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Hull Earth & Negative Bonding Questions


Morris

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I've tried to understand from the numerous threads on here but there are a couple of things I am not sure on.

 

1) On our boat the negative from the starter battery and a lead from a negative bus bar in the engine bay (cruiser stern) connect to the engine block.  Is a further negative bond to the hull necessary if there is continuity between negative and hull? Would a second bond increase the chances of corrosion?

 

2) If a further bond is not required should the 230V system earth bond be as close to the engine as possible (on the engine mounting rails for example) or is the bulkhead close enough?

 

3) If a stud is added to the bulkhead for negative bonding where should the wire come from? Would the negative bus bar be a good place in our case?

 

4) For the earth bonding is it ok to take it from the boat side of a GI as per drawing (clearly I have mad skills in paint) and are there any issues with using the same lug for the case of an inverter?

 

5) Off topic, but does the set up in the drawing fail in any other areas? Would common or garden 13A plugs and sockets be a problem for the BSS in this application?

 

 

230V Set Up.png

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Reading your post, trying to understand it and the diagram I would suggest that for your own and others safety you employ a professional.

 

From what I can gather it seems that you negative battery wiring is not the common way, but probably works. Normally the engine battery and the domestic bank negatives are joined at the batteries, not via a bus bar.

 

A negative bus bar is usually part of the domestic wiring, although some  marinisers (Beta) use an engine negative stud affair.

 

If you are saying the only negative bond is to the engine block then as long as the engine is NOT flexibly mounted and bolted down onto the steel beds it is probably OK. If it is flexibly mounted a bond needs to go between the engine block and the bed. Unless you can guarantee the whole hull is at both banks' negative potential a short circuit may well not blow the fuse or trip a circuit breaker.

 

As long as the AC "earth" is bonded to the hull it really does not matter where it is, although convention prefers it close to but not on the same stud as the DC earth bond point.

 

Your diagram seems to show a socket from the inverter and a separate socket from the shoreline. That is not usual, your inverter seems to show an AC shoreline input. That suggest it is a combi-unit so no need for the separate sockets. If it is not a combi-unit one normally uses some form of change over to select shoreline or inverter output - switch, relay, or wander plug.

 

I have no idea why you seem to have a mains plug feeding the RCD. It seems to leave both sockets unprotected. Visit this diagram again and show the three conductors as separate lines. As shown and without guessing your intention the galvanic isolator will do nothing. As I said you may do well employ a professional. Mains can and does kill.

 

 

 

 

 

  • Greenie 1
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1/ The aim is to have the hull connected to battery negative but without any circuit through the hull, ie a single point of connection. If the engine is not well connected to the hull then it would be advisable to connect a cable from the engine block to the hull somewhere adjacent. The engine may be connected to the hull via exhaust, control cables, stern gland etc but this might not be a very good connection and in the event of a major short circuit to hull, the resistance of these connections might cause problems eg control cables glowing!

 

2/ The mains earth hull connection should be adjacent to the battery negative one. I don't think this is too critical but it is specified in the relvant British Standard.

 

3/ Not from the bus bar as that introduces an alternative current path to the engine via the hull and then via exhaust, control cables etc to the engine. Take a cable from the engine to the hull

 

4/ Yes as per your drawing is fine.

 

5/ If this is a combi inverter then it is designed to be installed with shore mains at the input and the output connected directly to the rcd then domestic circuits. There is no need to any plug and socket switchery and the way you have drawn it, you could connect the inverter output to its input which it is not going to like! Please refer to the installation manual of your combi inverter. A combi inverter is designed to automatically switch between incoming shore power, and inverter power, depending on whether or not shore power is connected. You might want to have a socket or circuits connected to the incoming shore power directly for some devices that you only want to be able to use when on shore power, but in that case it must route via another RCD.

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30 minutes ago, nicknorman said:

2/ The mains earth hull connection should be adjacent to the battery negative one. I don't think this is too critical but it is specified in the relvant British Standard.

 

 

And that opens up another whole can of worms. Is this oat within the scope of the RCD/RCR? If so the OP should have access to and be familiar with all the BS/ISO that apply.

 

I don't want to get into a discussion about if Alan is correct that major work brings the boat within scope of the RCD/RCR but fiting a shorline and messing with the  DC circuits just might.

Edited by Tony Brooks
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Singular lack of an RCBO/RCD in that diagram.

I guess you are relying on the 13amp plug fuse for over current protection but there is nothing for earth leakage protection.

Suggest you get someone local that knows what they are doing to fit a safe system.

 

Going to be interesting plugging the combi input into the output, would that give you self charging batteries?🤭

Edited by Loddon
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Hi all, thanks for the replies.  Just a bit of background info to explain:

 

As most will have already realised I have very little knowledge in this area.  I know most will say 'get a professional in' and tbh I would be extremely happy to do this if I could guarantee that the person I was paying really knew what they were doing.  It is my experience that "professional" just means that a person is getting paid for doing the job, not that they necessarily are an expert at it.  That may sound cynical but I have noticed this in all areas of life and this has made me want to research and understand topics such as this as well as I can rather than just trusting someone to do a good job.

 

That being said I had no plans to work on anything involving 230V and then plug in without at least getting it checked over first.

 

The diagram is my attempt to replace like for like what is there whist wiring in a GI and adding an earth stud.  The boat currently has loose 16A plugs and sockets instead of the 13A in the diagram which are obviously unsightly.  I just thought the 13A sockets would be neater, I did think about the limiting aspect, but our power requirements are very small so thought it wouldn't be a problem.  And I obviously want to keep the self charging battery feature! Haha

 

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Agree about the concept of “professional”! But it isn’t rocket science, my advice is just to do what I would do which is to read the installation instructions for the inverter-charger, and then implement it.

 

If the inverter charger has previously been installed with the socketry you have shown, it was probably installed by one of those “professionals” that we both have limited regard for!

Edited by nicknorman
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10 hours ago, Morris said:

 

 

230V Set Up.png


I haven't read every post here so apologies if this has been covered.

 

Have you checked that the inverter output is neutral-earth bonded as it should be? A lot of cheap pure sine wave inverters sold on Amazon and eBay aren't and can't be modified as they have a floating earth.

 

It's a different question to hull bonding. It's simply down to the make/model of inverter, but if it's not N-E bonded your breakers may not work in the event of a fault.

Edited by blackrose
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22 hours ago, blackrose said:

Have you checked that the inverter output is neutral-earth bonded as it should be? A lot of cheap pure sine wave inverters sold on Amazon and eBay aren't and can't be modified as they have a floating earth.

 

It's a different question to hull bonding. It's simply down to the make/model of inverter, but if it's not N-E bonded your breakers may not work in the event of a fault.

 

Hi Blackrose, the inverter is a Sterling Combi so I believe it should be N-E bonded.

 

 

I was at the boat today so I took a couple of pics of the situation.  The plan was to create a hole in the plywood panel (x marks the spot), drill and tap two holes in the bulkhead and add a earth bonding stud inside the cabin and negative bonding stud in the engine bay. Does anyone think this sounds like a good or bad idea? The positive cable to the inverter in the engine bay looks like it would foul on a bolt in the position shown in the picture but the cable actually stands proud of the bulkhead by about 3 or 4 inches so wouldn't actually be very close to a short stud in that area.

IMG_20221127_101431.jpg

IMG_20221127_095958.jpg

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That would be ok but not ideal as the best practice is to have the two studs (dc negative to hull, and mains earth to hull) adjacent to each other. 
 

What I am much more concerned about is the dc installation in the pictures you have posted. Maybe the photo is misleading but the second photo seems show a fat red positive wire going from the battery isolator switch to the Combi, with no fusing AND the cable goes through a hole cut in the steel, with what looks like a bit of rag as protection. This is really bad practice and in fact, if the photo isn’t deceiving, I would say dangerous. Firstly the BSS and the Combi installation manual requires a fuse between the battery and the Combi, near the battery. The idea of the fuse is to protect against any issues that might cause excessive current through the wires that would cause an overheat and possible fire. In other words, it is there (or should be) to protect the cable.

 

Secondly when you have a cable at one voltage passing through steel at another voltage, in the presence of vibration and probable sharp edges, it is necessary to take proper precautions so that over time, the cable insulation doesn’t chafe causing a major short circuit and fire / spraying vaporised and moulted copper. Please don’t underestimate the energy stored in a domestic battery bank and how quickly that energy can be dumped into a short circuit. We are talking thousands of amps and tens of kilowatts. Mains voltage is dangerous, but so too can be high current DC wiring.

 

If it were my boat I would install a megafuse in that cable near the battery end, and a proper through-bulkhead grommet fitting where the cable passes through the steel.

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1 hour ago, nicknorman said:

That would be ok but not ideal as the best practice is to have the two studs (dc negative to hull, and mains earth to hull) adjacent to each other. 

 

 

ISO 13297 "Small Craft Alternating Current Electrical Sytems" 

 

4 General requirements

4.1 The protective conductor insulation shall be green or green with a yellow stripe. Neither colour shall be used for current-carrying conductors.

NOTE The equipotential bonding conductor of the d.c. electrical system (see ISO 10133) also uses green, or green with a yellow stripe, insulation and is connected to various exposed conductive parts of direct-current electrical devices, other extraneous conductive parts and the d.c. negative ground/earth.

 

4.2 The protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as practicable to the battery (d.c.) negative terminal.

NOTE If an RCD (whole-craft residual current device) or an isolation transformer is installed in the main supply circuit of the a.c. system (see 8.2), the negative ground terminal of the d.c. system need not be connected to the a.c. shore ground (protective conductor)

 

 

And for DC systems where a cable passes thru a bulkhead :

 

ISO 10133 "Small craft Electrical systems Extra-low-voltage D.C. installations"

 

7.4 Conductors which may be exposed to physical damage shall be protected by sheaths, conduits or other equivalent means. Conductors passing through bulkheads or structural members shall be protected against damage to insulation by chafing

 

 

 

 

Edited by Alan de Enfield
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26 minutes ago, nicknorman said:

That would be ok but not ideal as the best practice is to have the two studs (dc negative to hull, and mains earth to hull) adjacent to each other. 

 

Thanks Nick, just so I am clear, best practice would be to have the studs adjacent on the same side of the bulkhead, as per the right hand diagram rather than my proposed arrangement on the left (which I suppose would fit some interpretations of adjacent but not others). 

1425038622_studarrangement.png.53711d928f754aa2d0a009abc12b32e4.png

That being the case would it be best practice to route an earth wire to the engine bay or a negative wire to the cabin?

 

Re the positive wire going through the bulkhead, the protection that looks like rag is actually rubber hose, although it is split along both sides giving a splayed appearance.  I fully appreciate the possible dangers and will inspect it more closely next time I am at the boat.

The cable is fused though, that isolator switch is just for the combi - there are starter and domestic battery isolation switches in the cabin.  Sorry, I should have mentioned that the batteries and cables have been removed.  All the leisure batteries were connected (individually and involving large Anderson connectors!) to the bus bars.  The cable in the pic on the RH of the positive bus bar goes to a 500A fuse (just above blue hose in pic) then to the isolator switch and then to the combi. Does this fuse size and placement seem appropriate? The cable to the left of the positive bus bar goes to the domestic battery isolator switch in the cabin.

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It’s always difficult to be precise just based on photos, and also I slightly misread your post. I don’t think it matters that one stud points one way and the other points the other way. However I question how far away from the engine this all is and what you are connecting to what? The aim is to avoid creating multiple negative current paths between the engine and the batteries including one via the hull, otherwise a proportion of the current will route via the hull, which is bad from an electrolytic corrosion current point of view.

 

Ideally one would have the studs located near the engine, but the most important point is that the negative connection to the hull is from the engine, not from the battery negative, to avoid creating a current path between engine and batteries via the hull/exhaust/control cables etc. By having only a connection between engine and hull, there is no circuit formed, just a spur which would not have any current flowing through it (except under fault conditions). And bear in mind that eg with a fault in a positive cable protected by a 500A fuse, up to 500A could flow through this engine-to-hull cable so it needs to be chunky.

 

On the point about the dc wiring, fair enough I didn’t realise that was just an isolator for the Combi and it is fused. I’m not clear which Combi you have but 500A does seem a lot, the recommended fuse size in the Sterling manual, ie either 400A or 250A depending on the model. I suppose if it should be 400A and is actually 500A that isn’t the end of the world, the main thing is that it is fused.

 

93EC1C0F-67B8-4F8B-AC11-0FFE42C06498.jpeg.6b036d567299ca2a97b450384d4629c5.jpeg

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Thanks for the fuse size info, it is the 2500W model (marked up as 2100W but I checked with Sterling and this was apparently a mistake by the sticker manufacturer!) so the 500A fuse hopefully wouldn't be too much of a problem in the short term but I will change it to 400A. 

 

I am notoriously bad at giving context but hopefully if I fill this thread with pics I'll be able to convey the plan I have loosely formed in my mind!

 

Pic 1 - the general layout in the cabin

 

705921551_Cabin1.jpg.152921c87a992f83d134a2482a047e52.jpg

 

 

Pic 2 is my proposed site of hole sawing the plywood panel to gain access to the bulkhead (yellow circle), chosen because I believe/hope there will be little chance of there being any wiring behind the panel in this location,  

 

664373856_Cabin2.jpg.a7327f486fa312fa512da5cb8020029e.jpg

 

 

On the engine bay side of the bulkhead the area of the yellow circle would correspond to the blue circle shown here (I would secure the live cable out of the way to prevent any possible issues of chafing on the stud).  Would this be close enough to the engine?  It is roughly a foot from that closest engine mount. It could be lower and closer to the bed but my thinking was that a lower position would potentially expose the connection to damp, coolant spills etc. but maybe that's less of an issue than it being further away?

 

2110175841_EngineBay1.jpg.e06797da191ab9a7f216fd55bb775e0b.jpg

 

 

I figured that drilling and tapping two holes (I have read that M8 would be the best size?) in that area and arranging the studs to protrude either side of the bulkhead would allow the fitting of a negative connection between the engine block and a stud on the bulkhead in the engine bay and also allow the easy routing of an earth wire from both the RCD (in this thread WotEver suggests that taking the earth connection from the RCD would be better rather than from the GI as I suggested in the first post) and the stud on the case of the combi to a stud inside the cabin.

 

 

 

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Now we can see where the engine is (close!) I think that is fine. As you say, you don't want it too low as it could become submerged or at least get wet. Go for it!

M8 should be fine. Personally I prefer taking the hull earth from the first possible place within the boat to reduce the risk of it somehow being disconnected - ie the boat side of the GI, and this is how our boat came new from a prolific boat builder of good repute. However it has been said on this forum that it is better to take it from the consumer unit (RCD). Not quite sure why, just convention I think. Either way I don't think it matters too much.

 

Just a word on crossing cables of the positive and negative variety, perhaps slightly counter-intuitively it is best to firmly tie wrap togther any cables of different voltage that might touch each other. The enemy is relative movement from vibration etc causing chafing, and by tying them firmly together, relative movement and hence chafing is eliminated.

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39 minutes ago, nicknorman said:

Now we can see where the engine is (close!) I think that is fine. As you say, you don't want it too low as it could become submerged or at least get wet. Go for it!

M8 should be fine. Personally I prefer taking the hull earth from the first possible place within the boat to reduce the risk of it somehow being disconnected - ie the boat side of the GI, and this is how our boat came new from a prolific boat builder of good repute. However it has been said on this forum that it is better to take it from the consumer unit (RCD). Not quite sure why, just convention I think. Either way I don't think it matters too much.

 

Just a word on crossing cables of the positive and negative variety, perhaps slightly counter-intuitively it is best to firmly tie wrap togther any cables of different voltage that might touch each other. The enemy is relative movement from vibration etc causing chafing, and by tying them firmly together, relative movement and hence chafing is eliminated.

 

Thank you so much for your help and patience, I definitely owe you one. I will give it a go and report back with pics although I'm afraid I can't guarantee there won't be more questions before it's done!

 

Thanks for the tip about tie wrapping cables, I will bear that in mind.  Just got to figure out all the tools and materials required now.  It's always the stuff that isn't already on the boat, isn't it?! Or worse, stuff that I think is on the boat, but when I get there it isn't.

  • Greenie 1
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  • 1 month later...

Quick update as promised.

 

IMG3.jpg.c2585a3d0fb52ba2ff58b2afd05477ed.jpg

 

 

Fixed earth and neutral hull bonding studs, got combi and GI wired up, got an electrician in to check everything over and... with 230V going through it the combi has a Christmassy but entirely useless flashing coloured lights function. Hmm. Should have checked this before buying the boat.  Ah well. 

So now the 230V side of things is connected up sans combi and I'm looking for a battery charger.  Victron Ip22 30A single output seems to fit the bill.  Anyone any thoughts on these?

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3 minutes ago, AndrewIC said:

I have always wondered, why are two separate studs recommended?

 

Because if it is a single stud and for some reason it becomes detached from the hull while keeping the 12 & 240V earths connected a 240V fault to the earth wire would put 240V on the 12V DC negative and thus the hull.  I would agree with anyone who says that is not very likely but for the sake of another stud/nut & bolt why risk it.

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22 minutes ago, Tony Brooks said:

 

Because if it is a single stud and for some reason it becomes detached from the hull while keeping the 12 & 240V earths connected a 240V fault to the earth wire would put 240V on the 12V DC negative and thus the hull.  I would agree with anyone who says that is not very likely but for the sake of another stud/nut & bolt why risk it.

That makes no sense, in the scenario you outlined the stud had become detached from the hull?

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1 minute ago, AndrewIC said:

That makes no sense, in the scenario you outlined the stud had become detached from the hull?

 

OK, so you think a 240V and a 12V wire connected to a detached single stud can't put 240V  onto the 12V negative in the case of a 240V fault. I am not sure why you think that is not the case. Remember, although in theory there should only be one 12V negative bond point, in practice there are usually more on inland boats involving the engine electrics

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19 minutes ago, Tony Brooks said:

 

OK, so you think a 240V and a 12V wire connected to a detached single stud can't put 240V  onto the 12V negative in the case of a 240V fault. I am not sure why you think that is not the case. Remember, although in theory there should only be one 12V negative bond point, in practice there are usually more on inland boats involving the engine electrics

No, I didn’t say that, don’t put words into my mouth. I understand that a mains fault might put 240V onto mains earth, and in the scenario you described hence also onto the 12V negative line via the bonding cables that are still connected together (but no longer connected to the hull via the detached stud). What I did not appreciate - and what you could have explained without the sarcasm - was the assumption that there could be additional, perhaps unintended, 12v-to-hull connections which would thereby make the hull live (and/or get fried by fault current). That makes sense, thank you.

 

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Just now, AndrewIC said:

No, I didn’t say that, don’t put words into my mouth. I understand that a mains fault might put 240V onto mains earth, and in the scenario you described hence also onto the 12V negative line via the bonding cables that are still connected together (but no longer connected to the hull via the detached stud). What I did not appreciate - and what you could have explained without the sarcasm - was the assumption that there could be additional, perhaps unintended, 12v-to-hull connections which would thereby make the hull live (and/or get fried by fault current). That makes sense, thank you.

 

 

Sorry, but this has been explained so often in the past it gets very wearing having to type it all out again. On a proper sea boat with fully insulated return 12V systems there would be way the 240V could get to the hull in that scenario but very few inland boats are build like that because they tend to use automotive electrical equipment and that tends to be earth return. A few people like Lister use a relay to try to make it insulated return once the engine is running but that still leaves potential paths to the hull via unmodified radio aerial leads and such like.

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