AC ground to DC negative connection

[gasdave]

I am about to go onto a marina pontoon berth and wish to safely access the shore power facility.
I have purchased my galvanic isolator and RCD/MCB unit and been looking at ways to install, using the usual sources (web, "Electrics Afloat", and Mr Calder's big book). The installation seems to be straightforward but I would appreciate some response on one point which is the old question of whether to link the AC ground and DC negative

Opinion on this appears to be divided and the legal situation in the UK does not make it obligatory, although there was an older post here suggesting that this may change to bring UK into line with US and ? Europe (anyone know if this has happened yet )

Question: what do you all do about this? If you do connect them, can it simply go from the galvanic isolator output to the DC negative at battery - or engine?

Muchus gracias as always!

 

[Jegs]

Connection of dc neg & ac earth

The problem with answering a question like this is that nowadays one fears potential litigation. However, I followed the method indicated by Calder & connected both the dc neg & ac earth to the inboard side of the G I.

John G

 

[sailorman]

I am about to go onto a marina pontoon berth and wish to safely access the shore power facility.
I have purchased my galvanic isolator and RCD/MCB unit and been looking at ways to install, using the usual sources (web, "Electrics Afloat", and Mr Calder's big book). The installation seems to be straightforward but I would appreciate some response on one point which is the old question of whether to link the AC ground and DC negative

Opinion on this appears to be divided and the legal situation in the UK does not make it obligatory, although there was an older post here suggesting that this may change to bring UK into line with US and ? Europe (anyone know if this has happened yet )

Question: what do you all do about this? If you do connect them, can it simply go from the galvanic isolator output to the DC negative at battery - or engine?

Muchus gracias as always!

Do NOT link AC & DC negatives

 

[Maine Sail]

Do NOT link AC & DC negatives

Seeing as this is a discussion about AC & DC grounds perhaps you meant AC neutral and not the ground? If you meant AC ground, not neutral, perhaps you could expand on your statement as it goes against the current US industry approach/standards as defined by ABYC E-11. Perhaps you guys have a differing standard.

AC ground (not neutral) and DC grounds are tied together then to one universal ground point in the boat, usually the engine block to meet current US standards...

 

[Sans Bateau]

Mr Sailorman is right DO NOT CONNECT AC & DC negative together. However connect DC - with AC ground, thats fine.

If you think about it, a good many items in the boat will ultimately have DC - and AC ground connected anyway, as DC- often can be ground, the engine block for instance.

 

[Matt341]

I agree with what Jegs states about litigation etc.

If someone was to say there is no need to connect to the anode, and then a mains wire rubs through a metal deck or hull fitting the whole boat would become live, the RCD could fail and you could be killed.

Connecting the AC to the DC anode connection then involves the risk of galvanic action and eat away at your anodes and then move onto things like the prop and shaft but then it would be safer than not connecting.

At the same time, if you dont connect the earth up this gives a risk.

Our previous boat had a simple shorepower system with a socket in the cockpit, RCD in the cabin and then a feed from the RCD to a few sockets.

Our current boat has an electrician fitted system, which feeds things like the mains sockets, calorifier, battery charger etc. I was thinking that the system would be earthed with a GI but on inspection it is fairly similar to our previous boat, socket in the cockpit, RCD in the cockpit locker and then wires running through the boat to everything. I check regularly that the wires are fastened in place and that they are not rubbing on anything. I also test the RCD every week or so and use a plug in mains tester just to ensure that connections are ok.

The only thing that is connected to the boat anode is the Engine and shaft.

Im not saying that the system is 100% safe. Although the boat is fitted with a RCD as well as the pontoon they could both fail.

Ian.

 

[VicS]

1. I dont have Calders book so dont know excatly what he says but be aware that accepted practices in the US may differ from those in the UK.

2. Ignore any empahtic statements that the shore power earth should not be connected to the DC negative. They are contrary to current expert opinoin and the likely future revisions to the standard which applies

3 Provided we are not talking about metal hulls the CURRENT VERSION OF ISO 13297 does, despite the above, allow for the shorepower earth not to be connected to the DC negative provided a whole craft RCD (or isolation transformer) is fitted. Otherwise the connection should be made as close to the battery negative as possible.

The downside of connecting the shorepower earth to the DC negative is galvanic corrosion if the shorepower is connected for prolonged periods, even if not in use, however a Galvanic isolator should protect you from this.

 

[gasdave]

I am being persuaded to add a connection from AC ground (inboard side of GI) to DC negative. My next questions are:-
1. what size should this cable be - I am assuming it ought to be at least equal to the incoming mains earth cable?

2. is it better to go to a common ground (eg. engine) or to the battery negative? There seems to be an opinion that it is better to have all earthing going to a common ground to prevent the possibility of potential differences (and therefore stray currents) around the boat. However (and here is my ignorance again ) I would have thought that this only applies to bonding situations. The battery negative is not connected to the hull by any means other than the common ground which in my case is the engine. Someone please tell me if I've got this wrong!!

 

[goboatingnow]

I for one am not in favour of connecting the protective earth ( misnamed AC ground) to the DC negative, provided a whole boat RCD is fitted. Its worth pointing out that a Galvanic isolator in effects basically disconnects that anyway.

In my experience its often found that there are significant fault currents in the earth circuit, this can cause a GI to close, effectivly nullifying their existence. Equally if the earth ever goes hot, now your whole DC system is lifted to the fault voltage. exit DC infrastructure.

If someone was to say there is no need to connect to the anode, and then a mains wire rubs through a metal deck or hull fitting the whole boat would become live, the RCD could fail and you could be killed.

ifs and buts and wheretofores, how would a whole boat that is GRP become live. Equally even if the DC negative is connected to AC protective earth, your seanario above would not protect you. AN RCD would, but not the DC connection. Note saying an RCD could fail and youd be killed is nonsense, your common sense could fail and youd be run over by a bus ,etc. equally you could be stupid and hold AC neutral and live and be killed.....

Obviously this is not for metal boats, here an isolation transformer is the ONLY route.

 

[VicS]

I for one am not in favour of connecting the protective earth ( misnamed AC ground) to the DC negative, provided a whole boat RCD is fitted. Its worth pointing out that a Galvanic isolator in effects basically disconnects that anyway.

Maybe I misread you. Surely the GI does not disconnect the RCD isn't the whole point that it isolates low voltages (from galvanic sources) but allows current from higher voltage sources (the mains power supply) to flow so that the RCD and the MCB or fuse still give the normal protection

In my experience its often found that there are significant fault currents in the earth circuit, this can cause a GI to close, effectivly nullifying their existence.

The area of concern as I understand it is way in wich some switch mode power supplies operate, dumping RFI into the earth connection. There is something about it in the technical info on the Smartgauge website http://www.smartgauge.co.uk/galv_tran.html
In fact the whole of the technical info section is worth studying http://www.smartgauge.co.uk/technical1.html

 

[VicS]

1. what size should this cable be - I am assuming it ought to be at least equal to the incoming mains earth cable?

The same size would seem logical.

2. is it better to go to a common ground (eg. engine) or to the battery negative?

ISO 13297 to which I have provided a link says the connection to the DC should be as close as possible to the battery negative.

 

[goboatingnow]

Maybe I misread you. Surely the GI does not disconnect the RCD isn't the whole point that it isolates low voltages (from galvanic sources) but allows current from higher voltage sources (the mains power supply) to flow so that the RCD and the MCB or fuse still give the normal protection

yes sorry, I meant that the GI effectively disconnects the AC protective earth anyway. It doesnt do anything with voltages but what it should do is prevent low stray earth currents from entering the boat, coupling across into the dc negative and then attacking you underwater connected metals. What it does in practice is another thing alltogether.

 

[gasdave]

I for one am not in favour of connecting the protective earth ( misnamed AC ground) to the DC negative, provided a whole boat RCD is fitted. Its worth pointing out that a Galvanic isolator in effects basically disconnects that anyway.

In my experience its often found that there are significant fault currents in the earth circuit, this can cause a GI to close, effectivly nullifying their existence.

I think the point about the RCD is that there is a reported failure rate of (I think) 10 - 20% on testing. The device is safe when it works.

The GI is designed not to conduct under normal circumstances (unless it has a capacitor allowing small AC current flow - your "fault current") thus providing galvanic protection. When there is a voltage leak it is designed to close/conduct thus providing person protection. The voltage at which it closes is determined by the number of diodes.
My understanding is that the GI does not "disconnect" the RCD - it simply provides a safe return to earth of leaked AC. The RCD is still able to sense the difference in current between the live and neutral, so can trip.

Calder's and presumably the American argument for using the AC earth to DC negative connection includes the scenario where you may have an AC leak into the DC circuit (eg. fault in a battery charger). This may not generate enough current flow through the DC ground (water) to trip the shore or boat RCD and without the connection has no other means of returning to earth. You then have a very "hot" DC system, and potentially dangerous surrounding water.

There is also something about adequate lightening protection requiring the two grounding circuits being held to the same voltage potential.

The downside is that there is a risk of having your DC equipment "fried" in the event of a failure on the AC earth to shore return line at the same time as an on-board leak or short - when "all the holes in the Swiss cheese line up"! The upside is that you may survive it.

 

[Stu Jackson]

Some observations here:

http://c34.org/bbs/index.php/topic,4685.0.html

 

[Jegs]

Re RCD

FWIW,

A possible failure of an RCD was mentioned. In my limited experience, a total of three, they simply repeatedly trip: which is, of course, the way you'd want it.

John G

 

[William_H]

Earthing

Some observations here:

http://c34.org/bbs/index.php/topic,4685.0.html

Thanks Stu that was an interesting link (West Marine) Re that link what is a Ground Fault Circuit Interupter? Is that another name for RCD An RCD as I understand it compares current in the active to current in the neutral. Any difference is current via earth or through a person so shuts off.Or is that a core balance relay? A different device or different name?

I am not sure about the earthing details given in that article. I dont think many baots have the negative battery isolated from the engine. Mostly connection via stater alternator and pressure sensors mean engine is very much connected to batt negative. Only a few have isolated staters Alternators etc
I would not connect an earth wire to the chain plates as suggested for lightning protection unless you have galvanised steel wire. Stainless steel has a significant electrical resistance just enough to get really hot and lose its temper (heat treatment) with any large current. The loss of strength could see a failure some time after a strike. I think the earthing of the Al mast should be the only ground path for the lightning current. Of course you could say the low resistance of the Al mast should bypass lightning current from the stays. i think stays best left not earthed.

I am not at all sure about his assertions that salt water in a marina carries currents which can take an alternative path through interconnected skin fittings. I am no expert but doubt it.
Even well written articles need to be scrutinised.
For comment olewill