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Old 13-08-2013, 18:46   #106
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We need a white board to continue this discussion<g>.

I'll go out on a limb and suggest if using an ISO transformer then earth ground should not even connect to the boat anywhere. I don't know ABYC's view on that though.
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Old 13-08-2013, 19:20   #107
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I strongly disagree. In fact it is the best practice. If the case is not connected to boat ground then there is the possibility of shock within the boat which is the most likely danger. The case must not be allowed to be at a different potential than the boat ground and neutral.

Even if the shore hot comes in contact with the case you will not be shocked so long as you do not touch the case and shore ground at the same time. You would need really long arms to do that. The probability of shore hot contacting the case is remote anyway.

BTW, this fault condition is what can cause drownings in fresh water marinas and why many marinas are installing GFCI or whatever the acronym of the week is. It can also cause corrosion to the boat. But this type of fault is many times less likely than the marina sparky criss-crossing hot and neutral. The iso transformer makes that mistake a non event which is why they are a great safety device.

In your lab it is easy to touch the case and earth ground at the same time. Thus the case must be earthed. In a boat it is nearly impossible to come in contact with both at the same time.
Hang on , firstly there is no code that suggests that local AC earth is established. ABYC don't mandate it.

Hence only through inadvertent means does the output E wire attempt to establish a true neutral.

Hence the shock hazard I mentioned was incoming hot ( shore) touching the traffo case. Since the shore power is earth referenced and sea water is a return path , in this case merely touching the case results in an unprotected shock ( again see the smart gauge web site. )

What I trying to establish , is that current ABYC standards only use the traffo to provide galvanic isolation , no additional on board shock hazards are prevented by the transformer

In a europe , its actually code forbidden to establish an on board ground with shore power. aC and DC are not connected , so its much more likely that the output of the transformer is floating , this has actually issues for breakers , as no particular side is hot , both legs are. This would suggest that double pole breakers and RCDs are a must.

Dave
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Old 13-08-2013, 19:23   #108
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I strongly disagree. In fact it is the best practice. If the case is not connected to boat ground then there is the possibility of shock within the boat which is the most likely danger. The case must not be allowed to be at a different potential than the boat ground and neutral.
.
Whether the case is connected to either ground makes no difference in that regard. The actual potential isn't really relevant , its current that kills and once there is either a fault path or sufficient insulation resistance , potential in itself isn't relevant ( as the birds know).

Explain how a shore hot touching the case has a fault path where the case is connected to boat ground.

Dave
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Old 13-08-2013, 20:20   #109
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Potential is what causes current. You can't have current without it. If all objects in the boat are the same potential then you are like the bird sitting on the live wire. But if you connect something to Earth and let everything else float then the potential between things is no longer controlled and problems arise. To avoid that the boat neutral has to be connected to ISO transformer case and thus the purpose of the transformer is defeated.
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Old 13-08-2013, 20:29   #110
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Potential is what causes current. You can't have current without it. If all objects in the boat are the same potential then you are like the bird sitting on the live wire. But if you connect something to Earth and let everything else float then the potential between things is no longer controlled and problems arise. To avoid that the boat neutral has to be connected to ISO transformer case and thus the purpose of the transformer is defeated.
You keep repeating it mantra like , but you miss the point , connecting the case to either boat or shore ground , effectively " grounds it" in both cases. Both sides of the transformer are actually earth referenced , especially in US wiring practice. So your argument isn't valid , there is no significant common mode voltage potential , what I was arguing was that the connection of the case to the boats earth ground , exposes a serious fault sceanario. ( please read the smartgauge reference link)

In the ABYC wiring , the isolating transformer provide NO additional personal security ( as the advantage of floating is defeated), the traffo is there purely to prevent galvanic current corrosion.

As to potential with or without current, consider a direct battery short , zero potential ( in effect ) , massive current. Equally consider high tension wires 25kv with no current to ground as air is a good insulator.

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Old 13-08-2013, 20:38   #111
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Dave we'll have to agree to disagree. I'm pretty familiar with Maxwell and Faraday as I do it for a living. If you don't connect the boat ground to the transformer case you're creating possible danger. One should not have the whole boat just floating on the water with no reference potential and bring earth potential to one metal object. And as someone else already pointed out ABYC says the same thing.

The only reason I keep repeating myself is because I don't want anyone hurt. But I'll stop now if you like.
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Old 13-08-2013, 21:01   #112
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Re: Heads Up - Victron Isolation Transformer

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We need a white board to continue this discussion<g>.

I'll go out on a limb and suggest if using an ISO transformer then earth ground should not even connect to the boat anywhere. I don't know ABYC's view on that though.
The transformer screen needs to connect to shore power ground in order to trip the shore power breaker in the event of a primary winding insulation breakdown.
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Old 13-08-2013, 21:20   #113
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We can talk and talk but the thing is that the Europeans got it right. Floating transformer output is safety; every tech creates this on his bench when testing AC gear because he wants to be as safe as possible. The only thing you need shore ground for is to protect the shore power cable and primary windings of the transformer. That's where it should end. It is foolish to demand shore ground connected to boat ground and then allow galvanic isolators that interrupt ground.
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Old 13-08-2013, 21:50   #114
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Re: Heads Up - Victron Isolation Transformer

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It is foolish to demand shore ground connected to boat ground and then allow galvanic isolators that interrupt ground.
Exactly right up to the last phrase. Galvanic isolators don't interrupt "ground". They just prevent low voltage DC between boat and shore from creating DC current. They are designed to allow ground fault current to flow.

There is nothing wrong with connecting the shore ground to the electrostatic shield so long as it is insulated from the case and core. But I am skeptical that does much of anything when the insulation fails. Most insulation failures in transformers result in turn to turn shorts and seldom do the windings short to the electrostatic shield or the core.
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Old 14-08-2013, 03:08   #115
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We can talk and talk but the thing is that the Europeans got it right. Floating transformer output is safety; every tech creates this on his bench when testing AC gear because he wants to be as safe as possible. The only thing you need shore ground for is to protect the shore power cable and primary windings of the transformer. That's where it should end. It is foolish to demand shore ground connected to boat ground and then allow galvanic isolators that interrupt ground.
Later today , I will put up some diagrams. Because we need to " tease out" exactly where and what the faults are. ABYCs logic ( according to Calder) in destroying the floating output , is to prevent one of the hot secondary's raising the hull potential by say coming in contact with the engine. But this in itself doesn't create a shock hazard which is why I find ABYCs logic puzzling ( as does Calder )

If yiu are worried about common mode , then reference the output to earth using say 1meg resistor.

Diagrams to follow , maybe in a seperate thread as there actually three threads with this conundrum being discussed

Dave
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