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Old 23-10-2013, 16:43   #91
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I believe Chala is referring to a scenario where there is a failure at two different locations, where the first failure shorts L1 to metal plumbing lines and another failure shorts L2 to an appliance's metal housing. After all those failures that go unnoticed, a person comes and grabs the metal appliance with one hand, and a metal plumbing part with the other. This is why I wrote that being hit by a moonrock is more likely. I don't even have metal plumbing lines for example.

Also, the use of GFCI outlets will fully protect against this and the return current is through plumbing instead of through the outlet.
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Old 23-10-2013, 16:43   #92
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It would be a phenomenal coincidence that such a contrived case could happen. And anyway a conventionally wired system with these problems would also present a safety hazard without a GFI.
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Old 23-10-2013, 16:47   #93
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Originally Posted by pbiJim View Post
Dave, Are you saying that an appliance can not leak current to ground simply because it's power source is the floating secondary winding of an isolation transformer? In the case of a refrigerator that has a steel shell that happens to touch some metal part of the boat, if the compressor motor leaks some current to the metal shell, would that not be an example of current leaking to ground? If a hot plate were also in use & the other power leg leaked current in that appliance, would that not create a hazard for a person holding a pan to fry some eggs while touching a metal surface in the boat? My concern would be that just because a secondary winding was not grounded at the transformer, does not mean that an appliance down stream can not create a ground loop there.
That is the beauty of an isolation transformer: it is an energy source in no way referenced to it's input nor to ground. It galvanically isolates every conductor used in AC power, instead of just the ground like in a galvanic isolator.

So the only way to create a closed circuit is to connect to both output conductors of the transformer. You can take either one in your hand and jump in water or whatever you want but nothing will happen. Unless some idiot connected the other output to ground...
Sorry, couldn't resist that
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Old 23-10-2013, 17:43   #94
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Re: Isolating Transformers, The Earth wire connection conundrum

A couple of points if I may, its normal ship practice to check the earth lights between Line and hull the equivalent on a metal yacht would be the same on glass boat i'm not sure if all metal items are bonded together on a new boat or on a older boat if they remain bonded. Still as discussed both hot wires are floating therefore measuring voltage or earth lamps wouldn't be much use anyway.

Something that nobody appears to have stated that any switch should be double pole (my opinion), as changing a light bulb for example you wouldn't want the extra risk.

I haven't thought this thru but why not run a earth wire from your isolating transformer to your bonded equipment if RCD protection is your aim but not hull, earth lamps may then be used and would pick up any insulation breakdown.

But I'm not qualified. Comment please.
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Old 23-10-2013, 17:53   #95
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Quote:
Originally Posted by pbiJim View Post

Dave,
Are you saying that an appliance can not leak current to ground simply because it's power source is the floating secondary winding of an isolation transformer?

In the case of a refrigerator that has a steel shell that happens to touch some metal part of the boat, if the compressor motor leaks some current to the metal shell, would that not be an example of current leaking to ground?

If a hot plate were also in use & the other power leg leaked current in that appliance, would that not create a hazard for a person holding a pan to fry some eggs while touching a metal surface in the boat, or reaching to grab another egg out of the refrigerator?

My concern would be that just because a secondary winding was not grounded at the transformer, does not mean that an appliance down stream can not create a ground loop there.
An appliance fed by a fully floating supply from an IT cannot leak current to earth, earth is not in the circuit , what you are relating are faults in an earth referenced or earth return orientated mains supply system , like the one into your house. In that case the earth is a valid return circuit , which is why you can get a shock.

Hence your last paragraph cannot and does not apply.

Dave
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Old 23-10-2013, 17:55   #96
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I have never heard of "earth lamps" ... but yes, I recommend double pole breakers for AC although I must admit to only have single pole myself.
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Old 23-10-2013, 18:04   #97
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I have never heard of "earth lamps" ... but yes, I recommend double pole breakers for AC although I must admit to only have single pole myself.
I think the poster means the idiot lights many boats have that tell if the shore power is hooked up backwards.
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Old 23-10-2013, 18:04   #98
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I have never heard of "earth lamps" ... but yes, I recommend double pole breakers for AC although I must admit to only have single pole myself.
Well with an IT single pole are fine.

Dave
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Old 23-10-2013, 18:14   #99
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I think the poster means the idiot lights many boats have that tell if the shore power is hooked up backwards.
Ah! With an isolation transformer, you don't care about all that anymore, it has become irrelevant.
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Old 23-10-2013, 18:16   #100
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A couple of points if I may, its normal ship practice to check the earth lights between Line and hull the equivalent on a metal yacht would be the same on glass boat i'm not sure if all metal items are bonded together on a new boat or on a older boat if they remain bonded. Still as discussed both hot wires are floating therefore measuring voltage or earth lamps wouldn't be much use anyway.

Something that nobody appears to have stated that any switch should be double pole (my opinion), as changing a light bulb for example you wouldn't want the extra risk.

I haven't thought this thru but why not run a earth wire from your isolating transformer to your bonded equipment if RCD protection is your aim but not hull, earth lamps may then be used and would pick up any insulation breakdown.

But I'm not qualified. Comment please.
Reverse polarity lights are a peculiar US invention, rarely found elsewhere. In many European systems , AC is unpolarised and hence it not unusual to get " reverse" polarity. The EU schuko socket accepts unpolarised plugs as standard.

Furthermore any AC on board system should be designed to be safe in a reverse polarity system. Two pole breakers are the key to this. In my view no on board wiring should assume neutral is anywhere near earth potential and in fact should be regarded and wired as a " live" or hot wire.


Oceanride, you are correct in that you can establish an on board neutral , by connecting the on board "earth" wire, to a designated side of the IT secondary and this will cause an RCD/breaker to trip if the other live side of the IT touches the appliance.

However such earth wires can in effect inadvertently ground reference the output side of the transformer and in effect create a earth referenced supply , which reintroduces the earth conductivity failure modes and more reliance on safety back to fuses/breakers and RCDs. Better treat the boat like a " double insulated" appliance and not rely on ground wires at all. ( which is what SV Jedi is doing , with a floating supply )

Dave
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Old 23-10-2013, 18:34   #101
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Re: Isolating Transformers, The Earth wire connection conundrum

In reply to S/V Jedi
Trying to remember the diagram, Explanation given for 3 phase.
Earth lamps, these lamps rated at phase voltage, imagine these lights connected in star with the neutral point switched to earth (usually push button to close). therefore all 3 lamps will burn with equal brilliance. If a fault occurs say 1 line dead short to earth that lamp will go out the other 2 will burn more bright. if partial failure (insulation breakdown) there will be a imbalance in brilliance. The push button is used to close the circuit as a lamp check. These days they use resistance meters hard wired each AC circuit is alarm monitored. I've also seen them used on DC switch boards and each circuit
It will work here but only if isolation transformer is centre tapped or a couple of resistors are used to create a artificial neutral voltage.
In ship practice although we know no harm is being done by having the earth, you never know when that moon rock will fall, we tend to it immediately except on standbye. ie we take it very serious.
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Old 23-10-2013, 18:42   #102
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I
In ship practice although we know no harm is being done by having the earth, you never know when that moon rock will fall, we tend to it immediately except on standbye. ie we take it very serious.

Unfortunately harm can actually be done in certain configurations by having the earth wire. You really have to look at these things from first principles , and leave aside code that often was developed for other applications. When I hear people transferring house wiring practices to boats, I shudder.

You have to understand the provenance of the earth wire to realise why it's there at all.

Dave
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Old 23-10-2013, 18:47   #103
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Originally Posted by Oceanride007 View Post
In reply to S/V Jedi Trying to remember the diagram, Explanation given for 3 phase. Earth lamps, these lamps rated at phase voltage, imagine these lights connected in star with the neutral point switched to earth (usually push button to close). therefore all 3 lamps will burn with equal brilliance. If a fault occurs say 1 line dead short to earth that lamp will go out the other 2 will burn more bright. if partial failure (insulation breakdown) there will be a imbalance in brilliance. The push button is used to close the circuit as a lamp check. These days they use resistance meters hard wired each AC circuit is alarm monitored. I've also seen them used on DC switch boards and each circuit It will work here but only if isolation transformer is centre tapped or a couple of resistors are used to create a artificial neutral voltage. In ship practice although we know no harm is being done by having the earth, you never know when that moon rock will fall, we tend to it immediately except on standbye. ie we take it very serious.
Much harm is done by grounding an IT output. The ground should be exterminated when an IT is used. There is no added safety from ground behind an IT, only added hazard. The only error y'all make is by repeating info about polarized/grounded systems, which is all irrelevant behind an IT.

Edit: Dave is too quick for me
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Old 23-10-2013, 19:12   #104
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Re: Isolating Transformers, The Earth wire connection conundrum

To go boating now.
Don't know anything about your reverse polarity lights issue, other that on a ship you would check shore connection EVERYWHERE if you had motors on board or single pole switching.
Earth lamps are what makes Neutral earth viable on ships. otherwise more moon rocks would be more common.
I think we are getting concensus on the other matters, The situation called a conundrum by the OP is spot on, this situation is unique in that the issues haven't been sorted already, and some of ask What to do.
I see your point on ground loops but I wonder if such a neutral point could be isolated.
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Old 23-10-2013, 19:24   #105
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Re: Isolating Transformers, The Earth wire connection conundrum



Interesting discussion, Jedi and GBN have it correct that earth/ground is not needed on a single phase IT. To add a earth to a IT would defeat part of the safety feature that the IT gives. Its Isolated power, not connected to shore or an earth loop, for goodness sake. Completely different then 3 phase shore power on a larger metal ship, which has its own problems.
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