D.C. A.C. Grounds On Steel

[FORTUNATA]

On the night shift last night I was going over my marine electrics book and there's something not quite 100 per cent clear. The writer ( a professional boat engineer) seems to suggest that if you have A.C. and D.C. on a steel boat, the D.C. circuit is best left totally isolated and not grounded. However, the A.C. he says must be grounded to a fixed grounding point below the hull or around the hull.If I understood him right, that means the D.C. circuit is left isolated but protected by circuit breakers and fuses and the A.C. functions apart from the D.C. but is grounded. Of course, the A.C. electrics also have RCD devices and MCB's. That is residual current devices and miniature circuit breakers.

Here is where I get confused, though: If I read it right, the writer stated you cannot link D.C. negatives to any A.C. negative. Then he mentions something about a common ground point on the boat (maybe below the hull) where you can ground both A.C. and D.C. grounds jointly. In other words, check the A.C. ground isn't connected to the D.C. negative on the battery. Apparently, in that instance, if a fault occurs on the A.C. ground, the D.C. circuits can become alive and dangerous.

This guy wires up and checks boats for surveys so I think he knows what he's talking about. However, sometimes I'm not sure I quite understood him 100 per cent. Is it correct to ground A.C. and D.C. together at a point below the hull?

Edited October 4, 2008 by FORTUNATA

[Tony Brooks]

On the night shift last night I was going over my marine electrics book and there's something not quite 100 per cent clear. The writer ( a professional boat engineer) seems to suggest that if you have A.C. and D.C. on a steel boat, the D.C. circuit is best left totally isolated and not grounded. However, the A.C. he says must be grounded to a fixed grounding point below the hull or around the hull.If I understood him right, that means the D.C. circuit is left isolated but protected by circuit breakers and fuses and the A.C. functions apart from the D.C. but is grounded. Of course, the A.C. electrics also have RCD devices and MCB's. That is residual current devices and miniature circuit breakers.

Here is where I get confused, though: If I read it right, the writer stated you cannot link D.C. negatives to any A.C. negative. Then he mentions something about a common ground point on the boat (maybe below the hull) where you can ground both A.C. and D.C. grounds jointly. In other words, check the A.C. ground isn't connected to the D.C. negative on the battery. Apparently, in that instance, if a fault occurs on the A.C. ground, the D.C. circuits can become alive and dangerous.

This guy wires up and checks boats for surveys so I think he knows what he's talking about. However, sometimes I'm not sure I quite understood him 100 per cent. Is it correct to ground A.C. and D.C. together at a point below the hull?

 

 

As I understand best practise:- ground the AC to a sturdy part of the hull like an engine bed. If you are going to ground the DC (see later) do it close to, but not on the same fixing as the AC (because if the fixing fell off AND an AC fault developed you would put mains through the 12/24 volt system.

 

I know of no readily available modern engines that can be wired as insulated return. They all have certain components grounded to the block - even if it looks as if you have a negative connection. This means that the 12v DC will be grounded to the engine block and things like control cables stand a very good chance of grounding that to the hull. Personally I would run an additional negative cable from the main negative/ground point on the block to another one close to the AC ground.

 

Ensure all ground connections make a good contact, are kept tight and ar secure.

 

 

Wire all 12/24 volt appliances back to a negative busbar and then back to the battery negative. The engine - domestic battery negative link with make the single domestic system ground.

[Guest]

Much as I have read. If I recall correctly, the DC earth (ground) should be within 3 inches of the AC earth, but not on the same stud. I haven't gone from the engine block to earth, but from the battery negative post. I can see no reason why it shouldn't go from the engine block though.

[bottle]

We are on the dangerous ground (sorry) of the same name for different items.

 

AC has 'live', 'neutral' and 'earth' also known as 'ground'.

 

DC has 'live' and 'neutral' also known as 'ground'.

 

To be clear,

 

On AC the 'earth' wire (green and yellow in colour) should be connected to the hull.

 

On DC the 'neutral' (black in colour) should also be connected to the hull but the hull must not be used as the return (neutral)

 

For example if wiring a DC lamp then it should have a 'live' and a 'neutral' from the distribution box (sometimes called the 'fusebox')

 

The AC 'earth' and the DC 'neutral' should be connected to the hull near each other but on their own stud.

[FORTUNATA]

I see. The boat I'm having surveyed has shore power electric fittings to plug into a marina. Therefore, I imagine it will have an isolation transfomer. Certainly it has D.C. electrics too and I imagine there will be a ground point for that - the hull. Inverters I know very little about and, in fact, never heard of one till last week. There is one of those too.

I'll return to my electrics book tonight on the night shift where D.C. and A.C. grounds are discussed. It's a bit vague in some places which is why I thought I'd ask.

 

 

We are on the dangerous ground (sorry) of the same name for different items.

 

AC has 'live', 'neutral' and 'earth' also known as 'ground'.

 

DC has 'live' and 'neutral' also known as 'ground'.

 

To be clear,

 

On AC the 'earth' wire (green and yellow in colour) should be connected to the hull.

 

On DC the 'neutral' (black in colour) should also be connected to the hull but the hull must not be used as the return (neutral)

 

For example if wiring a DC lamp then it should have a 'live' and a 'neutral' from the distribution box (sometimes called the 'fusebox')

 

The AC 'earth' and the DC 'neutral' should be connected to the hull near each other but on their own stud.

[steelaway]

I see. The boat I'm having surveyed has shore power electric fittings to plug into a marina. Therefore, I imagine it will have an isolation transfomer. Certainly it has D.C. electrics too and I imagine there will be a ground point for that - the hull. Inverters I know very little about and, in fact, never heard of one till last week. There is one of those too.

I'll return to my electrics book tonight on the night shift where D.C. and A.C. grounds are discussed. It's a bit vague in some places which is why I thought I'd ask.

 

 

The quality inverter will also have an earth connection - if it doesn't - dont buy it.

 

Alex

[colin stone]

Any AC power source must have one of the live wires grounded at source, this then becomes the neutral. The other live and the new neutral go to the AC panel and an 30ma RCD. So a generator alternator will have in the connection box one of the AC wires grounded. The other wire is the live. The earth/ground point should then have dedicated well sized earth cable to a ground bar. Likewise in any isolation transformer on the boat side, one of the transformer secondary coil lives will be grounded. An inverter is the same. My earth cables back to the ground bar are at least 4mm2 and mostly 10mm2.

 

DC negative bar in any distribution panel and box should go back to the same ground bar, to which is also connected the battery negative. I use a long copper bar 25mm x 6mm firmly bolted to clean steel on the hull.

 

My prop shaft is also connected back to the ground bar.

 

Ground bar is here - http://www.luxe-motor-kei.co.uk/internal/page/image59.html

 

and an example of a distribution panel - http://www.luxe-motor-kei.co.uk/internal/page/image97.html

The vertical copper bar on the right is the negative for the distribution panel and is connected straight to the ground bar with a 35mm2 cable.

[alan_fincher]

I see. The boat I'm having surveyed has shore power electric fittings to plug into a marina. Therefore, I imagine it will have an isolation transformer.

I'd be amazed!

 

Isolation transformers are very much a rarity, and far more likely to be found on upmarket boats, in my experience.

 

The strong likelihood is that there will be neither isolation transformer, nor galvanic isolator, and that the on-board 240 volts will be directly connected to the shoreline, with no device between the two.

[FORTUNATA]

I saw a galvanic isolator advertised in a canalboating magazine - about 80 pounds, I think. As for the isolation transformer, the book I've been working through was really aimed at sea yachts and private sea vessels. The writer makes a living wiring up or repairing yachts so isolation transformers must be standard practice at sea marinas. Only the case of the isolation transformer is earthed to your boat.

What I also noticed is when D.C. electrics are isolated on these boats (but the A.C. grounded), they use double pole protection on D.C. circuits. That is, a circuit breaker on the positive and negative. However, in cases where the D.C. is grounded to the engine block, only positive polarity circuit protection is used.

It's not that I'm planning to have a go at wiring a boat up, of course. It's just a case of keeping myself busy on a night shift so a book on marine electrics keeps the brainbox from rusting up.

 

 

I'd be amazed!

 

Isolation transformers are very much a rarity, and far more likely to be found on upmarket boats, in my experience.

 

The strong likelihood is that there will be neither isolation transformer, nor galvanic isolator, and that the on-board 240 volts will be directly connected to the shoreline, with no device between the two.

 

 

Thanks for explaining it in detail. I guess if my boat does pass its survey I can have a look round and see if I can work out the wiring system.

 

Any AC power source must have one of the live wires grounded at source, this then becomes the neutral. The other live and the new neutral go to the AC panel and an 30ma RCD. So a generator alternator will have in the connection box one of the AC wires grounded. The other wire is the live. The earth/ground point should then have dedicated well sized earth cable to a ground bar. Likewise in any isolation transformer on the boat side, one of the transformer secondary coil lives will be grounded. An inverter is the same. My earth cables back to the ground bar are at least 4mm2 and mostly 10mm2.

 

DC negative bar in any distribution panel and box should go back to the same ground bar, to which is also connected the battery negative. I use a long copper bar 25mm x 6mm firmly bolted to clean steel on the hull.

 

My prop shaft is also connected back to the ground bar.

 

Ground bar is here - http://www.luxe-motor-kei.co.uk/internal/page/image59.html

 

and an example of a distribution panel - http://www.luxe-motor-kei.co.uk/internal/page/image97.html

The vertical copper bar on the right is the negative for the distribution panel and is connected straight to the ground bar with a 35mm2 cable.

[chris w]

Only the case of the isolation transformer is earthed to your boat.

 

Wrong.........the case of an isolation transformer should NEVER be earthed to your boat but to the shore earth. ONE of the isolation transformer's two OUTPUT leads should be earthed to your boat to provide a neutral, which will result in the other output lead being a live.

 

Chris

[Guest TerryL]

Wrong.........the case of an isolation transformer should NEVER be earthed to your boat but to the shore earth. ONE of the isolation transformer's two OUTPUT leads should be earthed to your boat to provide a neutral, which will result in the other output lead being a live.

 

Chris

 

Wrong.....If you take any shore supply to the boat it should be earthed to the hull, end of. Any fault can liven up the hull. To isolate, the transformer needs to be on shore.

[alan_fincher]

Here we go again..........

[Guest]

This should be good.

 

Gentlemen: choose your weapons.

 

Edited October 5, 2008 by Guest

[Guest TerryL]

This should be good.

 

Gentlemen: choose your weapons.

 

 

A thousand paces!

 

[chris w]

Wrong.....If you take any shore supply to the boat it should be earthed to the hull, end of. Any fault can liven up the hull. To isolate, the transformer needs to be on shore.

 

NOT if you take the shore supply through an isolation transformer (IT). Regardless of where the IT is physically sitting (on-shore or on the boat) the connections are always the same, viz:

 

1. Shore live and neutral to transformer inputs.

 

2. IT casing earthed to SHORE

 

3. One IT output earthed to hull (becoming therefore the neutral)

 

4. Other IT output becomes the de facto live

 

 

I have also plagiarised the following from Gibbo's site: (see: http://www.smartgauge.co.uk/galv_tran.html which also includes diagrams)

 

"......note that there is a "safety screen" between the primary winding and the secondary winding which is connected to shorepower earth. This is so that any fault current in the primary (for instance from insulation breakdown) is returned to shorepower earth to trip the circuit breaker or RCD rather than electrifying the boat hull.

 

In effect, the transformer recreates a totally isolated AC supply just like having your own mini AC power station. Totally isolated from anythng else."

 

 

Quad Erat Demonstrandum

 

Chris

[Guest TerryL]

name='chris w' date='Oct 6 2008, 01:46 AM' post='278291']

NOT if you take the shore supply through an isolation transformer (IT). Regardless of where the IT is physically sitting (on-shore or on the boat) the connections are always the same, viz:

 

1. Shore live and neutral to transformer inputs.

 

2. IT casing earthed to SHORE

 

3. One IT output earthed to hull (becoming therefore the neutral)

 

4. Other IT output becomes the de facto live

 

 

I have also plagiarised the following from Gibbo's site: (see: http://www.smartgauge.co.uk/galv_tran.html which also includes diagrams)

 

note that there is a "safety screen" between the primary winding and the secondary winding which is connected to shorepower earth. This is so that any fault current in the primary (for instance from insulation breakdown) is returned to shorepower earth to trip the circuit breaker or RCD rather than electrifying the boat hull.

 

In effect, the transformer recreates a totally isolated AC supply just like having your own mini AC power station. Totally isolated from anythng else."

 

 

Quad Erat Demonstrandum

Chris

 

Wrong.............As I said before, if you take any shore supply to a boat the earth must be connected to the hull. This has nothing to do with the transformer internals but all about the wiring to it and the physical presence of a shore supply and a shore earthed appliance onboard a boat which is supposed to be electrically isolated from shore, it is not and you overlook this. If a first fault to the hull develops from the shore supply and there is no equipotential bonding on the hull it will become live and there will be a potential between the hull and shore/water and the transformer casing and hull, that is not acceptable in any circumstances.

 

The transformer will only isolate the boat if the secondary is completely isolated and the supply is on shore and the earthed casing is untouchable from the boat otherwise all you are doing is removing the shore supply earth to the hull with live wires and a metal cased appliance still onboard! To galvanically isolate a boat, a secondary role, it must first have a primary role and be electrically isolated from the presence of all shore supply connections not just remove the earth! This is a fundimental misunderstanding of the role of an isolating transformer. Changing or ignoring the rules for the convenience of installing the transformer onboard is a dangerous precedence. You do understand equipotential bonding don't you?

[chris w]

You do understand equipotential bonding don't you?

 

It is YOU who clearly doesn't understand Isolation Transformers on boats. Check manufacturers' sites, suppliers' sites, even Gibbo's site. eg: http://www.smartgauge.co.uk/galv_tran.html. If you have a direct connection between the shore earth and the hull, you will get galvanic corrosion which is what the IT is supposed to prevent!!. You must break the direct connection by either using a galvanic isolator or by using an IT earthed as I (and everyone else involved in IT's) suggest.

 

Victron state, "Connecting the ground wire of the shore-side supply to the metal parts of the boat will result in galvanic corrosion" See: www.victronenergy.com/upload/documents/Datasheet-UK-Isolationtransformer.pdf

 

If you think through the issue logically, you will see why and will also see why the rest of us are correct.

 

Quote from Mastervolt, the other large supplier of on-board electrical systems: "To prevent this corrosion you need to remove the earth-connection from shore to the yacht’s hull. " They wire their isolation transformer in exactly the same way as advocated by me and others.

See: http://www.mastervoltusa.com/support/faq.a...on_Transformers

 

Chris

Edited October 6, 2008 by chris w

[Guest TerryL]

It is YOU who clearly doesn't understand Isolation Transformers on boats. Check manufacturers' sites, suppliers' sites, even Gibbo's site. eg: http://www.smartgauge.co.uk/galv_tran.html. If you have a direct connection between the shore earth and the hull, you will get galvanic corrosion which is what the IT is supposed to prevent!!. You must break the direct connection by either using a galvanic isolator or by using an IT earthed as I (and everyone else involved in IT's) suggest.

 

The noticable exception is Victron who wire it as you suggest but, because of this, tell you to add an additional link when the boat is out of the water on dry land and to remove it when the boat is in water. Definite scope for excitement if one forgets about the link. If you think through the issue logically, you will see why and will also see why the rest of us are correct.

 

"Eat more lamb.... a million hyenas can't be wrong."

 

Chris

 

And here we reach the limitations of (most) electronic engineers knowledge, many think they know it all! You obviously can't answer me at all and it's all over your head. As I said, there is a fundumental misunderstanding about isolation transformers. I'm fully aware of what others say but it does not make them right, they can't even agree amongst themselves. This is about safe electrical installations not convenience.

 

Victron do not wire as I suggest, what they do is just as dangerous, the IT and shore supply should be isolated physically and electrically from the boat, read isolation in the dictionary. It is YOU who is not thinking logically and don't have sufficient electrical engineering knowledge, you're out of your depth.

[chris w]

And here we reach the limitations of (most) electronic engineers knowledge, many think they know it all! You obviously can't answer me at all and it's all over your head. As I said, there is a fundumental misunderstanding about isolation transformers. I'm fully aware of what others say but it does not make them right, they can't even agree amongst themselves. This is about safe electrical installations not convenience.

 

Victron do not wire as I suggest, what they do is just as dangerous, the IT and shore supply should be isolated physically and electrically from the boat, read isolation in the dictionary. It is YOU who is not thinking logically and don't have sufficient electrical engineering knowledge, you're out of your depth.

 

Clearly, by your logic, Gibbo is also out of his depth as he and I are at one on this topic. We are but mere mortals not a God like you of course. You're talking through your arse as usual.

 

To repeat, the shore supply earth should NOT be bonded directly to the hull if one wants to prevent galvanic corrosion. The IT earth should be connected to the shore earth and one of the outputs of the IT should be connected to the hull to provide a neutral in order that the on-board RCD will still function. This method will provide safety and freedom from galvanic corrosion.

 

Chris

[smileypete]

Clearly, by your logic, Gibbo is also out of his depth as he and I are at one on this topic. We are but mere mortals not a God like you of course. You're talking through your arse as usual.

 

To repeat, the shore supply earth should NOT be bonded directly to the hull if one wants to prevent galvanic corrosion. The IT earth should be connected to the shore earth and one of the outputs of the IT should be connected to the hull to provide a neutral in order that the on-board RCD will still function. This method will provide safety and freedom from galvanic corrosion.

 

I think the problem is that if the shoreline chafes through and a live conductor in the shoreline contacts the hull, the hull can become live.

 

Gibbos site doesn't cover this important safety aspect.

 

cheers,

Pete.

[chris w]

I think the problem is that if the shoreline chafes through and a live conductor in the shoreline contacts the hull, the hull can become live.

 

Gibbos site doesn't cover this important safety aspect.

 

cheers,

Pete.

 

I understand perfectly the situation. The question though is that IF this is the overriding factor in one's mind, then there is no point in one's having an Isolation Transformer in the first place if a direct shore connection to the hull is made, as galvanic corrosion will then occur. If the insulation breaks down, within the IT, the safety shield earth will still blow the shore RCD before the hull becomes live.

 

Chris

Edited October 6, 2008 by chris w

[Scotty]

Wrong.....If you take any shore supply to the boat it should be earthed to the hull, end of. Any fault can liven up the hull. To isolate, the transformer needs to be on shore.

So what are your views on Galvanic Isolators then?

[FORTUNATA]

John C Payne indicates that only the casing of the isolation transformer should be earthed to the hull of the boat. The actual core of the transformer is earthed to the marina earthing point, isolated from the hull. There are all sorts of regulations about the cables to be used and the way they're fitted. I pretty much trust what Payne has to say as he can quote the I.S.O. electrical safety standards texts pretty much blindfolded. However, electrics is really quite a complex topic. Added to that, perhaps many of our boats don't even have isolation transformers.

I'm just off in a moment to meet with the surveyor as today is the survey on the boat I plan to buy. Maybe I'll remember to ask the surveyor if isolation transformers are used more these days.

 

Here we go again..........

[colin stone]

The casing of an IT and how and where it is earthed, has been a topic of debate in many fora, including a number or articles in the excellent magazine Professional Boat Builder. From all the research I have read and "what iffing" the various failure points with the IT, I have concluded that the case should not be earthed at all but completely isolated, effectively achieving a double insulated device, so that if it does become live, it cannot touch any part of the hull. Only the safety screen inside the transformer windings goes to shore earth.

 

The only transformers that, until recently, are potted to achieve this are US ones and pricey. There is now a Brit transformer that should do the job - http://www.airlinktransformers.com/transfo...transformer.asp

 

I have yet to get one, but for the time being my IT is mounted on a wooden plinth and the case does touch any thing else. I think I would be a very strange defect that energises the case without any indicators anywhere else, but a simple neon could be used as a check.

[Guest TerryL]

Clearly, by your logic, Gibbo is also out of his depth as he and I are at one on this topic. We are but mere mortals not a God like you of course. You're talking through your arse as usual.

 

To repeat, the shore supply earth should NOT be bonded directly to the hull if one wants to prevent galvanic corrosion. The IT earth should be connected to the shore earth and one of the outputs of the IT should be connected to the hull to provide a neutral in order that the on-board RCD will still function. This method will provide safety and freedom from galvanic corrosion.

 

Chris

 

Yes it would appear so, you've covered transformer faults ok but not external wiring faults. How can I alone be talking out of my arse when Gibbo and presumably you agree with him, state quite clearly and correctly that the shore earth should be earthed to the hull. Why does installing a transformer change that requirement? It doesn't, galvanic isolation does not override earthing safety. You and him are contradicting your own advice and yes it makes a nonsense of fitting the IT onboard because it should never be there. You might just as well do away with the earth altogether to isolate it and save money on the transformer, you'll be just as unsafe! The point is that the transformer should never be fitted onboard in the first place so how you wire it is irrelevent except on shore away from the boat where you would be correct.

 

Not connecting the earth gives galvanic isolation but it is not safe, how can it possibly be? You or Gibbo state the earth should be connected to the hull, make up your mind. You cannot change this just because it fits in with the convenience of putting it onboard, can you? Electrical safety comes before galvanic isolation and the only way you can combine the two is to have a shore based or a fully insulated inline transformer, that's tough but there are some things like this that are just not possible to do and still be safe. The manufacturers have only got away with this unsafe practice because there was no regulation to stop them and you've all fallen into the same trap of believing it is ok. The fact that there is so much argument about how it should be done and people scratching their heads makes it even more obvious that it cannot be done safely.

 

This is about standard electrical safety practice, not something I've made up, why don't you answer my concerns about equipotential bonding if you think I'm wrong instead of just repeating yourself and ignoring the safety aspects that I've highlighted. If you are as clever as you make out then you know I'm right and should be prepared to admit it.