Shore earth and battery negatives

[Guest]

We have a Dahler 36 CWS. We have just discovered that the shore earth is connected to the battery positive output inside the charger. This results in 14 V DC between the calorifier with shore powered immersion heater and the diesel tank. This would not normally be a problem until you try and remove a valve from the calorifier and touch the diesel tank with the spanner!!! The question is: Why is the shore earth connected to the 14V positive output inside the charger? Should it be connected to the 14V negative output so that the boat ground is at shore earth? That is how I read the diagram in last months PBO?

 

[andyball]

that sounds dangerous......you're sure that's how it's connected?.

get it checked professionally if you're uncertain.....240V mistakes can be painful at best.

 

[jfkal]

Do you have an original wiring diagram? Some rare installations use 12 V + ground but then it needs to be consistent. The "normal" wiring is 12V Neg to boat grounding to shore ground (via a ground isolator to prevent corrosion through stray current).

 

[gtmoore]

A quick question on isolators

I'm just putting shore power into my boat at the moment and read about these isolators. Are they easy to get hold of as I haven't seen them in the normal chandlery catalogues doing the rounds.

The boat is kept in a marina and I would hope to keep the power on permanently for the battery charger - am I looking at a very rapidly corroded anode if I don't fit one?

Thank

Gavin Moore

 

[dickh]

Re: A quick question on isolators

PBO or was it Sailing Today has an article on Shore Power connections in the current Issue - I red it last week.
Dick

 

[ccscott49]

I have kept my 24volt and mains wiring completely seperate, even at the chargers, I dont like any connections to earths of any kind apart from earth on mains on boat to earth on the pontoon mains. My engines are completely insulated from any returns in the 24volt system. The only connections for my anodes are to the stern gear. I like it this way and do not have corrosion problems. I still don't understand why you would want the earth of a mains system connected to your batteries. If you get an earth leak, wouldnt it put 230 volts through your batteries?

 

[vyv_cox]

I have always agreed with you on this, but the PBO article did make me think a little. Its main stumbling point for me was the "double fault" scenario that I, and probably you, have always been taught to exclude in Hazops.

If the earth on the pontoon is not working and your electrical equipment on board develops a fault that allows a live to earth connection, then touching something earthed through the DC side while holding the faulty equipment will electrocute you. But virtually every 240 V item on my boat is double insulated anyway, and I don't know why we should suspect pontoon earths. All the ones I use have earth leakage breakers and you can test them for yourself. So I shall continue to keep my 240 and 12 V systems completely separate. I think I may add an earth to my onboard breaker though, just to be sure.

Any comment from the electrical experts?

 

[jfkal]

Re: A quick question on isolators

They are not so readily available and if at very high cost. I built my own with 4 diodes of the 50 A 400 V type.

One pair ---->|--->|-------
Second pair -----|<---|<-------

The pair have to be connected in parallel and should be assembled on an ample heat sink. What they do is:

a, Block any DC voltage below 1.4 volts thus preventing galvanic corrosion
b, Conduct any AC voltage above 1.4 volts allowing for a faulty circuit to ground blowing the fuse )

 

[Paulka]

I fully agree.
I was very surprised by what I saw in PBO.
On my boat, both circuits are completely separate, except of course through the battery charger.
.... And I'm still alive!

;-))

Paul

 

[Guest]

Re: A quick question on isolators

I got one from Index marine at just over 100 quid. It is the type with a capacitor as well as diodes.

 

[Guest]

It should not be so. There must be a fault in the charger - I would not use it until someone qualified has checked it out properly. Sounds like the AC side is shorting across somehow.

The standard hookup is to connect AC earth to DC ground. If the AC earth were connected to DC positive, it would short! Not a handy design feature!

 

[Guest]

I have connected DC ground to AC earth. A lot of people do not, but I am fairly convinced of the arguments - one of the best being it provides a clear route to AC ground for any stray current that escapes into the DC, thus making sure the boat RCB trips reliably (in addition to providing the normal earthing function).

If the AC ground is not functioning on the pontoon, then you are in trouble anyway (you can test it to some extent with a meter). To avoid this, I intend to use an isolation transformer if/when I upgrade to a bigger boat to avoid such external problems, polarity issues, corrosion etc.

 

[ean_p]

If you didn't have your ac/dc connected then maybe you wouldn't have loss to your dc side, and fitting an isolation transformer removes you from the shoreside altogether including the earth unless you bring this round the transformer with you.
When we put our circuit together we basicly had two priorites . The first was to protect people and then to protect the boat (its steel!!!) we decided that the best system for the boat was to bring the shore supply on through an isolating transformer but leave the earth on the shoreside so that the boat now was floating and in theory you could touch the line conductor and the hull/water and not receive a belt as the 240v had no relationship with 'earth', hence no stray leakages....but this left people unprotected in the event of a line/neutral short and so as in a domestic situation we fitted fast responce mcb's but really still lacked the best protection of an rcd. So in an effort to correct this we had the isolation transformer wound 110/110 with a centre tapping so now we pull 240 from across the two 110's and connect appliance earth (only appliance's via the earth pin of the plug and not in any way anything connected to the boat as the boat is still 'floating') to the centre point so should an appliance have an earth short the loss to this centre would unbalance the rcd and trip it quicker then would be the responce from the mcb's. As to the dc side its 24v and fully insulated all round with no ground or any equipotential bonding as the only non ferous underwater being the propshaft and prop and both these are covered by anodes in very close proximity.....incidently when we first bought the boat the dc side was grounded and the engine mount nearest the earthing strap from engine block to hull was totaly destroyed....the others being in excellent condition.....

 

[jtwebb]

Re: Shore earth and battery negatives on Dehler 36

I also have a Dehler 36 CWS and found some 'confusion' over mains live and return which I still have to resolve. The Dehler should have the electrics isolated from the metal parts of the boat and does not have an anode. I am unclear about the engine isolation and hence the prop so that now has an anode on it! It is a topic I intend looking at more but I have not had this boat for long.

J Webb

 

[Paulka]

Just to be more precise :
The battery charger is the only "black box" connected to both AC and DC circuits.
Of course they should be separated by the transformer, unless its an autotransformer ..... for price reasons!
Another connection could be through the casing, which should be earthed in the case of the Victron products, according to theyre notice (no double insulation!). But again only in the case of an accidental (or deliberate, but then why?) contact between the casing, and any one of the DC + or - wires.

Stop because of headache.

;-))

Paul

 

[HaraldS]

Given that your diesel tank is on negative 12V, we can assume that your engine is too and hence you should have a normal negative ground DC system.

Normally your AC ground would also be connected to the ship earth and that would effectively connect AC ground and DC negative. If now your charger connects DC plus with shore AC ground, you would have a short across your DC. Since that doesn't seem to be the case, your shore ground seems to not be intentionally connected to anything, or maybe accidentially to the wrong side of the charger.

Though the charger would normally not be the point to make such a connection.

Most likley conclusion: Your charger is broken.

But it worries me that Dehler wouldn't connect AC ground to ship earth, as otherwise they would not meet CE requirements, unless of course you have an isolation transformer.

Your described situation seems dangerous from two aspects:

The possibility of a DC ground short, say from engine or diesel tank to your AC ground or calorifier could lead to a battery short without any fuse in the circuit, (not sure if your charger plus is fused) after the connection with AC ground).

The second problem could be electric shock hazard.

And finally you might have a serious electrolysis problem since you essentially put 12V straight through the water: On land the water has connection (hopefully) to AC-ground, on your boat that part is wired to plus 12V and your minus 12V is connected to the engine block and goes to the water through shaft and prop. (do you still have the prop?)

If you are not comfortable troubleshooting and fixing electrical systems, then you should get somebody to checkout your boat very soon.

 

[HaraldS]

One argument against having AC appliances on shore AC ground, floating relative to your ships earth, is that the AC ground could take on a differnt potential to your boat.
Some faulty wireing on the shore side could cause that, and then your ground fault wouldn't trip, as it is just the voltage between the two grounds that could become dangerous. The further you sail from civilaztion, the more likley you find some weird shore power. I personally would NEVR trust the shore system.
Another method, I think in combination with a ground fault breaker it's even legal by some standard, is to not bring the shore AC ground on board at all, but make up your own AC ground from ship ground. That puts the chassis of your appliances on the same level as the ship ground and if some noticable leakage to ground happens, the RCB will trip.
Downside is that leakage currents below the trip level of the RCB will go through the water, and while AC isn't know to cause electrolysis problems other than by biggy packing DC, it can be very dangerous to swimmers. You need a lot less current than the usual 30mA of the RCB to paralyze a swimmer. A low current will not kill directly, but you might be unable to keep above water. (swimming in a marina might kill you anyway ;-)
I think the only clean solution is an isolation transformer. After the transformer you have a couple of decent options:
You can ground one end of the transformer output and create your own neutral, or you can leave them both unrelated to ground. I would still have an AC ground system, connected to ships ground in one place, as some devices have noise filters that depend on AC ground and if unearthed the filters don't work and might nergize the chassis slightly over a capacitor and give you a nice tickling feeling.

 

[Guest]

Thanks for the advice. I am comfortable with electrical issues and I will progress this one to a suitable conclusion. I have asked Dehler why they connected the charger mains shore earth to the the battery positive.
The shore earth is only connected for a short time when I work on the boat and plug in, so the electrolysis problem dos not seen to have occured. We still have a prop. I will isolate everything and open up the charger to see how it has been wired.

 

[merlin1]

Re: Shore earth and battery negatives on Dehler 36

Hello,

I am considering buying a dehler 38 and have seen your notice on the YM forum.

Can you give me any advise on this boat and do you know of any reports that have been compiled. I have tried YM's back issues but no success.

Many thanks,
John.

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