johnhill
New member
I understand that it is now the practice to bond the earth of a shore supply to the negative side of the 12 volt. Is this correct and why? Is it simply a wire connection between these two terminals
Earth Bonding
- Thread starter johnhill
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VicMallows
New member
It's argueable. Why not do a search using the forum search facility and read the numereous comments on this subject? You wil find innumerable different views. Then ask us a specific question. .... but still be prepared for lots of different views!
Vic
Vic
pvb
Well-known member
Yes...
Earth bonding as you describe it is recommended for safety reasons. It's now part of the standards laid down by the EU's Recreational Craft Directive, and is recommended also by the BMEA, ABYC, USCG, etc. The downside is that galvanic corrosion may be accelerated - this can be prevented by installing a galvanic isolator (about £60).
This question has been raised many times before, and you may find that people will disagree with this answer. Unfortunately, the authorities and experts don't share their view. If you're doubtful about the facts, just PM me and I'll let you have links to websites which will confirm the recommendation.
Earth bonding as you describe it is recommended for safety reasons. It's now part of the standards laid down by the EU's Recreational Craft Directive, and is recommended also by the BMEA, ABYC, USCG, etc. The downside is that galvanic corrosion may be accelerated - this can be prevented by installing a galvanic isolator (about £60).
This question has been raised many times before, and you may find that people will disagree with this answer. Unfortunately, the authorities and experts don't share their view. If you're doubtful about the facts, just PM me and I'll let you have links to websites which will confirm the recommendation.
Ships_Cat
New member
Re: Yes...
Unfortunately, the authorities and experts don't share their view
That is very misleading as it is incorrect.
The engineers in the yards I work with building high quality vessels are of the view that isolation of the AC earth from the DC negative is the most desirable approach, especially in metal vessels. You will also find that the rules of the classification societies for smaller vessels specifically allow for it. I have every expectation that the authorities in EU would accept any small vessel built to the practices that such rules set out (such as a vessel built to Lloyd's Special Service Craft) even if the vessel is not classed.
For the USA case of your mention of the ABYC rules they do indeed say to bond the AC earth and DC negative however those rules are decidedly behind the times. I am confident that you will find no mention of that in the commonly used ABS's Guide for Building and Classing Racing Yachts. It is, as everywhere else, also common practice to build boats in USA where the AC earth is not bonded to the DC negative (I have seen them).
John
Unfortunately, the authorities and experts don't share their view
That is very misleading as it is incorrect.
The engineers in the yards I work with building high quality vessels are of the view that isolation of the AC earth from the DC negative is the most desirable approach, especially in metal vessels. You will also find that the rules of the classification societies for smaller vessels specifically allow for it. I have every expectation that the authorities in EU would accept any small vessel built to the practices that such rules set out (such as a vessel built to Lloyd's Special Service Craft) even if the vessel is not classed.
For the USA case of your mention of the ABYC rules they do indeed say to bond the AC earth and DC negative however those rules are decidedly behind the times. I am confident that you will find no mention of that in the commonly used ABS's Guide for Building and Classing Racing Yachts. It is, as everywhere else, also common practice to build boats in USA where the AC earth is not bonded to the DC negative (I have seen them).
John
steverow
New member
Re: Yes...
Earth bonding for mains supplies is a complex subject in itself, and not helped by the fact that different countries use different systems of network bonding.
In the UK we use the PME system, where the household earth is bonded to back to neutral at the substation, and relies on an individual consumer earth at each distribution point.
In a marina shorepower environment this can give rise to problems, leaving you at the mercy of other peoples earth leakage problems as well as your own, particularly if you have bonded your mains earth back to your DC negative and threfore presumably to your anodes.
You are effectively setting up a path to everybody else's dodgy supply issues in the marina, or at least those who have bonded AC earth to DC neg.
IMHO you should try to isolate the DC and AC systems as much as possible, and leave the marina earth to do it's job protecting the AC supply, and your anodes protecting your engines and metalwork.
There are possible safety issues as well as a host of corrosive/galvanic issues to be considered.
Steve.
Earth bonding for mains supplies is a complex subject in itself, and not helped by the fact that different countries use different systems of network bonding.
In the UK we use the PME system, where the household earth is bonded to back to neutral at the substation, and relies on an individual consumer earth at each distribution point.
In a marina shorepower environment this can give rise to problems, leaving you at the mercy of other peoples earth leakage problems as well as your own, particularly if you have bonded your mains earth back to your DC negative and threfore presumably to your anodes.
You are effectively setting up a path to everybody else's dodgy supply issues in the marina, or at least those who have bonded AC earth to DC neg.
IMHO you should try to isolate the DC and AC systems as much as possible, and leave the marina earth to do it's job protecting the AC supply, and your anodes protecting your engines and metalwork.
There are possible safety issues as well as a host of corrosive/galvanic issues to be considered.
Steve.
Colvic Watson
Well-known member
Re: Yes...
I am no expert - at all!!! But I think in general terms there are two types of shore power installation:
1. A simple mains line aboard to run a single appliance (drill, heater, de-humidifier, battery charger) and is connected for shortish periods of time. For this you need only ensure that you have used a simple consumer RCD socket/plug (about £8 from B&Q) at your end of the supply. No need to connect the earth to the -ve on the boat.
2. A mains line intended for connection whenever you are in a marina (yours or one you are visiting). In this case a proper consumer unitwith individual MCB's and a fixed RCD and crossed polarity protection. If you bond the earth to the neutral a galvanic isolator is essential.
I like Douglas Adams' view on electricity - The only way humans have found to power a light bulb is to stretch two long pieces of wire from a furnace or nuclear reactor about 70 miles away. Then they get a third bit of wire and stick it in their garden outside the back door. Then they get all cross when it dosen't work.
I am no expert - at all!!! But I think in general terms there are two types of shore power installation:
1. A simple mains line aboard to run a single appliance (drill, heater, de-humidifier, battery charger) and is connected for shortish periods of time. For this you need only ensure that you have used a simple consumer RCD socket/plug (about £8 from B&Q) at your end of the supply. No need to connect the earth to the -ve on the boat.
2. A mains line intended for connection whenever you are in a marina (yours or one you are visiting). In this case a proper consumer unitwith individual MCB's and a fixed RCD and crossed polarity protection. If you bond the earth to the neutral a galvanic isolator is essential.
I like Douglas Adams' view on electricity - The only way humans have found to power a light bulb is to stretch two long pieces of wire from a furnace or nuclear reactor about 70 miles away. Then they get a third bit of wire and stick it in their garden outside the back door. Then they get all cross when it dosen't work.
Lee_Shaw
New member
Re: Yes...
[ QUOTE ]
The engineers...are of the view that isolation of the AC earth from the DC negative is the most desirable approach, especially in metal vessels.
[/ QUOTE ]
Forgive me if i am being naive but how is it possible to seperate the ac & dc if the former is required to charge the batteries of the latter?
[ QUOTE ]
The engineers...are of the view that isolation of the AC earth from the DC negative is the most desirable approach, especially in metal vessels.
[/ QUOTE ]
Forgive me if i am being naive but how is it possible to seperate the ac & dc if the former is required to charge the batteries of the latter?
steverow
New member
Re: Yes...
Battery chargers as a rule are not grounded these days, and in any case use a transformer which isolates, so the secondary of the transformer and it's associated
rectifiers and smoothing circuits will be at 12V Neg potential.
If an AC earth is fitted, it is usually connected to the metal case of the appliance
and to the screen of the transformer only.
Even a switch mode unit will normally have an isolated output.
Steve.
Battery chargers as a rule are not grounded these days, and in any case use a transformer which isolates, so the secondary of the transformer and it's associated
rectifiers and smoothing circuits will be at 12V Neg potential.
If an AC earth is fitted, it is usually connected to the metal case of the appliance
and to the screen of the transformer only.
Even a switch mode unit will normally have an isolated output.
Steve.
srm
Well-known member
Re: Yes...
I think battery charging is where the need for a common earth becomes an issue - for other uses you can keep the two systems totally separate and rely on the mains earth system. Unless, of course, you disconnect both sides of your batteries from the vessel's wiring during charging so that you can not get a path from the mains into your DC earth.
Do use a RCD in the circuit for your own protection, water and electricity can produce very strange effects as well as being lethal.
Sean.
I think battery charging is where the need for a common earth becomes an issue - for other uses you can keep the two systems totally separate and rely on the mains earth system. Unless, of course, you disconnect both sides of your batteries from the vessel's wiring during charging so that you can not get a path from the mains into your DC earth.
Do use a RCD in the circuit for your own protection, water and electricity can produce very strange effects as well as being lethal.
Sean.
AlexL
Member
Its a complex and thorny subject, however the concensus is that bonding AC and DC earths is Safer, and not bonding them is better for galvanic corrosion. Personally I prefer not to have huge hunks of unearthed metal (like an engine) on my boat. If the AC earth is not connected to DC for example you could have a mains lead in contact with the engine, making it live and as it is not earthed, then a) your entire DC system would float up to 240 volts (including any metal things like the mast which may be used as a DC earth return) and b) there would be no protection as there is no route to earth untill some hapless soul touches a live bit, or goes swimming next to the boat. Not Good.
If you bond it then this cannot happen, however as has been mentioned you may be exposing your underwater fittings to the vagaries of other peoples dogy wiring, however you can get a galvanic isolator which avoids this problem and most saildrive and propshafts are now electrically isolated from the engine so that they are protected underwater.
Also remember that the main earthing wire must be capable of carrying the full fault current, sufficient to blow the fuse although an RCD shouldl provide the first line of defence on most boats.
If you bond it then this cannot happen, however as has been mentioned you may be exposing your underwater fittings to the vagaries of other peoples dogy wiring, however you can get a galvanic isolator which avoids this problem and most saildrive and propshafts are now electrically isolated from the engine so that they are protected underwater.
Also remember that the main earthing wire must be capable of carrying the full fault current, sufficient to blow the fuse although an RCD shouldl provide the first line of defence on most boats.
boatmike
Well-known member
Re: Yes...
Again John, I believe you are correct, and Steverow is too.
When people talk about AC supplies however they fail to discern between shore supply, current generated on board and that taken from an inverter. These are all different cases and Steve only addresses shore supply.
As we have discussed before the ideal installation for all ship electrics keeps battery return (negative) isolated from the hull and seawater. This is never achieved completely though and does not need to be to make the attempt valid. If nothing else an engine cooled by seawater is grounded naturally through the cooling system.
I think the situation with an inverter illustrates the desirability of seperate ground very well as it has a DC side and an AC side. The DC should be protected by a suitable fuse on the positive side and negative returned to battery. The AC side should have the ground connected to the hull and RCCB fitted across the AC terminals. Then the maximum protection is obtained for safety and short circuit. If the inverter is grounded to battery negative, in cases of internal short circuit AC current could flow in the DC battery circuit of the inverter making all DC systems live and shoving 240v AC up all your 12v kit. Major damage could occur. AC and DC circuits should NEVER be interconnected even on the earth side.
Therefore DC should remain as far as possible isolated from hull grounding and AC should be grounded to hull. There is an argument that says hull grounding is ineffective but if it achieves nothing else it stops static build up in the casing of AC appliances. Connecting to DC negative could be dangerous.
Again John, I believe you are correct, and Steverow is too.
When people talk about AC supplies however they fail to discern between shore supply, current generated on board and that taken from an inverter. These are all different cases and Steve only addresses shore supply.
As we have discussed before the ideal installation for all ship electrics keeps battery return (negative) isolated from the hull and seawater. This is never achieved completely though and does not need to be to make the attempt valid. If nothing else an engine cooled by seawater is grounded naturally through the cooling system.
I think the situation with an inverter illustrates the desirability of seperate ground very well as it has a DC side and an AC side. The DC should be protected by a suitable fuse on the positive side and negative returned to battery. The AC side should have the ground connected to the hull and RCCB fitted across the AC terminals. Then the maximum protection is obtained for safety and short circuit. If the inverter is grounded to battery negative, in cases of internal short circuit AC current could flow in the DC battery circuit of the inverter making all DC systems live and shoving 240v AC up all your 12v kit. Major damage could occur. AC and DC circuits should NEVER be interconnected even on the earth side.
Therefore DC should remain as far as possible isolated from hull grounding and AC should be grounded to hull. There is an argument that says hull grounding is ineffective but if it achieves nothing else it stops static build up in the casing of AC appliances. Connecting to DC negative could be dangerous.
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