220V on board

[wiggy]

I am thinking of installing a 220V system on board. I will use a combined RCD/MCB consumer unit. Do I need to link the earth with that of the 12V system if all I want is two 220V sockets?

<hr width=100% size=1>

 

[sailorman]

no earth using the shore lead.
12v & 240 v dont mix very well

<hr width=100% size=1>

 

[steve_cronin]

But there is more

Link to earth via a galvanic isolator or your boat, the pontoon and other boats will make a nice little galvanic cell and you could be at the wrong end of the electrochemical series and end up with de-zincified hull fittings which will fail and drop off - result obvious.

Steve Cronin

<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[mriley]

Re: But there is more

I have bought a separate RCD and galvanic isolator, which I'm waiting to fit (after I've finished the 101 other jobs that need doing in the next fortnight). Should the RCD, or the galvanic isolator, go next in line to the shore power connection.


<hr width=100% size=1>

 

[Birdseye]

Re: But there is more

the only sensible answer to your question is to advise you to get someone properly qualified to do it for you. there are very few jobs on a boat where you ned to do this, but messing around with the mains is one. dont end up sat on a cloud wishing you were down here sailing!

<hr width=100% size=1>

 

[pvb]

Yes, it should be linked...

The earth wire of the mains shorepower should be connected to your 12v negative ground, ideally at the engine. This is for safety considerations. To avoid corrosion problems though, you'll need to fit a galvanic isolator in the earth wire, just after it enters the boat.

<hr width=100% size=1>

 

[Quartet]

The RCD/Circuitbreaker unit should be fitted in a dry position as close as possible to the boat mounted socket into which you plug your shorepower cable. We try to fit these in such a position as to ensure the absolute minimum of unprotected cable run from this socket to the rcd, but if there has to be some, try and route it in such a way that the cable cannot be fouled by locker contents, kedge etc. Don't use domestic 240v cable, use the proper low temperature blue shrouded cable and protect it where it passes through bulkheads etc.

<hr width=100% size=1>

 

[MainlySteam]

Re: Yes, it should be linked...

<<<The earth wire of the mains shorepower should be connected to your 12v negative ground, ideally at the engine. This is for safety considerations.>>>

While ABYC still says this (I believe) it is increasingly being regarded as bad practice because of the possibility of introducing electrolysis problems, especially on metal boats, with no upside. To fully achieve it though, does introduce the problem of isolating the engine from DC if it is not an isolated model.

It is actually the topic of conversation with repect to a new build we are involved in where the electrical designer has bonded the negative to the ac earth and even though that bond is continually and automatically monitored for stray currents (which is not something one would find in the average pleasure vessel) both the shipyard and myself find this contrary to best practice (it is a metal vessel though). The shipyard follow their practice even on large vessels both commercial and pleasure (eg an around 100 m LOA powered superyacht) of their own design.

The only reason any of us have been able to come up with for bonding the negative to the ac earth is for the case where one gets an ac short into the dc system, a case we all regarded as exceptionally weak.

John

<hr width=100% size=1>

 

[wiggy]

Re: Yes, it should be linked...

are you saying keep two systems seperate, the boat lives on a swinging mooring and will oly occasionally be linked to 220V when in marinas.

<hr width=100% size=1>

 

[MainlySteam]

Re: Yes, it should be linked...

Yes, in my opinion and that seems in common with those I work with. But especially with a metal boat. It may be difficult if ones engine is not dc isolated (alternator, starter motor, instrument transducers do not have their cases at dc negative) as the engine is normally at ac earth in non metal boats (and some metal ones too) so the negative and the ac earth are unfortunately automatically connected.

But in case is needed to be said, remember you must have an ac earth on the boat for the ac systems safety, even, if you have no ac supply on the boat (even if no invertor for example).

John

<hr width=100% size=1>

 

[pvb]

Which is more important?

I suppose it boils down to whether you think it's more important to guard against electrolysis or to guard against electrocution. For most people, that decision is simple.

The ABYC and USCG have recommended linking the AC earth to the 12v ground for some time. I believe the BMEA now also recommend this. They all justify the practice on the basis of safety. Equally, they acknowledge that measures need to be taken to minimise electrolytic corrosion. For most boats, a simple galvanic isolator will do the job. For metal boats, isolation transformers are usually required.

As with any recommendations, there are always people who will disagree.

<hr width=100% size=1>

 

[MainlySteam]

Re: Which is more important?

Except that a galvanic isolator (or an isolating transformer) has nothing to do with preventing the problems that bonding the DC negative to ac ground may cause. The not bonding of DC to ac earth is all about avoiding on vessel problems, whether a shore connection exists or not.

John

<hr width=100% size=1>

 

[Davydine]

Re: Which is more important?

I also am thinking about fitting a shore power system to the boat. My understanding was that you only ran a risk of Galvanic corrosion if the 220v system was connected to the 12v system, for example, through a battery charger. In my case I would not be using a battery charger (at least not with the battery still connected to the system) I just want to plug in a small heater and run my tools so I can work on the boat.

I dont believe that I need a galvanic isolator in this case... Am I right?

David

<hr width=100% size=1>

 

[MainlySteam]

Re: Which is more important?

No, you should install a galvanic isolator if you install a shorepower connection unless, if you only wish to run tools or a heater on board then you may be able to run a lead from the shore onto you boat directly to the tool or heater. In that case you do not need a galvanic isolator. However, using leads in that way is generally frowned upon unless for temporary use only.

Regarding your reference to battery chargers, I suspect that the opposite is the case, in that there are situations where non marine chargers may negate a galvanic isolator if the charger has a non isolated transformer in it (I do not know if such chargers are permitted in the UK though, but if so they will be found among the automotive chargers). Such chargers have safety concerns on board as well.

John

<hr width=100% size=1>

 

[pvb]

Not sure I understand you...

Not sure I understand you, John. I mentioned the need to guard against electrolysis simply because that was your first objection to the recommendation that the AC earth should be linked to the DC negative. Incidentally, in addition to the ABYC, USCG and BMEA, I believe that the EU's Recreational Craft Directive now says that linking is required.

You said that "The only reason any of us have been able to come up with for bonding the negative to the ac earth is for the case where one gets an ac short into the dc system, a case we all regarded as exceptionally weak." Sadly, it's a fact that tragedies often happen when highly unlikely events occur. As an analogy, in well over a million miles of motoring, I've only once needed the protection of my seat belt. But that doesn't stop me wearing it every time I get into the car.


<hr width=100% size=1>

 

[Davydine]

Re: Which is more important?

I think I may have got this wrong, but I thought that there was only a risk of Galvanic corrosion if there was an electrical connection between the 240v system and the 12v system - which there would be if I was using a battery charger. If there is no electrical connection then then where in the circuit should the galvanic isololator be fitted.

If I am only running a heater or dehumidifier then I thought the only risk would be if the boat started to sink - in which case the RCD would trip and prevent anyone from swimming in the marina from being electrocuted....

David

<hr width=100% size=1>

 

[MainlySteam]

Re: Which is more important?

Not sure if you are referring to use of an extension lead onto the boat direct from the marina supply to your heater or humidifier, or to a shorepower connection to cabling in the boat to outlet sockets.

What you say is correct if you do not have any installed ac cabling on the boat and just use an extension lead direct from the marina supply to the appliance. As long as the frame of appliance on board is not connected in any way to the boats underwater metal fittings then galvanic corrosion due to interconnection with other boats, etc is not possible and you do not need a galvanic isolator. But, as I said, that practice is generally frowned upon for other than temporary use, just as in the same way and for similar reasons that running an extension lead from ones house to an outside workshop to provide a permanent connection would be.

If you are talking about a proper shorepower connection on your boat then, just like a house, it has to be cabled as a proper ac system with line, neutral and earth in the boat. The earthing requirement means that you have a connection through your boats metal fittings to the sea and galvanic corrosion is then possible unless you us a galvanic isolator or isolating transformer.

John

<hr width=100% size=1>

 

[MainlySteam]

Re: Not sure I understand you...

Well, I can only suggest that if you want to keep safe that you stay away from any modern vessel, especially aluminium ones (and most small to midsized commercial passenger vessels are aluminium these days). While I live 12,000 miles away I know through involvement with the building shipyard of one recent large passenger ferry in the UK that it does not have its negative bonded to the neutral - it was entered into class and met the requirements of MCA. I suspect that there are many more.

With respect to the Recreational Craft Directive I think the information may be incorrect. It is some time since I had anything to do with it, but if I recollect correctly the Directive only refers to equipment meeting relevant standards and practices. It does not set out specifically how to build the boat.

With respect to USCG it is news to me that they require the neutral and DC negative to be bonded. I have just had a quick scan through Code of Federal Regulations 46, which sets out the requirements for commercial vessels, and cannot see any reference to such a requirement. It may be so, perhaps someone can direct me to it. I do not believe that there is any likelihood that there is a special requirement for US pleasure vessels as I see reference to both approaches being used in them.

John

<hr width=100% size=1>

 

[steve_cronin]

Re: But there is more

This isn't rocket science as some would have you think but if you're not sure, run it through with a qualified marine professional or seek the help of someone you know you can trust.

However, since this is a PRACTICAL Boat Owners' forum and not an arena for immature scholboys to score points off of each other:-

The live and neutral go to the RCD feed. The incoming ground goes to the "Shore" terminal of the Isolator and then a new green/yelow wire from the "Boat" terminal to the earth connector of the RCD. All boat earths like the grounding plate are connected to the boat side of the ioslator and/or the earth on the RCD. NOTHING but the earth connection of the shore power goes to the "Shore" terminal on the isolator. This is MOST important.

All terminals should be good and it is unwise to simply loop the conductors of the wires around the studs. use proper terminations. Also hold the base nut on each stud with one spanner whilst tightening the other nut with a second spanner otherwise internal damage can be done to the G.I.

Steve Cronin

<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[MainlySteam]

Re: But there is more

<<<However, since this is a PRACTICAL Boat Owners' forum and not an arena for immature scholboys to score points off of each other>>>

I trust that you are not referring to me. In the case that you are, some asked questions related to galvanic isolation and I gave professional answers, some asked for expansion on those. PRACTICAL does not mean AMATEUR - if the discussion is beyond your understanding then just ignore it.

In any event you will have a quiet week from me, I will be overseas providing advice in shipyards for the rest of it - good luck with your banking.

Regards

John

<hr width=100% size=1>