Grounding methods of marine installations - Discussion

captainslarty

captainslarty

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Re some recent healthy discussion, I believe this deserves it´s own thread. so here goes.
The main reference for this work is Calder, I am using ed 3
I fully support the reasons mentioned in his excellent book.
The only ground neutral connection on a boat should be at the marina plug on the pontoon - or wherever it ends up.
If using a portable genny on board INSTEAD of the ac supply, then the neutal and ground should be connected at the genny case itself.
Joe.

Please feel free to contribute.
 
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The only ground neutral connection on a boat should be at the marina plug on the pontoon - or wherever it ends up.

[/ QUOTE ] I am not quite sure what that means, possibly because it is quoted rather out of context.

The neutral is earthed at the electricity supply substation. A earth connection should run from the supply earth to the ship and all major metal lumps should be connected to it as well as all electrical equpment that is not doubly insulated. (however see a later comment * ) That will, in most cases, mean that the 12 (or24) volt onboard system has its negative connected to the mains supply earth. Maybe it should be deliberately earthed anyway. It also means that anodes are also connected to it as well. (then you'll need a galvanic isolator)
A continuity of the earth connection from the shore to all metal bits is the only way to guarantee the full protection that the RCDs should give, and in their absence (or failure) ensure that a fuse blows or mcb opens in the event of a full live to earth fault occurring.

I agree that the genny neutral should be earthed to the genny case but that must in turn be connected to the ships earth system if the genny is to be used to power all the onboard mains voltage equipment. I think that the most satisfactory way of using a portable genny would be to plug it in to the shorepower socket in place of the shorepower lead.

* later comment: A few weks ago I was talking to an electrician who does a fair amount of work connecting power supplies to outside events. If I understood him correctly he said that in those cases, or in the case of outbuildings (eg your detached garage/workshop) the earth connection in the main building should not be extended to the satellite location. They should be provided with a separate earth via an earthing rod (and all that that entails.) Not really practical in the case of teporary installations of course. He wondered if the installation on a boat should follow the same idea. I guess that would entail a substantiall earthing plate on the hull. Hardly possible to connect a boat to an earth rod in a pit of coke or whatever!
It was brief conversation as we both should have been busy with other tasks and he had not previously given any though to boat installations although he is a boat owner.

I have given a bit of though to having an isolation transformer. It's big, expensive and IMHO, although it isolates the ships syten from the shore, it offers very little advantage in terms of safety.

My final comment is that IMHO boat systems should not be like this (domestic) system anyway. They should be like an industrial system with a 110 volt supply with a centre earth. That is where each side of the supply is at 55volts above earth potential, not with either one side or the other being earthed. Lots of practical disadvantages of course.

Sorry a rather complex "olewill" style post. Also sorry I have taken ages to compile this so probably now repeats what others have said.
 
Hi Sir, In the UK, the neutral is earthed at the substation, not in the US and not in many areas of Europe.
You are absolutely right that the genny earth strapped to Neutral should be fed to the boats marina socket or at least to the same infrastructure.
The boat does NOT need substantial grounding, the normal ´to anode´is fine, a lot of marinas are earthed at the pontoon feed, ie, the neytral is strapped there, it is the reason why you ideally have the facility to check the connections prior to hooking up.
The basics are to have NO earth to Neutral ON the boat, only at the supply. If you cant check the supply, then really dont use it to be safe.
Basically if using a genny strap the E and N together at the genny case, plug it into the shore power socket, keep a handle on your overall wiring, or get it checked if one cannot do it yourself.
The elcb or rcd type breakers specced for UK and US are a bone of contension too, the yanks favour much lower trip values with unlimited time. Many faults have been recorded in RCD type devices that do not show up in normal use., hence the recommendations.
Joe.
 
I don’t want to go off on a tangent, but the IEE/IET Wiring Regulations are finally catching up with marina power supplies.

These come into effect from 1st January 2008, but from what I understand these cannot be backdated, i.e. they would only affect new installations from the new year. This could affect any future marina/ shore power installations.

Details of the new regulations can be found on the IEE website:

http://www2.theiet.org/Publish/WireRegs/...r_locations.pdf

If anyone wants to discuss this in more detail, please feel free to PM me.
 
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Details of the new regulations can be found on the IEE website

[/ QUOTE ] The link did not work!
 
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Hi Sir, In the UK, the neutral is earthed at the substation,

[/ QUOTE ]

Not always. If you have a TN-C-S supply (not uncommon in the UK), then the Neutral and Earth are combined into one conductor (the CNE conductor - Combined Neutral and Earth). At your installation, the Main Earth Terminal is connected directly to the incoming CNE conductor. So the only wires coming into your premises is a Live and [Neutral+Earth] conductors.
 
Thanks for that excellent reference. I found wikipedia 'earthing' a useful reference to interpret the meaning of PME, TT, TI etc.

Now for the examination: Answer?

The vessel should never use the earth provided by a substation. It should always use an earth downstream of an isolating transformer.

The isolating transformer may be provided within the marina supply. If the boat does not have an isolating transformer, the earth cable on the boat socket may be wired, but it should also be earthed to sea within the boat supply. Earth should not be connected to neutral on the boat. With this arrangement, there is some risk of electrolytic corrosion to earthing points on the vessel.

If the boat has its own isolating transformer, earth will not be wired at the input socket. Instead, earth (to water) will be connected to the neutral output from the transformer. The risk of electrolytic corrosion is minimal.

If you connect to the local taverna's electric supply, you won't know which is neutral and which is live . . . until you plug in and note a tingle . . . or a jolt . . . or the RCB trips

Did I get that right?
 
I am so bloody confused.....

I have a get set and shore power, have absolutely no idea how it is wired! don't know where to start - but I have not got a shock yet. The distribution box appears to be a standard household fuse box with 4 swithces which flip up for on - the first is the master switch and is double the size (double pole?) and 3 circuit switches. Are these switches RCDs?

On the basis - it ain't broke - I am going to leave well alone! (unless you persuade me otherwise.
 
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Not always. If you have a TN-C-S supply (not uncommon in the UK), then the Neutral and Earth are combined into one conductor (the CNE conductor - Combined Neutral and Earth

[/ QUOTE ] Our house, built in the 1960s was wired like that until a few years ago. An inspector coming round after a new meter had been fitted spotted it and altered it so that the house earth is now connected to the sheath of the incoming cable. I assume the CNE syetem is no longer used in new installations and is changed when it is spotted. It was something I had not come across before but I was told that it was common in some areas.

BUT surely the CNE conductor is earthed at the substation it would not be a combined neutral and earth if it was not.
 
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[ QUOTE ]
Hi Sir, In the UK, the neutral is earthed at the substation,

[/ QUOTE ]

Not always. If you have a TN-C-S supply (not uncommon in the UK), then the Neutral and Earth are combined into one conductor (the CNE conductor - Combined Neutral and Earth). At your installation, the Main Earth Terminal is connected directly to the incoming CNE conductor. So the only wires coming into your premises is a Live and [Neutral+Earth] conductors.

[/ QUOTE ]

Whats the difference? If you have 3 core wire coming in, two of the wires are connected at the sub station and possibly other places along the route anyway. So really you only have live and {neutral+earth} anyway.
 
I assume you mean a 240v small petrol gen set? If so, this outputs a voltage difference between two lines. At this stage, neither line is live or neutral.

One line may be internally connected to the chassis - and the earth line outlet. You can check this by connecting your ohm meter to the earth outlet and each of the supply lines in turn (with the genny not running!). If connected, there will be no resistance between earth and one line - and that will be your nominal neutral line.

Nominal neutral only, because the earth line at this stage isn't earthed (or watered in this context).

If your boat has an isolating transformer, this doesn't matter as far as the boat's electrics are concerned (they'll be independently earthed and neutral will be neutral). But your boat's mains input plug is unlikely to have a connected earth, so it does leave you with a potentially live genny chassis. But it's only live relative to the live genny output line. That small danger can be removed by earthing the chassis to water, and making sure that the nominal neutral is connected to the earth. Belt and braces.

If your boat does not have an isolating transformer, your boat earth is probably wired through to the mains input socket, so connecting your genny (if it's earth and neutral are connected) will automatically earth through the boat's system. There is no problem adding an additional earth wire to the water - it just duplicates what's already there.

Slight snag if your genny has a continental output - some of those can be put in either way up! So, 'wrong' way up your genny live feeds the boat neutral feeds the earth feeds the genny neutral . . . and the short instantly trips the genny. So with continental plug output it's best not to interconnect your neutral to earth within the genny, unless you have a proper 'one way only' plug and socket.

I hope that hasn't confused you further . . . but you don't have to read the bits which don't apply to your setup. Which makes it very easy if you're a diesel genny guy with automatic switching between mains and genset . . . leave well alone!
 
>I hope that hasn't confused you further . . . but you don't have to read the bits which don't apply to your setup. Which makes it very easy if you're a diesel genny guy <

I am a deisel genny guy! - I keep trying to remember my physics from school.... that was a looong time agoooo
 
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Then scroll down to "Marinas and Similar Locations". It's under the title "Issue 23 Spring 2007"

[/ QUOTE ] Still can't get it. I get a blank page and an error message on the progress bar or what ever it's called.

Perhaps I need a newer version of Acrobat reader!
 
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