marine install when the entire point is to isolate from the Shore power Earth. (A reason why I like Galvanic Isolator) Some Iso tx's and best for a marine install, can have a lifted Earth. The Mains Earth is connected to the core of the Tx. That keeps the Tx safe should a short occur. The Secondary earth is made by bonding to the Neutral. Thus all 2 conductors are isolated from shore power supply.

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this is a schematic from ABYC guidelines showing a marine transformer with an isolated earth per Wheels description

 

note this is from USA so wire colours and voltage are different but connections are the same.

My preference where possible is a transformer of this type as they maintain the ground isolation from the shore. A galvanic isolator is effectively a one use item, typically once it has conducted a fault current it does not provide protection and should be replaced. Just to confuse matters there are 2 types, one fails open after use, the other fails closed.

 

 

TX schematic.png

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I’ve seen a similar schematic in practical sailor but could not understand the logic. I posted it and then deleted owing to copywrite. The mere fact that you have a isolating transformer means that you have to contact two conductors to make a circuit. Not just live an earth. Ok, so it doesn’t self regulate faults and you would  need an RCD on the secondary of the Tx to do that. Other than a decent short tripping the tx's fuse or cb of course. (old school style)

I get the no polarity relative to dock AC, but with secondary neutral to earth to DC ground and you put your meter on secondary live and a probe in the water I reckon a good many boats would register a pd.

That doesn’t thrill me and was similar to what CD was on about.

 My old school iso Tx earths are not tied, although primary earth  to metal case is (of course). Secondary N  is isolated from earth.(By design....old)

I am reluctant to bond them in the boat, for many reasons. Including using AC only for charger and dbl insulated fridge. (12V/230V)
Give me the rationale for this to be wrong?

 

Edited as previous done on phone and was ambiguous.
 

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There is ALWAYS some danger when it comes to Power. You can never make it completely safe, apart from disconnecting it. If we get in between the Phase and Neutral in some way, then as electricity see's it, we are just another load.
I don't know about today, but it used to be that in some countries, Earth was not used. But that causes other issues and dangers, mainly in th area of stray voltages doing weird things. So for us in NZ, we use the MEN system. The Earth, even though it is connected to Neutral, is the life line for us and stops those stray voltages. Very simply, the copper conductor conducts the Electricity much better than we do, so the Phase shorts directly to Earth and ruptures the Fuse, instead of running through us as if we were yet another load. The TX simply isolated us from the ability to get between Earth and a live conductor.
In the case of an RCD, this is the best modern day means of detecting a fault and disconnecting it before our hair gets permanently permed, The RCD works by sensing that fault current running through the Earth conductor and triggering and disconnecting the Power.
For a land based Iso Tx, the secondary side does not actually need an Earth. Simply because most systems were designed to run nothing much more than a Power Tool. For a Boat though, those weird stray voltages can run wild. So we need a way of tying them down and the best way to do that, is by reinstating an Earth.
The ONLY thing the Isolation does, is disconnect the boat from the Shore power, which then disconnects the Boat from the other boats in the Marina. POOR CONNECTIONS cause stray currents and all other boats poorly connected by Salt water allow the voltages to occur. Disconnect the boats, and the voltages cannot exist. That is all the Tx and Galv ISO does.

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Pictorially I like what you said Wheels, but.

Imo, having an earth bonded to neutral encourages stray current by providing an alternate path. By definition.

ie by giving earth a polarity

And without rcd's on secondary (ABYC above) that is unwise, if you want instant fault trip.

But, thats my logic, and I could be convinced otherwise.

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