Why bond earth to DC

[rogerthebodger]

Via the battery , or via any DC equipment switched on at the time, to the negative and then to earth via the bonding and /or any adventitious paths, such as engine, stern gear and water, when they exist

Wouldn't that cause catastrophic damage to the DC equipment and/ro battery with the possibility of explosion or fire.

If that is a possibility I for one would not even contemplate that connection and the ISO would be guilty of creating a potential dangerous installation. I would also hold the surveyor and insurance company totally responsible to any result.

Vic any Idea what the ISO or other standards say about separate mains/DC wire runs and the insulation of any equipment connected to mains and DC circuits.

I think that anyone who is adamant that the DC neutral and mains be connected should connect 240 VAC mains across a 12DC battery and report the results an the forum.

 

[BabaYaga]

Via the battery , or via any DC equipment switched on at the time, to the negative and then to earth via the bonding and /or any adventitious paths, such as engine, stern gear and water, when they exist

Via the battery – is there connectivity between positive and negative within the battery? Not a very good one, at least not for system voltage DC (self discharge?). Perhaps 230 V AC would be different?
Via DC equipment – yes I suppose, as long as something is switched on. No DC equipment on = no path to ground?

Anyway, I am not convinced that the eventuality discussed in the later posts of this thread is the main reason for the ISO requirement to connect AC PE to the grounding point.
A much more likely dangerous situation would be a fault in the shore cable PE connection – or in a galvanic isolator – in combination with a live-to-case fault in a piece of AC equipment.

But there could be several reasons, of course.

 

[Boo2]

I would really like to understand this. Could you please describe the path to ground for an AC current applied to a DC positive battery terminal?

As I said the DC -ve terminal is the same voltage as the DC +ve terminal minus 12V. Therefore the AC voltage can be found at the DC -ve terminal directly (offset by 12V). The path to ground from the DC -ve terminal is direct from the DC -ve terminal to ground through your body. Of course there will be a current into the +ve terminal as well, and of the same size as the current that kills you.

Boo2

 

[Boo2]

Wouldn't that cause catastrophic damage to the DC equipment and/ro battery with the possibility of explosion or fire.

If that is a possibility I for one would not even contemplate that connection and the ISO would be guilty of creating a potential dangerous installation. I would also hold the surveyor and insurance company totally responsible to any result.

Vic any Idea what the ISO or other standards say about separate mains/DC wire runs and the insulation of any equipment connected to mains and DC circuits.

I think that anyone who is adamant that the DC neutral and mains be connected should connect 240 VAC mains across a 12DC battery and report the results an the forum.

This is just silly. Nobody is suggesting that mains voltage *should* be applied to a battery +ve terminal, the question is what happens if that fault does arise ?

There are 2 possible scenarios as follows (note that in both cases the mains is earthed and an RCD is fitted :

1. The battery -ve terminal has been joined to the mains earth : The RCD (if it is working) trips at the point that the battery +ve terminal becomes live (ie at the time the fault develops).
3. The battery -ve terminal has not been joined to the mains earth : The RCD trips at the point that a human being bridges the mains earth and the battery -ve terminal (or anything connected to it like the engine block etc).

I want my boat (or any boat I visit) to be type 1 above because it is less likely to kill me.

Boo2

 

[mjf107]

Bonding is a safety system to limit touch voltage between exposed metal parts in the event of a live to earth fault, along with MCBs and RCDs and earthing conductors. It is used where there is a perceived extra risk of electric shock. You could look at it as a last resort if the other devices fail or the earthing system becomes compromised, best to have more than one safety system, RCDs can and do fail

 

[Heckler]

The RCD would trip




I don't belive that all. If this was the intention then the requirement would be to connect to some external underwater earthing plate . The equivalent of an earthing rod.
My boat has no conducting part in contact with the water when the outboard is raised so bonding to the DC system would not be an effective way of providing an alternative route to earth

Exactly, my MD22 is deliberately fitted with non earthed starter and alternator so that the engine is not connected to earth and so there is no path to the oggin for any "stray" stuff.
S

 

[VicS]

Exactly, my MD22 is deliberately fitted with non earthed starter and alternator so that the engine is not connected to earth and so there is no path to the oggin for any "stray" stuff.
S

Dunno if your Md22 is fitted to a sail drive ??


The trouble with the MD22 is that it is not ( cannot easily be ? ) insulated from the SD as many other small VP engines are. As a result the engine electrics are fully isolated, In these circumstances the DC negative is not bonded to the shorepower earth. The ISO standards make note of this situation.

IIRC it was primarily due to a short between the engine block and the starter wiring that led to the complete destruction of Nigel Mercier's sail drive leg. Not "stray stuff", or even galvanic stuff, in this case but pure and simple electrolysis.

 

[Ruffles]

I think most people who have proper fitted mains battery chargers bond the DC -ve to earth. After all a galvanic isolator is a small additional cost.

Our shore power is a single cable from the transom to a double socket so I haven't bothered.

I think in practice there would be enough natural leakage between the battery negative were it to become live and the sea to trip an RCD . But I wouldn't bet my life on it!

 

[rogerthebodger]

I think most people who have proper fitted mains battery chargers bond the DC -ve to earth. After all a galvanic isolator is a small additional cost.

Our shore power is a single cable from the transom to a double socket so I haven't bothered.

I think in practice there would be enough natural leakage between the battery negative were it to become live and the sea to trip an RCD . But I wouldn't bet my life on it!

But what fault would cause the battery negative to become live in real life in a properly installed mains installation?

 

[Boo2]

But what fault would cause the battery negative to become live in real life in a properly installed mains installation?

Same faults as any other installation ? You might as well ask what fault would cause a metal equipment box to become live - any misplaced or broken wire end could do it. Or someone puts a fastener into a panel with a wire run behind it and cuts the insulation of ac and dc circuits.

Boo2

 

[rogerthebodger]

Same faults as any other installation ? You might as well ask what fault would cause a metal equipment box to become live - any misplaced or broken wire end could do it. Or someone puts a fastener into a panel with a wire run behind it and cuts the insulation of ac and dc circuits.

Boo2

But AC and DC circuits should be in totally separate conduit/trunking. If this is not specified in the ISO standards it is a great and major error.

Metal cased equipment would have a collection to the mains earth wire without being connected to the DC negative.

What else then.

 

[BabaYaga]

But AC and DC circuits should be in totally separate conduit/trunking. If this is not specified in the ISO standards it is a great and major error.

It is specified, even though the ISO leaves several alternatives for how the separation can be done.

In my view, there is probably no definite answer to which precautions that should be taken to avoid every more or less likely fault condition. Each to his own. But it could still be interesting to try and find out the reasoning behind the ISO requirements, i.e. what are the dangers perceived by the authors?

More specifically in this thread, is the DC system being accidentally 'contaminated' with live AC 230 V such a danger and is this the reason for the requirement to connect PE to the crafts grounding point?

One indication for the answer being no is what the ISO 13297 has to say about AC in a boat with a metallic hull AND a fully insulated DC system (i.e. where the negative is not connected to ground). In that case the AC PE should be connected to the hull. Period.
Which would leave the DC system unprotected by the PE.
This makes it clear, to me at least, that AC contamination of the DC system is not a major consideration for the ISO authors.

 

[Boo2]

What else then.

OK, it's completely impossible and no such fault could ever happen and therefore not earthing the -ve rail of your DC supply could never kill you.

Boo2

 

[Lon nan Gruagach]

Dual power equipment like 12V/240V fridges is a possible failure point.
Human error is another.

Why bonding 12V negative will trigger RCD on live to 12V positive fault:
As the mains voltage exceeds battery voltage, the 12V negative will have current flowing due to the battery charging:
live -> 12V +ve -> battery -> 12V -ve -> bonded PE
AS the mains voltage drops below battery voltage the mains live becomes a load on the battery, again causing current to flow in the PE.
In both cases the source of the current in the PE is the live conductor, so the RCD will trip.
Incidentally, since mains neutral also has a voltage on it (~4V ac in most domestic situations) then the RCD will also trip on a neutral to 12V system fault.



Now, what happens when you connect 240V ac across a battery?
Lets start with a little demonstration....

First test @160A ohh look, steam, thats why the candles are there, to show that no free hydrogen escapes, the battery has a catalyst to recombine it with the oxygen.
What fault current might you expect?
From shore power:
Shore power cable : 20m of 2.5mm^2 gives fault current of 1000A which will trip in 0.01 seconds.
From inverter: Sterling Power Products quote a surge rating of just 3 times rated continuous load, so of the order of 30A.

And what happens to your electronics?
Apart from the circuit where the fault occurs, then probably not a lot. You would have 240V (315V max) on a thin piece of wire connected via a trip to the 12V distribution bus which has, by comparison a massive cable to the battery, which as we have seen will stubbornly refuse to let the voltage exceed ~14V.

Worst case you are left with engine, lights, handheld VHF, compass, ohh and your smartphone.

The choice is remarkably simple
a)risk a ludicrously painful electric shock, possibly involving broken bones.
or
b)spend a bit of money and possibly fry some electronics that should be no more than a navigation aid.

 

[Boo2]

Now, what happens when you connect 240V ac across a battery?

That video doesn't actually show mains across a battery but a 160A overcharge ?

Boo2

 

[rogerthebodger]

It is specified, even though the ISO leaves several alternatives for how the separation can be done..

Ok that is good to know. is it compulsory or optional.

But it could still be interesting to try and find out the reasoning behind the ISO requirements, i.e. what are the dangers perceived by the authors?

I wholeheartedly agree I always like to know the reasoning behind behind any safety requirement unless it is self evident to me.

More specifically in this thread, is the DC system being accidentally 'contaminated' with live AC 230 V such a danger and is this the reason for the requirement to connect PE to the crafts grounding point?

This is where I have a problem in that how this is going to happen in a properly installed mains boat installation.

One indication for the answer being no is what the ISO 13297 has to say about AC in a boat with a metallic hull AND a fully insulated DC system (i.e. where the negative is not connected to ground). In that case the AC PE should be connected to the hull. Period.
Which would leave the DC system unprotected by the PE.
This makes it clear, to me at least, that AC contamination of the DC system is not a major consideration for the ISO authors.

This is where the standards are contradictory because if it was such a high potential danger that would not be allowed.

Having a steel hull boat myself this is how my boat is wired as if there is a leakage of AC mains to any metal cased appliance , the hull could become live which is a very dangerous situation. On a GRP boat not such a danger as the basic structure is non conductive. Only some items on a GRP boat could become live.

It would appear that only appliances that have any connections to both mains and the 12VDC system that could cause a contamination or the DC system with 240VAC. or damage to conductors carrying AC and DC in the same enclosure/trunking. This latter one would be eliminated by following the ISO standards and having the two sets of conductors totally separated as I have done.

Only 2 appliances so far have been identified battery charges and duel voltage refrigerators. These should/must have isolation transformers with a grounded screen between primary and secondary windings.

I am open to any real reasons for the mains earth to DC negative with a real danger not just a vague possibility.

Again I am concerned that if the mains and DC negative are connected and a mains live shorted to the DC positive what would happen to the battery and any other DC equipment that may be switched on at the time.

The video posted Dougal Tolan still does not show 240 VAC mains connected across a battery only what happens when over charged with 160 Amps ans a direct short. Both are quite dramatic and I would not wish either to happen. Having fuses in my main battery connections as close to the battery would reduce that happening.

 

[l'escargot]

I think your circumstances are rather unusual in having a steel hulled boat and aren't really relevant to the vast majority on here.

The reasons for bonding are exceptional, just as they are in a house - my steel draining board isn't bonded for any obvious reason but just in case...

 

[Lon nan Gruagach]

The video posted Dougal Tolan still does not show 240 VAC mains connected across a battery only what happens when over charged with 160 Amps ans a direct short. Both are quite dramatic and I would not wish either to happen.

And the calculations afterwards show that a mains short circuit can only produce 1000A on an average shore power cable. At that current your MCB must trip within 0.01 seconds and so your battery will only have to absorb the equivalent of 1A for 10 seconds. its not even going to get tepid, never mind hot or explode.

 

[Lon nan Gruagach]

The reasons for bonding are exceptional, just as they are in a house - my steel draining board isn't bonded for any obvious reason but just in case...

Except for when you are doing the dishes as a rat chews a cable that falls and hits the copper pipework. If the sink isnt bonded, you get quite a zap. My favourite "you really have to bond that?" is the chromed plastic insert in a plastic shower tray that has a plastic drain pipe, but then some people will take a hair dryer into the shower.....

 

[nigelmercier]

... but then some people will take a hair dryer into the shower.....

That's what Darwin is for.

In my opinion, isolation is better than earth bonding.