Galvanic Isolator?

Norman_E

Norman_E

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My boat is to be marina based and connected to shorepower much of the time. I know that this can cause galvanic corrosion and that two boats in the marina where mine is currently based had to have propellers & shafts replaced, after they were moored in one particular spot. I have decided to fit a galvanic isolator, but there seem to be a fair number available. I have seen that Seashore Marine make a heavy duty looking unit rated for shore power connections from 3 to 63 amps, 110 to 240 volts. Has anyone any experience of this or any other recommendation?
 
Definitely worth fitting one. Last year we had all anodes down to about twenty or thirty percent left and damage to all four props. This year I'd estimate that there was only about twenty percent of the anodes used.

I made my own using a couple of bridge rectifiers mounted on a heat sink. Can't recall the exact cost but think it was in the region of twenty pounds. Also had the comfort of knowing the components were up to the job.
 
Hi

We bought one from Seashore Marine on Ebay. Cheaper than their shop prices, we paid about £40.00 inc p&p, and if you don't have an electric thinking brain /forums/images/graemlins/confused.gif its a lot easier . Easy to fit too. Can't comment on how the anodes are doing as we are not lifting the boat 'til June/July next year.

David
 
Thank you. I will probably go the commercial route, but would be interested to see the circuit diagram for your isolator, though I do not understand how bridge rectifiers can block small voltages from passing down the earth wire, whilst still allowing 220 volts to pass in the event of a mains short to earth. I thought that would need some fancy electronics to stop the small galvanic voltages passing in either direction, but if it can be done simply, then I am all for saving about £50.
 
The reason they stop the small voltages is that a silicon diode has a forward voltage of about 0.7 Volts. (Forward as in when its conducting). If you put the diodes back to back, they will conduct higher voltages in either direction, but under normal circumstances isolate everything that is less than 0.7v potential difference.

This is the 'isolation' that the devices refer to.

The essential element of building your own is to ensure that the diodes are sufficiently robust so that in the even of a short circuit, the fuse or circuit breaker blows before the diode does. Small silicon devices blow faster than fuses, due to the thermal runaway inherent with an overloaded silicon junction.
 
The use of bridge rectifiers was for the ease of construction of the isolator as they come nicely packaged with a hole for easy mounting on to a heat sink. In this application they are not used as a bridge, only two connections are used. I can't lay my hands on the diagram I used, it was downloaded from here, under their projects section though they seem to have removed the circuit diagram now and supply their own isolator. They do have a manual which gives details of how to install and test the isolator.

From memory the bridge rectifiers were connected in parallel, I think the DC outputs were connected together and the mains inputs removed. This gives you the effect of having two diodes in series which will block the low DC voltage (about 1.2 volts) you're after yet still pass any high voltages should a fault develop. The bridge rectifiers were sourced so as to be capable of handling the maximum current of the circuit breaker individually. I think they were rated at 40 amps each with a peak of something like 400 amps.

I stored a copy of the circuit diagram and tech specs of the bridge rectifiers within the electrical panel of the boat. If you're interested let me know and I'll scan them and email them to you. Probably won't be down to the boat until the New Year though.
 
DIY isolator instructions...

The assembly instructions are still online at the yandina.com website, although they seem to have removed the link - see here for the instructions.
 
Re: DIY isolator instructions...

Thanks to all of you who replied. I have actually put a bid on Ebay for a commercial one, But if the bid fails (and I am mean enough) I will see if I can get the right rectifiers and build my own.
 
Correct me if I'm wrong please but my uderstanding is that galvanic isolators are designed to allow ac current to pass whilst not allowing a dc component.

Surely if the ac component is over the 0.7 required for the diode to conduct then a dc componant would also pass through the diode? thus a dc current could be present and galvanic action continue if on a cyclic basis as the ac exceeds 0.7 volts on each cycle.

If you could explain in simple terms how your set up isolates dc components on an ac circuit I'd be interested.


I'm also surprised at the interest in saving money for something that protects your shafts, props, P brackets, legs.
I would tend to go for the best product on the market not the cheapest. They are a high current device and what I know of electronics is that heat contributes a large part to component failure, so I would have thought that bigger and probably more expensive is better?
 
There\'s no AC current normally...

The galvanic isolator is only installed in the earth wire. There's normally no AC current flowing in the earth wire, so galvanic corrosion is prevented. If there's an electrical fault, the galvanic isolator has to be meaty enough to conduct the full fault current without failing, so as to ensure that the marina power trips out properly.
 
Re: There\'s no AC current normally...

Not wishing to detract from anything here but:-

(1) IMHO all boats should be equipped with onboard ELCB (Earth Leakage Current Breaker) and that should be a 30mA type.
(2) IME these things trip really quickly when there's a fault, so quickly that I really didn't feel a thing when I was the earthed part...
(3) They work by measuring the current in the live and neutral wires. If there's a difference it's going somewhere it trips. If there's a difference then it's on the boat and so probably very little goes down the ship / shore earth wire.

If I'm correct then even the DIY types will be good enough. In any event the diodes , sorry, bridges are available from Maplin - about £1.50 each.
 
In it's simplest form, the problem is caused by a voltage in the marina earth plug, in respect of the true earth ground. As the marina piles are in the sea bed at true earth, and the boat is at marina plug earth that could be say 1 volt, you get a current path through skin fittings, via bonding to mains earth at the plug. The gavanic isolator puts a 1.2 volt drop inline, thus stoping the current flow, and the electrilytic corrosion.


Brian
 
Brian,
what would be the potential if the marina used PME? where the neutral and earth are the same cable in, and the earth is taken off of the neutral?

What I am trying to ask, is what would happen within the boat, vis-a-vis the galvanic isolator, if only a PME supply was used at a marina.

BTW, how are the switch panels coming along? Are you going to make any galvanic isolators?
 
Modern galvanitic isolators include a capacitor to allow AC volts to bypass the diodes (capacitors won't let DC pass). Hence if there is a small AC leakage to earth the boat will still be galvantically protected.

On the more expensive ones there’s also a monitor to show if and what currents are passing (in the US it’s the law that all must have a monitor). All this is well covered in Calder's book.

I’m sure an electronically minded person can make a safe and well built isolator and put volt and ammeters in the circuit. The advice to get a professionally designed and built one does however seem to make particular sense where mains electricity is concerned.

If nothing else it doesn’t give an insurance company a get-out clause if there should be a fire!
 
As there are two diodes the voltages would have to exceed around 1.2V to 1.4V before they conduct. Should you have an ac fault such that the diodes start conducting then I think you're probably correct and the isolator would not protect against electrolosiys. That said, I don't think this is the job of the job of the isolator, that fault should be taken care of by the earth leakage protection on the onboard mains circuit breaker.

As for cost if I though that a commercially available isolator provided better protection I'd have bought one. Mine is constructed from components that would handle currents in excess of what the shore power cable is designed to see. Judging purely on physical size I'd also hazzard a guess that I'm using a more efficient heatsink than the commercially available devices and could could maintain a high current a say 60A to 70A for longer. Anything over 20A would cause the breaker to trip in any case.

Cheers

Mark
 
PME something I'll have to think about. We did a lot of playing around late 80's with Sealine and the 450. Before the 450 very little bonding was done and very little problems, but on the 450 they bonded in the props, and they started having props disappear, always the one near the pile. This was when we found the variation between shore / earth spike and the marina plug voltage.
Just tossing a thought in the pot, a RCD is a two wire device, therefore if you connect ship earth to neutral shore side of the RCD, and do not connect bonding to mains earth. The RCD will work if you have a leakage to boat earth, if you touch a skin fitting and live the RCD will trip as you have a path that does not go through the neutral, and the skin fitting would be at a common voltage, that is also common to the sea. Thus you would not need a galvanic isolator. Don't try this it's just a passing thought.
I wonder if a galvanometor would me more use? it would tell if you had a proplem, rather than hid a possible one.
Should have the panels after Christmas, just on final tweeks.

Brian
 
If I understand Calder correctly, using an RCD and not connecting bringing the shore earth on board is legit in the UK (AFAIK a 30ma RCD trips whenever there is a 30ma or difference between the live and the neutral current flowing through it, i.e. when there there is a greater than 30ma leak anywhere in the installation). In theory therefore no earth is needed, however if the RCD isn't working correctly and there's a short somewhere it will kill you when you touch the hot piece of equipment as there's no other path back to earth. Frankly i'm not brave enough to trust just an RCD, especially one that's been sat in a damp marine environment for a while.

I'm the proud new owner of a Fulmar, but the survey (and therefore the underwriter) has insisted the shore power system is replaced with a new RCD protected system, plus galvanic protection before I can use it. This has lead to a dilemma.

A galvanic isolator is cheaper, but without a monitoring system (as stated above, a legal requirement in the US but not here in the UK) i'd have no way of knowing the diodes were working properly. I'm aware that if they blow in the 'open' position I have no galvanic protection (possibly expensive) or if they blow in the 'closed' position I have no earth (possibly lethal). Problem is, an isolater with monitor costs same ballpark figure as an isolation transformer, in which case I may as well get one of those.

Am I being overcautious here, and just how common is diode failure in a galvanic isolator anyway???

Andy
 
Its nearly 10 years since I retired from the Electricity Supply Industry, but as I remember we could not provide a PME connection to marinas or caravan sites. Perhaps the memory declines with age, or the supply regulations change.

j
 
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