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I have said it on here in another thread but an Isolating Transformer is far superior to Galvanic Isolation (with or without monitoring)

 

Yes, new here so sorry if i offend :rolleyes:

You don't offend, it's an old debate. My point would be "is a galvanic isolator adequate?". If not, why not? And "why not" based on proven fact, not anecdote.

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Not at all, I'm quite familiar with the issues (and of course that text is one man's opinion, not backed up with any hard data).

 

If you consider that a GI is not adequate for occasional use with shore power, or semi-permanent use with shore power when the boat is unattended, please say why. Then we can have a debate (if necessary).

Edited by nicknorman
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That's an old article.

 

My tests (more recently) showed that his statement "it won't happen in saltwater" is true but "in freshwater it is a possibility" is relatively misleading. A possibility it may be, but for a steel boat in a real canal or river the possibility is extremely close to zero. Even Gibbo had to concede this in the ancient debates.

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The top GI from safe shore is £155 so for the extra £95 would it not make sense to go the it way in case if I sell the boat the new owner decides to live in a marina with hook up.

 

Neil

 

Then let him worry about it and pay for the isolation transformer. wink.png

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I've tried to take a picture of my 7 year old anodes in the centre of the boat - the cut's quite clear today - but it's not happening. This boat is always on shore power, the anodes are in great condition. I've got a sterling 70amp GI. Is this conclusive proof of anything? No, except maybe that I don't need an IT.

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I've tried to take a picture of my 7 year old anodes in the centre of the boat - the cut's quite clear today - but it's not happening. This boat is always on shore power, the anodes are in great condition. I've got a sterling 70amp GI. Is this conclusive proof of anything? No, except maybe that I don't need an IT.

 

Add "up to now" and I would tend to agree with you. However because of the huge fault currents that may flow I would rather trust an IT than a GI. Even if the GI has inbuilt monitoring I am far from clear that the monitoring circuit will stand up to the voltdrop across the diodes that may result from faults.

 

I agree with Nicknorman but would add that if I needed to use a GI I would make sure I could and, more to the point, would frequently test it independently of any of its own LEDs or meters. At least the latest latest "plug in" models form Safeshore make that easer and safer even I I do not like the fact they do now seem to comply with the ABYC standard.

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Add "up to now" and I would tend to agree with you. However because of the huge fault currents that may flow I would rather trust an IT than a GI. Even if the GI has inbuilt monitoring I am far from clear that the monitoring circuit will stand up to the voltdrop across the diodes that may result from faults.

 

I agree with Nicknorman but would add that if I needed to use a GI I would make sure I could and, more to the point, would frequently test it independently of any of its own LEDs or meters. At least the latest latest "plug in" models form Safeshore make that easer and safer even I I do not like the fact they do now seem to comply with the ABYC standard.

 

I think it is worth contemplating what would happen if the GI failed from over-current.

 

To get such an overcurrent you need a short between Live and Earth whilst on shore power, with the short occurring inside the boat (ie not in the cable). The GI has nothing to do with anything when you are not on shore power of course. A short from live to neutral will of course just trip the MCB either on the bollard or on the boat.

 

So let's say there is such a short between live and earth which blows the diodes before an RCD or MCB can trip. Now boat's hull is live by virtue of the bond between earth and hull inside the boat. Depending on the nature of the water, it may well be that more than 30mA or so flows from the boat via the water back to earth, or via any contact points between the boat and earth, and this would trip the RCD either at the boat or at the bollard. In the unlikely event that the water is so pure that less than 30mA flows, you now have a live hull.

 

I suppose it depends on the water again, but with a live hull I can't help thinking there would be a bit of anode fizzing going on.

 

If you touch hull and earth you are likely to get a nasty shock but to get that you have to touch the hull, not the paint. Of course there is exposed metalwork such as the tiller extension but in general most of the boat you are likely to touch is painted and therefore insulated. When you do touch the tiller arm you have to make a circuit, so you have to be touching something connected to earth at the same time, ie something not on the boat such as the pontoon, and in a reasonably conductive way such as bare feet, with your other hand etc. When this happens you create a current imbalance so the RCD on the shore bollard should trip and save you. Even if it doesn't you have to make a fairly good circuit to be fatal, say one hand on the tiller extension, the other hand on a shore-side metal railing, with wet hands.

 

So to get yourself killed you have to:

 

Have a dead short between live and earth

Have a GI that can't stand that surge current and blows before any of the protective devices do (MCB on bollard, MCB on boat, RCD on bollard, RCD on boat)

Be in sufficiently pure water that less than 30mA flows via the water to earth

Not notice a bit of fizzing of the anodes

Not check your GI until:

 

One day you are holding the brass part of the tiller and grabbing a pontoon rail in drizzle with the shore power connected and with a faulty bollard RCD that doesn't blow.

So yes it could happen, but really, how probable is it? - and that is probably why we never hear of folk dying as a consequence of GI failure.

 

In the case of a failure short circuited, well you may get a little bit of slow corrosion until the next time you check the GI, but lots of folk seem to survive on shore power with no GI anyway.

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So could a IT fail the same way that a GI can.

 

Neil

Not in the same way, no. But ITs have their own issues. I'm not an expert on what those failure modes are, but for example with an IT all the power is passing through it, so when it is at max load it will get quite warm or even hot. Therefore I suppose overheating is a possibility. Whereas a GI normally has no power going through it.

 

They can hum annoyingly if the core is a bit loose. If they fail, chances are it will take out the whole supply rather than the protection (you may consider this a good or bad point, depending on when it happens). They are also of course a lot heavier, harder to install (and easy to install incorrectly) and more costly.

 

If certainty of avoiding even the slightest risk of death is your major priority then get an IT but be careful that you don't get run over when crossing the road to pick it up as that is a more likely cause of your untimely end.

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I think it is worth contemplating what would happen if the GI failed from over-current.

 

To get such an overcurrent you need a short between Live and Earth whilst on shore power, with the short occurring inside the boat (ie not in the cable). The GI has nothing to do with anything when you are not on shore power of course. A short from live to neutral will of course just trip the MCB either on the bollard or on the boat.

 

So let's say there is such a short between live and earth which blows the diodes before an RCD or MCB can trip. Now boat's hull is live by virtue of the bond between earth and hull inside the boat. Depending on the nature of the water, it may well be that more than 30mA or so flows from the boat via the water back to earth, or via any contact points between the boat and earth, and this would trip the RCD either at the boat or at the bollard. In the unlikely event that the water is so pure that less than 30mA flows, you now have a live hull.

 

I suppose it depends on the water again, but with a live hull I can't help thinking there would be a bit of anode fizzing going on.

 

If you touch hull and earth you are likely to get a nasty shock but to get that you have to touch the hull, not the paint. Of course there is exposed metalwork such as the tiller extension but in general most of the boat you are likely to touch is painted and therefore insulated. When you do touch the tiller arm you have to make a circuit, so you have to be touching something connected to earth at the same time, ie something not on the boat such as the pontoon, and in a reasonably conductive way such as bare feet, with your other hand etc. When this happens you create a current imbalance so the RCD on the shore bollard should trip and save you. Even if it doesn't you have to make a fairly good circuit to be fatal, say one hand on the tiller extension, the other hand on a shore-side metal railing, with wet hands.

 

So to get yourself killed you have to:

 

Have a dead short between live and earth

Have a GI that can't stand that surge current and blows before any of the protective devices do (MCB on bollard, MCB on boat, RCD on bollard, RCD on boat)

Be in sufficiently pure water that less than 30mA flows via the water to earth

Not notice a bit of fizzing of the anodes

Not check your GI until:

 

One day you are holding the brass part of the tiller and grabbing a pontoon rail in drizzle with the shore power connected and with a faulty bollard RCD that doesn't blow.

So yes it could happen, but really, how probable is it? - and that is probably why we never hear of folk dying as a consequence of GI failure.

 

In the case of a failure short circuited, well you may get a little bit of slow corrosion until the next time you check the GI, but lots of folk seem to survive on shore power with no GI anyway.

 

That reads like an argument, probably unintentional, to not bother bonding the ac earth to the hull at all. At least that's how I'm reading it.

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IT failure modes.

In 40 years of working with transformers from 1kVA single phase up to 150kVA three phase. I have only ever seen one type of failure and that is due to overload where the transformer will go open circuit and not pass any power. So a fail safe if a bit inconvenient.

As for Nicks scenario it doesn't have to be the same fault that takes out the GI that is the dangerous one.

You could easily have a fault that trips a breaker that could take out a GI and weeks later have another one that gives you a shock as you step on the boat grabbing the handrail that has a scratch on it.

There are many reports of people being killed in marinas in the USA whilst swimming due to electrocution and that's with fibreglass boats!

Finally in my travels I regularly see voltages up to and beyond 5v on the earths in marinas its 3.6v here which is why I use an IT

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IT failure modes.

In 40 years of working with transformers from 1kVA single phase up to 150kVA three phase. I have only ever seen one type of failure and that is due to overload where the transformer will go open circuit and not pass any power. So a fail safe if a bit inconvenient.

As for Nicks scenario it doesn't have to be the same fault that takes out the GI that is the dangerous one.

You could easily have a fault that trips a breaker that could take out a GI and weeks later have another one that gives you a shock as you step on the boat grabbing the handrail that has a scratch on it.

There are many reports of people being killed in marinas in the USA whilst swimming due to electrocution and that's with fibreglass boats!

Finally in my travels I regularly see voltages up to and beyond 5v on the earths in marinas its 3.6v here which is why I use an IT

So being practical about it, you should always check the GI after a fault that trips an RCD or MCB. But in 4 years we have never had such an event so it is not too onerous.

 

If you are in the habit of swimming around your boat in the marina, then sure, get an IT but also make sure you are vaccinated against Weil's disease and the rest! The case is weakened when you have to resort to a scenario that doesn't normally occur on the inland waterways of the UK, to make your point!

 

As to the earth voltage, that's why I would always recommend a GI with some sort of gauge or LEDs.

 

That reads like an argument, probably unintentional, to not bother bonding the ac earth to the hull at all. At least that's how I'm reading it.

Not really! If you don't bond the hull to earth then you risk a faulty piece of equipment (which is now live, with the GI blown) electrocuting you when you touch it and something metallic inside the boat. It is definitely better to be in an equipotential zone! Edited by nicknorman
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If you are in the habit of swimming around your boat in the marina, then sure, get an IT but also make sure you are vaccinated against Weil's disease and the rest! The case is weakened when you have to resort to a scenario that doesn't normally occur on the inland waterways of the UK, to make your point!

 

I wasn't using it to "make my point" I was meerly using it as another example of the dangers of electricity.

You have to remember that inland canals are a very small part of UK boating and the world doesn't revolve round them.

Irs going to be intersting when the next version of BS7671 comes out as that will extend its coverage of marinas.

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Finally in my travels I regularly see voltages up to and beyond 5v on the earths in marinas its 3.6v here which is why I use an IT

 

Now that's an interesting, and worrying, observation. I presume you're doing some kind of work related checks to discover this and therefore you're giving us the benefits of a professional opinion, which is great to have. With that in mind....

 

Am I right in thinking it's down to someone else on shore supply with a fault causing this or are there other factors involved?

 

If it's a boat with a fault it ought to be fairly readily identifiable, right? I mean, we just need to disconnect boats until the fault disappears and the (probably unknowing) culprit is highlighted?

 

I take it you inform the marinas in question - do they take any action? If yours is 3.6v perhaps it's more of a challenge than I think!

 

When you see these values, which are way beyond the levels a GI can help with, I wonder whether you might share the knowledge and identify those mainas with uncorrected faults so that we might lobby the owners, unplug or avoid them?

 

Thanks

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At our marina, the gauge on our GI deflects just enough to be detectable which reassuringly tells me the GI is not short circuit.

 

I'm the same Nick, just about centre. At Crick I was deflected to almost the end of the green zone, so I too am reassured. A bit. I think.

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Now that's an interesting, and worrying, observation. I presume you're doing some kind of work related checks to discover this and therefore you're giving us the benefits of a professional opinion, which is great to have. With that in mind....

 

Am I right in thinking it's down to someone else on shore supply with a fault causing this or are there other factors involved?

 

If it's a boat with a fault it ought to be fairly readily identifiable, right? I mean, we just need to disconnect boats until the fault disappears and the (probably unknowing) culprit is highlighted?

 

I take it you inform the marinas in question - do they take any action? If yours is 3.6v perhaps it's more of a challenge than I think!

 

When you see these values, which are way beyond the levels a GI can help with, I wonder whether you might share the knowledge and identify those mainas with uncorrected faults so that we might lobby the owners, unplug or avoid them?

 

Thanks

Its not just marinas

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