D.C. A.C. Grounds On Steel

[churchward]

Actually, Terry, this is rather the impression I got of you.

 

Where's Gibbo when you need him? I am sure he would have some direct and informative comment to make.

 

I know very little about electrickeryt but I do like the passions and wide ranging views that it generates on here.

[smileypete]

I would have thought if not already covered as outdoor supplies then my reading of BS7671:2008 would imply that they are. Although this does apply to new installations.

 

You're right, what I meant and should have said that it's not retrospective.

 

It's covered by the IET article here:

 

http://www2.theiet.org/Publish/WireRegs/Wi...r_locations.pdf

 

Looks like it comes under 17th edition of the wiring regulations.

 

cheers,

Pete.

[Guest TerryL]

The rest is ISOLATED from the shorepower, and it has its own independent earth by connecting one of the outputs to the hull. That is, a fault in the wiring 'dowstream' of the isolation transformer will be dealt with just as safely as any other earthed installation. Simply not earthing the boat installation at all is very different, and not what anyone is recommending.

 

I am fully aware of that, I was refering to the onboard shore installation as the rest.

[Guest TerryL]

You're right, what I meant and should have said that it's not retrospective.

 

It's covered by the IET article here:

 

http://www2.theiet.org/Publish/WireRegs/Wi...r_locations.pdf

 

Looks like it comes under 17th edition of the wiring regulations.

 

cheers,

Pete.

 

This really refers to plastic or wood boats and assumes a double insulated transformer as it shows no transformer earthing. Steel should be treated quite differently and is probably where people are going wrong assuming the same applies. This was issued to the 16th Edition and may be out of date now with this years 17th Edition. I recall something about this being issued in response to enquiries made early last year but it didn't help the steel boat owners that made them! I don't think the onboard transformer earthing is quite right either.

[Guest]

Is that it then?

[bottle]

I bloody well hope so

 

Till the next time.

[Guest TerryL]

Is that it then?

 

I'm doing me best, must have been the Bren!

[Guest TerryL]

I would refer you to my answer some time ago. It appears that YOU cannot understand the counter argument. I fully understand the point you are making but you seem to miss the point that if one follows your "advice" one would never install an IT on one's boat because it would be doing nothing in terms of galvanic protection.

 

Still catching up on replies here.

 

You don't have a counter argument and if you fully understand the point I'm making you will see that it is exactly what you say, one should not fit the transformer onboard because for it to be of any use you have to leave the boat unearthed, or fit a GI. I've been saying this all along.

 

I'll put it another way. Do not fit an IT onboard because if your boat is connected to the shore earth as it should be for safety, it will not isolate and will be useless.

 

If your point is one should remove a hull earth, thereby creating a potential hazard to life, for the sake of convenience in installing a device for saving sacrificial anodes, then YOU are barking and don't understand my point at all.

[chris w]

Still catching up on replies here.

 

You don't have a counter argument and if you fully understand the point I'm making you will see that it is exactly what you say, one should not fit the transformer onboard because for it to be of any use you have to leave the boat unearthed, or fit a GI. I've been saying this all along.

 

I'll put it another way. Do not fit an IT onboard because if your boat is connected to the shore earth as it should be for safety, it will not isolate and will be useless.

 

If your point is one should remove a hull earth, thereby creating a potential hazard to life, for the sake of convenience in installing a device for saving sacrificial anodes, then YOU are barking and don't understand my point at all.

 

I would refer you to my answer, even further above now, where I explain everything very simply and have never advocated having an unearthed hull. The OUTPUT of the IT is bonded to the hull to ensure the boat's RCD will trip in the event of a fault. If you bond the INPUT of the IT to the hull, the IT becomes a very large and expensive paperweight.

 

Chris

[Scotty]

Still catching up on replies here.

I can post it again if it helps:

I don't have a shoreline on my boat so it is never 'earthed' all mains voltage generated onboard is bonded to the hull and protected by RCD

If I do need a shoreline then I will fit an IT on the boat, the isolated supply will also be bonded to the hull and protected by RCD

 

Then your boat will be unearthed from the shore supply if it is not connect to it, your risk! It is very dramatic if a fault develops you, your family and friends would be most at risk from electrocution! You could possibly get away with a faulty installation for years but one day the circumstances will change to the wrong conditions and...zap! I've seen it before and nearly lost my brother because the earth was disconnected by a new plastic water pipe and not reinstalled properly. The electricity board engineer who came to fix the line fault said they get a fatality once a week with unearthed installations, is that dramatic enough?

 

Where is this fault likely to develop?

 

There's the cable from land to the socket on the boat, then onboard there's the cable from socket to the Isolation Transformer

 

Even if there were damage to one of these short pieces of cable, the shoreline would be protected by an RCD on the 'shore', won't that trip as soon there's an imbalance?

[FORTUNATA]

There's a very fundamental principle in philosophy that goes as follows: "In the quest for knowledge and enlightenment, seeking the truth makes being right or being wrong irrelevant."

In other words, hearing out different views while making a rational, polite counter argument is beneficial.

Now, if you think these guys are bad you should try discussing boats with my dad. He's now, as it happens, an expert in all things marine yet has never even owned a boat. His favourite line is, "I worked as an engineer for years and years!" Then he tells me that when choosing a boat I should look for an inboard petrol engine!! In fact, my old man reminds me of Rigsby in Rising Damp.

It's true he did train as an engineer but I'll bet if you asked him how many diodes in a rectifier he wouldn't know.

 

Never start a thread on electrics they are never friendly or polite.

[Guest]

There's a very fundamental principle in philosophy that goes as follows: "In the quest for knowledge and enlightenment, seeking the truth makes being right or being wrong irrelevant."

In other words, hearing out different views while making a rational, polite counter argument is beneficial.

Now, if you think these guys are bad you should try discussing boats with my dad. He's now, as it happens, an expert in all things marine yet has never even owned a boat. His favourite line is, "I worked as an engineer for years and years!" Then he tells me that when choosing a boat I should look for an inboard petrol engine!! In fact, my old man reminds me of Rigsby in Rising Damp.

It's true he did train as an engineer but I'll bet if you asked him how many diodes in a rectifier he wouldn't know.

A very good signature IMHO.

[chris w]

His favourite line is, "I worked as an engineer for years and years!" Then he tells me that when choosing a boat I should look for an inboard petrol engine!! In fact, my old man reminds me of Rigsby in Rising Damp.

 

There are many people nowadays who call themselves "engineers" when they mean "technician" or "mechanic". IMHO, an engineer is someone who is formally qualified not only to "fix" things but to have the training, knowledge, ingenuity and intelligence to think through a problem, to which the answer is not obvious, and come up with a good "engineering" solution that works. Often this involves "thinking outside the box" which is a skill in which most (but not all) of the so-called "marine engineers" I have met are patently lacking.

 

Chris

[Guest TerryL]

I can post it again if it helps:

 

Where is this fault likely to develop?

 

There's the cable from land to the socket on the boat, then onboard there's the cable from socket to the Isolation Transformer

 

Even if there were damage to one of these short pieces of cable, the shoreline would be protected by an RCD on the 'shore', won't that trip as soon there's an imbalance?

 

I seem to have missed you Scotty, too many posts.

 

You could use the same argument for having no earth at all onboard and some do, to actually work an RCD requires an earth as a return for fault currents and then it might not work in a hostile environment so then the only safety measure left is the fuse or trip which requires an even better earth. If there is no earth the boat can remain live and anyone touching it will get a shock, the RCD should then trip out in time but again may not. If you are onboard this will be reversed and the earthed transformer case and all shore earth wiring, which could be exposed, and the shore will appear to be live.

 

Flexible leads are vulnerable and a chaffed, cut, trapped or stretched shore lead is the most obvious problem and it does happen, just two layers of thin plastic between you and potential death if there is no earth. Faults inside are more likely to be in switchgear, plugs and sockets and the transformer wiring depending on how it is done, water ingress is also a problem. There were reports of a Victron with a loose wire livening up a barge. However low the risk you think there is, it is never worth taking, you are at extra risk on a metal boat if it is not earthed to carry any fault currents which is what the regulations require for your safety.

[chris w]

Two points you are missing.

 

1. Scotty is earthing one of the OUTPUTS of his IT to the hull so that the RCD will work.

 

2. There is an equal probabability that, if the mains input lead should be chafed as in your scenario above, it could be the earth lead that is cut through and not the live so the boat could still be unearthed.

 

Chris

[Guest]

We used to fit an armoured flex onto welding sets to prevent chafing when working around metal. It was still highly flexible, but performed very well when fabricators accidentally dropped sheet metal etc. on it. Sounds to me to be a possible solution for anybody with an onboard IT who may have concerns about live shoreline cables coming into contact with the hull?

 

Edited October 7, 2008 by Guest

[Guest TerryL]

I would refer you to my answer, even further above now, where I explain everything very simply and have never advocated having an unearthed hull. The OUTPUT of the IT is bonded to the hull to ensure the boat's RCD will trip in the event of a fault. If you bond the INPUT of the IT to the hull, the IT becomes a very large and expensive paperweight.

 

Chris

 

I know all about the output earthing so forget that and concentrate on the onboard shore supply. You are saying it should not be earthed to the shore earth so logically you are advocating an unearthed hull from the shore supply which is just as important to earth as the output earth if you bring it onboard so that the shore RCD and fuses can operate. How many more times do I have to say yes it will be useless when properly wired for safety. You are trying to justify the impossible.

 

How do you think the onshore safety devices will operate without an earth connected to the hull if there is a fault to the hull? Please just answer me that.

 

 

Two points you are missing.

 

1. Scotty is earthing one of the OUTPUTS of his IT to the hull so that the RCD will work.

 

2. There is an equal probabability that, if the mains input lead should be chafed as in your scenario above, it could be the earth lead that is cut through and not the live so the boat could still be unearthed.

 

Chris

 

1. The output earthing will not allow operation of the shore RCD only the onboard RCD.

 

2. Yes the earth can be damaged but no excuse for not having it in the first place.

 

 

We used to fit an armoured flex onto welding sets to prevent chafing when working around metal. It was still highly flexible, but performed very well when fabricators accidentally dropped sheet metal etc. on it. Sounds to me to be a possible solution for anybody with an onboard IT who may have concerns about live shoreline cables coming into contact with the hull?

 

 

It would still have to be earthed and as an exposed earth an even bigger hazard if the hull were not earthed as it would become a lose connection on deck with a possible dangerous potentional between it and the deck. Bond it all up then no problem.

[Scotty]

You could use the same argument for having no earth at all onboard and some do, to actually work an RCD requires an earth as a return for fault currents and then it might not work in a hostile environment so then the only safety measure left is the fuse or trip which requires an even better earth. If there is no earth the boat can remain live and anyone touching it will get a shock, the RCD should then trip out in time but again may not. If you are onboard this will be reversed and the earthed transformer case and all shore earth wiring, which could be exposed, and the shore will appear to be live.

 

I don't understand,

if there was damage to the cable and the live was exposed and I touched the live I would only get a shock if I was also in contact with a return path, but surely then the imbalance will trip the RCD?

[Guest TerryL]

I don't understand,

if there was damage to the cable and the live was exposed and I touched the live I would only get a shock if I was also in contact with a return path, but surely then the imbalance will trip the RCD?

 

In that situation which is different to the one I described, you would get a shock and trip the RCD according to the earth loop impedence through the water to the nearest earthing point on shore which could be some distance away. This would be in effect a TT supply. As the resistance through the water can vary considerably there is no way of telling what will happen. If the resistance is low then you will get a shock and trip the RCD, if it works! In the other extreme if the resistance is high and the current through you is lower than the tripping current of the RCD you may not get a shock at all or will get one of less but still lethal current, a more dangerous situation. Also you will be connecting yourself to the hull and if anyone else touches the hull and shore earth in some way as previously described at the same time as you, then both will get a shock and may trip the RCD more readily.

 

To avoid all these doubts and varying conditions the regulations require you to simply connect the earth to the hull and provide a good positive return path for all fault currents. This as safe as you will get anywhere for a direct live contact, you will still get a potentionally lethal shock and the RCD is more likely to work but live/earth faults will be less likely to be a hazard according to the length of earth wire regardless of the RCD operation.

[carlt]

the regulations require you to simply connect the earth to the hull and provide a good positive return path for all fault currents.

 

Which regulations are those, please?

[chris w]

Here the illogicality of what TeryL is arguing:

 

1. What TerryL is arguing is that one should not install an IT on board the boat because one can only earth the IT's output to the hull and not its input otherwise the IT is giving you no galvanic protection at all. Earthing the OUTPUT of the IT will provide galvanic protection and will allow the RCD to trip. Earthing the input will totally negate the effect of the IT and provide no galvanic protection at all.

 

2. His reason for being concerned about leaving the input unearthed to the hull (although the input will still be connected to shore earth) is the slight possibility that the input cable could get chafed through and potentially the live cable could touch the hull.

 

3. HOWEVER, whilst I accept that what he says COULD happen, if we felt it to be anything more than an extreme possibility, we would have no shore mains cable at all on the boat with or without an IT. This is because even with a normal shore mains lead with the shore earth bonded to the hull (ie: no IT) the shore mains cable could still be chafed through, as in the example above, severing the shore earth bond and allowing the live to touch the hull.

 

4. Thus, logically, there is no increased danger from having the IT. The danger of a chafed cable is equally present in both cases with or without an IT. Ergo, this is where I dismiss his overall argument because it is not logical.

 

5. Personally, I use a galvanic isolator (GI), for cost and space reasons, not an IT . However, despite their being sanctioned for boats they are inherently more dangerous if they fail, because an open-circuit failure would leave the hull with no earthing at all so the on-board RCD will not trip. Yet they are sanctioned and the vast majority of boaters who have galvanic protection use them in preference to an IT (for the same reasons I do, no doubt). My GI has a 25,000uF bi-polar capacitor installed in parallel with it to allow any earth leakage current (even at 50Hz) from switched-mode chargers etc to be bypassed around the GI thus precluding its being switched on by these stray currents and negating the effect of the GI.

 

6. All the manufacturers of IT's like Victron and Mastervolt explain in some detail in their manuals that the IT's INPUT must NOT be earthed to the hull for the reason explained above.

 

Chris

[Guest TerryL]

Which regulations are those, please?

These would be the 17th Edition IET regulations for special locations, where not specifically applied to a situation the general requirements apply i.e. they should be followed as the guidance for good practice or as a means of complying with a duty of care depending on who the installer is i.e. professional or amateur.

[carlt]

These would be the 17th Edition IET regulations for special locations, where not specifically applied to a situation the general requirements apply i.e. they should be followed as the guidance for good practice or as a means of complying with a duty of care depending on who the installer is i.e. professional or amateur.

I assume you mean the 17th edition IEE regulations which don't actually apply to boats, so there is no "requirement" to follow them and, in many instances, because of the peculiar circumstances, of boat electrics, it would be inadvisable to do so.

 

Edited to say: I am aware that they apply to Marinas, btw.

Edited October 8, 2008 by carlt

[bottle]

Here the illogicality of what TeryL is arguing:

 

1. What TerryL is arguing is that one should not install an IT on board the boat because one can only earth the IT's output to the hull and not its input otherwise the IT is giving you no galvanic protection at all. Earthing the OUTPUT of the IT will provide galvanic protection and will allow the RCD to trip. Earthing the input will totally negate the effect of the IT and provide no galvanic protection at all.

 

2. His reason for being concerned about leaving the input unearthed to the hull (although the input will still be connected to shore earth) is the slight possibility that the input cable could get chafed through and potentially the live cable could touch the hull.

 

3. HOWEVER, whilst I accept that what he says COULD happen, if we felt it to be anything more than an extreme possibility, we would have no shore mains cable at all on the boat with or without an IT. This is because even with a normal shore mains lead with the shore earth bonded to the hull (ie: no IT) the shore mains cable could still be chafed through, as in the example above, severing the shore earth bond and allowing the live to touch the hull.

 

4. Thus, logically, there is no increased danger from having the IT. The danger of a chafed cable is equally present in both cases with or without an IT. Ergo, this is where I dismiss his overall argument because it is not logical.

 

5. Personally, I use a galvanic isolator (GI), for cost and space reasons, not an IT . However, despite their being sanctioned for boats they are inherently more dangerous if they fail, because an open-circuit failure would leave the hull with no earthing at all so the on-board RCD will not trip. Yet they are sanctioned and the vast majority of boaters who have galvanic protection use them in preference to an IT (for the same reasons I do, no doubt). My GI has a 25,000uF bi-polar capacitor installed in parallel with it to allow any earth leakage current (even at 50Hz) from switched-mode chargers etc to be bypassed around the GI thus precluding its being switched on by these stray currents and negating the effect of the GI.

 

6. All the manufacturers of IT's like Victron and Mastervolt explain in some detail in their manuals that the IT's INPUT must NOT be earthed to the hull for the reason explained above.

 

Chris

 

Bl**dy H*ll

 

I understood that and on the basis of all the other posts that I have partially understood, will be having a GI and if I can affford it an IT instead.

[chris w]

Bl**dy H*ll

 

I understood that and on the basis of all the other posts that I have partially understood, will be having a GI and if I can affford it an IT instead.

 

You are hereby promoted to bosun's mate

 

Chris