Galvanic Isolator?

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Matt341

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Hi All,

Decided to keep the boat plugged in from now on and in the winter will have a dehumidifier on to save bringing everything home.

I have been advised that a galvanic isolator is peace of mind.

The shorepower system on the boat is simple, inlet in the cockpit locker, lead to fused switched, then to RCD then feeds to charger, sockets etc.

There is nothing wired to the boats anodes etc.

I have been told that the isolator just wires into the earth wire between the inlet and the trip switches and therefore prevents any damage to engines etc.

I assume that I could put the isolator closer to the inlet socket and therefore turn the fused switch off but the isolator would stay on even though nothing would be drawing off the system when the switch was off?

I assume an isolator will also protect against other boats plugged if they dont have an isolator? Will it work even though nothing is connected to the anodes/rigging etc.?

I hope this doesn't sound all to confusing.

Many Thanks
Ian
 
Is your shorepower earth grounded to your 12V system or are the two entirely independent ?
 
It needs to go close to the inlet.
It is in the earht line as you stated which is not switched - so it is never on or off it is just there.
The earth of your 240v, after the isolator, needs to be bonded to your 12v negative and thus will be bonded to your anodes etc.
There is no great science to these isolators, all they do is prevent small currents flowing and allow large currents to flow as normal.
Small currents cause galvanic erosion and can be cuased by all sorts of issues not on your boat. Large currents only flow down an earth wire in a fault condition, causeing the fuse to blow and thus protectiong you and your boat.
 
Hi Both, Thank you for the replies.

Just to confirm the shorepower was fitted originally with the boat in 1979, last year I re wired as we didn't want to risk using it. I followed the original pattern which was inlet in cocpit, wire to fused switch and then to RCD. No earth is connected to the 12 volt system. The 240v is an independent system. Does this mean I wouldn't need an isolator or the isolator wouldnt work?

Many Thanks
Ian
 
The galvanic isolator should be the first thing in the earthing circuit after the shorepower incoming plug.

If the shorepower earth is not connected to any part of the boats systems, Dc negative, anodes etc etc then a GI serves no useful purpose.
If on the other hand there is a connection then it is likely to be necessary to prevent galvanic corrosion and in particular rapid wastage of anodes that can result if the shorepower is left connected even if not actually in use.

Whether or not the Shore-power earth should be connected to the DC negative anodes etc is a subject of great debate and argument compounded by the fact that ISO 13297 says it need not be provided the whole craft is protected by an RCD while other standards, ABYC for example, say that it must be.

I used to argue that the DC system etc should be earthed but now rather sitting on the fence.

The theory of the GI is not quite as Elessar suggests. It blocks current flow from very low voltage sources ,such as the galvanic ones that cause the corrosion associated with dissimilar metals , but allows current from higher voltage sources, including the AC mains, to flow thus maintaining the safety protection that an earth confers.
 
Earth bonding is absolutely essential and we use it as an exam question for our engineers. If AC finds its way into the DC system (or some 'floating' piece of metal - like a guard rail), the shore RCD won't trip - until you make a connection between said piece of metal and ground (ie you). Therefore getting a shock (or not) is determined by the trip time and correct functioning of the RCD.

We always recommend a galvanic isolator, RCD and correct earthing of the system.

This worries me as a supplier in the marine industry - it will take just one accident for AC electrics to become like gas or your AC systems at home - where it is mandatory for people to have the system worked on by a qualified engineer.

James
 
Thanks for the replies, this is a very confusing topic. My dad was an electrician and I picked up a lot along the way but this is a completely different kettle of fish.

So to check, is the current system with nothing connected to the anode dangerous???

I spoke to the tech guy at the marina and he advised that the EHU fitted to the marina has a very sensitive earth leak and trips out the hook up on any chance of a leak, is this a different thing to connecting upto the anode or will the marina power box do this?

If not connected to the anode does this mean there will be no corrosion to anything underwater?

Many Thanks
Ian
 
The DC negative should be connected the the anode. The AC earth should be connected to DC negative. A Galvanic Isolator goes in the earth wire - this should be as close as possible to the incoming AC connector. You'll have to carefully peel the cable insulation back and pop the AC wire out. Nothing should be connected to the AC earth upstream of the isolator.

This is one example - you can download the wiring diagram http://www.power-store.com/view-item.asp?itemid=1332&id=221&

Regards

james
 
I remain puzzled why ISO 13297 says that the DC negative need not be bonded to the shorepower earth provided an RCD protects the whole vessel while you say it is so essential.

I note your website says it is illegal for it not to be. Can you please tell me which bit of legislation makes it illegal for a privately owned small craft.
 
Thanks for the replies. Still quite confused though :-(

Have made a few calls this afternoon and one was to the local chandlery, the guy has done a few jobs on the boat and has a wealth of knowledge, he reckons there is no need to connect the AC earth to any part of the boat as long as the boat has an RCD with earth leakage detection, also the pontoons have earth leak detection.

Is this good advice or not?

Another question which is nagging me, if I dont connect any of the ac system to the anodes etc. will this mean I wont have any corrosion???

I had the anode/wiring all checked before we were put back in and they used meters etc. to check the anode and shaft and all was well.

As far as i am aware though the only thing connected to the anode is the shaft and engine components. I am sure there is nothing to do with the DC system connected?

I also have a plug type thing which I use to check all sockets when we connect up.

So by hooking upto power 24/7 and not having any connections to the Boat earth will this prevent any corrosion?

Many Thanks again
Ian
 
[ QUOTE ]
A small privately owned boat is classified as a domestic installation where all metal parts shall be bonded. The same rule that applies to your pipes in your home... strange but true

[/ QUOTE ]


I would like to see references to back up that statement, I have been doing extensive research into the earthing/not earthing issue and have come to no great conclusion.

The implications of having the regs relating to houses being carried over to boats are horrendous.
 
They already do apply - it is of course arguable that a boat is not a domestic environment. Strictly speaking you should have an NECIEC ticket - even though NECIEC doesn't cover marine installations. We had a range of battery chargers removed from sale due to them not being certificated for use in a domestic environment (even though it was a lifeboat). I don't have the specific regulation details to hand (too busy working to find them) but this is a real situation that happened to my business - a cost of in excess of £150'000.

There is NO arguement about to bond or not to bond. Bonding is the safest method of installation of an AC power system on board.

As I've said. (this is theoretical, don't do this at home!) - take a piece of metal on a rubber sheet - attach a live 230V wire to it. Does the RCD blow. NO! touch the metal plate and something grounded. Does it blow? HOPEFULLY!. Now connect the piece of metal to ground, reconnect the live wire - does the RCD go (if its working YES).

For those who think that bonding is not essential - try the above experiment, I bet you don't becuase you'll say "maybe the RCD isn't functioning correctly and I may kill myself"...

Sorry but I've been in the business for 25 years now - don't believe everything you read on websites or so "experts" books either. Ignore AC/DC bonding at your peril.
 
[ QUOTE ]
A small privately owned boat is classified as a domestic installation where all metal parts shall be bonded

[/ QUOTE ] Now that is something I have never seen stated on here during any of the past debates and arguments on the subject. I have never seen it sated anywhere else for that matter. It puts a different complexion on the whole issue.
What constitutes all metal parts? Guardrails, mast, rigging, cleats and fair-leads cooker? You run into trouble when it comes to observing advice that non ferrous skin fittings should not be bonded to the same anodes as other items!

Still curious though to know what legislation states that my boat shall be classed as a "domestic installation" though. Arguments in the past have always been along the lines that there are no laws compelling a private owner wire his own boat in this way.

Happy with the rules about bonding pipework in the home especially without the whole house installation being protected by an RCD.
 
[ QUOTE ]
Is this good advice or not?

[/ QUOTE ] Apparently not according to James Hortop!

I am still bewildered by the fact that you can, it seems, have an installation that complies with ISO 13297 but still be considered unsafe and, to boot, be illegal!
The problem is that you are relying on an RCD to trip if the need arises. Can you trust them or not? I'd not trust the pontoon RCD but my own I would test regularly.

[ QUOTE ]
if I dont connect any of the ac system to the anodes etc. will this mean I wont have any corrosion???

[/ QUOTE ] Provided that there is no connection between the anodes and other underwater bits and pieces and the shorepower earth, deliberate or accidental, you should not suffer corrosion as a result of the shorepower connection.
 
[ QUOTE ]
They already do apply - it is of course arguable that a boat is not a domestic environment. Strictly speaking you should have an NECIEC ticket - even though NECIEC doesn't cover marine installations.

[/ QUOTE ]

I presume the "law" you refer to is BS7671 - the IEE Wiring Regulations?

This was covered in the Winter 2008 IEE Wiring Matters magazine (pdf file ) (EDIT: SEE LINK BELOW)

Where they state that BS7671 is applicable to houseboats (with the definition of "a permanently moored vessel used as a dwelling which will not move from its berth other than for long-term relocation.") but not leisure craft.

They also clarify that boats are not within the scope of Part 'P' of the building regulations. I therefore can't see why NICEIC would be required (or relevant).

[ QUOTE ]
There is NO arguement about to bond or not to bond. Bonding is the safest method of installation of an AC power system on board.

[/ QUOTE ]

I don't necessarily agree with that statement (whole different discussion), and believe that ISO13297 would be the relevant standard for the boats that most of the people here are talking about; which (by popular opinion) doesn't require the connection of the DC circuit to the 'mains earth'. This specific point isn't clarified in the article (as far as I can see), but the protection characteristics for '240v' circuits required by ISO13297 are listed.

[ QUOTE ]
... take a piece of metal on a rubber sheet - attach a live 230V wire to it ... touch the metal plate and something grounded. ... try the above experiment, I bet you don't becuase you'll say "maybe the RCD isn't functioning correctly and I may kill myself"

[/ QUOTE ]

I agree it would be foolhardy to try it, but in truth, the tripping current and disconnection times of the relevant type of RCD are specified to avoid harmful effects in this type of situation.

[ QUOTE ]
Sorry but I've been in the business for 25 years now - don't believe everything you read on websites or so "experts" books either. Ignore AC/DC bonding at your peril.

[/ QUOTE ]

The statements you've made above differ from the 'popular' advice given here in the past, which is why several people have challenged these statements. I feel there's a lack of clear guidance on this subject that's available to the responsible boat owner/fixer, and hope that this thread can come to a clear statement of what the legal requirements in the UK are, and what the best practice is, but I (and others, I suspect) like to know where these requirements stem from.

To the OP: Very sorry for the thread drift /forums/images/graemlins/blush.gif

To anyone else reading this, I strongly recommend you read the article I linked to above - it would be very difficult to produce anything more succinct which didn't introduce ambiguities (IMHO).

With the greatest respect,

Andy

(Edited to correct ISO10297 to ISO13297 - a typo that I subsequently copied /forums/images/graemlins/blush.gif)
 
Thanks Vic - Yes I did mean 13297 - I'll edit the post. The link to the article worked OK for me in preview, but now it's stopped working for me, too. Don't know what's gone on there.

Andy
 
VicS - You hit the nail on the head - do you trust the RCD on the pontoon or not???

There seems to be a lot of typical internet "Know everything, understand nothing" on here. You talk about "popular advice" from non-qualified, non-expert sources.

If you don't agree, try the experiment I suggested, try it with the RCD's in your local marina - I expect in a large marina, at least 1 won't be functioning correctly. If you want to risk your life and those of the people around you based on the hope the RCD functions correctly thats up to you!

ISO are currently consulting with BMEA on standards for boats - the requirement to bond will be in the new regulations to counter the ambiguity that you are referring to.

James
 
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