Galvanic Isolators

[sidney]

Another (big) vote for safeshore. Bought one for my last boat (steel) and a second with the new boat. Very pleased with results first time around and would put the longevity of the anodes down to the GI. Certainly I have no other explanation (and neither did the boat yard who lifted twice) why the anodes were lasting longer after a GI was fitted. Have just fitted same type to a glass fibre boat so too soon to report. On both occasions I have gone for the larger GIs - may be overkill, but for a few extra pounds took a view it was worth it and have been pleased with the results.

 

[VicS]

.......would put the longevity of the anodes down to the GI. Certainly I have no other explanation (and neither did the boat yard who lifted twice) why the anodes were lasting longer after a GI was fitted.

Thats the whole object of the GI. Thats why they are also known as "zinc savers"

 

[maby]

The current rating of a GI has nothing to do with the current rating of your shore power connection. It is the current at which the diodes will burn out - but that is the continuous current - in a mains wiring failure condition, the current through the GI could easily be thousands of amps for a few milliseconds until the circuit breaker trips out. Those are current levels that will destroy any diode in production if maintained for more than a small fraction of a second - the higher the current rating of the GI, the better the chances of it surviving long enough to trip the circuit breaker. Personally, I would assume that any GI which has been through a short circuit resulting in a circuit breaker trip is now scrap. It might still test OK, but the junctions in the diodes will have been under stress - will you be able to trust it to survive a second pulse long enough to trip that breaker? If not, you could find yourself with dangerous mains voltages on exposed contacts.

 

[VicS]

The current rating of a GI has nothing to do with the current rating of your shore power connection.

But the choice of GI current rating is based on the current rating of the shorepower connection , or at least on the installed MCB rating.

ABYC recommend GIs which have a fault current handling capability at least 33% above the maximum current available at the shore power outlet.

 

[maby]

But the choice of GI current rating is based on the current rating of the shorepower connection , or at least on the installed MCB rating.

ABYC recommend GIs which have a fault current handling capability at least 33% above the maximum current available at the shore power outlet.

But those are fairly meaningless guidelines - the critical factor is how quickly the circuit breakers can trip and whether or not the GI diodes can survive the thousands of amps passing through them for that long.

 

[VicS]

But those are fairly meaningless guidelines - the critical factor is how quickly the circuit breakers can trip and whether or not the GI diodes can survive the thousands of amps passing through them for that long.

So suppose I am thinking of installing a shorepower system which will include a galvanic isolator .

Looking at only 3 suppliers websites I have found 9 differently rated galvanic isolators ranging from 16 amps to 110 amps
I obviously dont want to buy one that is neither more highly rated than necessary, physically larger than necessary, nor more expensive than necessary.

There is no mention in the specs of thousands of amps and I have no knowledge how large these transient fault currents could be or their duration anyway.

You have rubbished the ABYC recommendations so how do I choose a suitable one?

 

[maby]

So suppose I am thinking of installing a shorepower system which will include a galvanic isolator .

Looking at only 3 suppliers websites I have found 9 differently rated galvanic isolators ranging from 16 amps to 110 amps
I obviously dont want to buy one that is neither more highly rated than necessary, physically larger than necessary, nor more expensive than necessary.

There is no mention in the specs of thousands of amps and I have no knowledge how large these transient fault currents could be or their duration anyway.

You have rubbished the ABYC recommendations so how do I choose a suitable one?

I don't think I've rubbished the ABYC recommendations - but they are an oversimplification. All galvanic isolators work perfectly well as "zinc savers" but they do so by compromising, to some extent, the protection that the earth wire gives you. You are effectively putting a fuse into your earth wire - diode junctions do not take kindly to excess forward currents. What size fuse are you happy putting in the wire that may be responsible for saving your life? 16A? 32A? Personally, I chose the highest I could find. Circuit breakers are electro-mechanical devices and take multiple milliseconds to respond - during that period the diodes in your GI are operating way outside their design criteria - the manufacturers of those components will wash their hands of them! You just overspecify to the limit of your budget and hope that the poor junctions hold up long enough.

 

[Barbican]

I don't think I've rubbished the ABYC recommendations - but they are an oversimplification. All galvanic isolators work perfectly well as "zinc savers" but they do so by compromising, to some extent, the protection that the earth wire gives you. You are effectively putting a fuse into your earth wire - diode junctions do not take kindly to excess forward currents. What size fuse are you happy putting in the wire that may be responsible for saving your life? 16A? 32A? Personally, I chose the highest I could find. Circuit breakers are electro-mechanical devices and take multiple milliseconds to respond - during that period the diodes in your GI are operating way outside their design criteria - the manufacturers of those components will wash their hands of them! You just overspecify to the limit of your budget and hope that the poor junctions hold up long enough.

So if I were to choose an Isolation Transformer as a means to achieve the "zinc saver" goal whilst providing flexibility for say US 110v shore power, how would this type of equipment fair in similar circumstances? PS. I am completely non techie......

 

[maby]

So if I were to choose an Isolation Transformer as a means to achieve the "zinc saver" goal whilst providing flexibility for say US 110v shore power, how would this type of equipment fair in similar circumstances? PS. I am completely non techie......

I haven't paid much attention to isolation transformers, so I'm not sure how boats with one fitted are wired up. The problem that a GI is trying to solve is not particularly the electricity supply to your boat, it's the interaction between your boat and neighbouring boats while they are linked through the common earth wire. That being the case, I don't see how the isolation transformer solves the problem unless you don't carry the earth connection through to the internal boat wiring. Perhaps someone with first-hand experience can explain how the transformer is used?

 

[VicS]

So if I were to choose an Isolation Transformer as a means to achieve the "zinc saver" goal whilst providing flexibility for say US 110v shore power, how would this type of equipment fair in similar circumstances? PS. I am completely non techie......

I haven't paid much attention to isolation transformers, so I'm not sure how boats with one fitted are wired up. The problem that a GI is trying to solve is not particularly the electricity supply to your boat, it's the interaction between your boat and neighbouring boats while they are linked through the common earth wire. That being the case, I don't see how the isolation transformer solves the problem unless you don't carry the earth connection through to the internal boat wiring. Perhaps someone with first-hand experience can explain how the transformer is used?

An isolation transformer isolates you totally from the from the shoreside power supply, including the earth as the boat wiring is not connected to it.

Since there is no connection to the shoreside earth the problems that a GI might be fitted to solve should not exist.

Electrical safety is re-established by creating a "neutral" by earthing one side of the secondary to the boats own internal earth system or hull in the case of a metal hulled boat. RCDs will then operate in the same way as they do normally as of course will overcurrent protection devices.

If the boat is wired to US standards maybe the isolation transformer secondary is centre tapped and earthed giving 55-0-55 volts on a 110volt supply. I dont know about that .. I would have to look into how an isolation transformer would be connected in the US.

Downside is the weight of the transformer. Also you have to remember that a mains voltage power supply in the boat even from an isolation transformer can be just as dangerous as it is on shore. It solves the galvanic isolation issue but IMO does not make a huge contribution to safety.

 

[tonygibbs]

I am slightly confused about G.I's, so hope I can explain this properly. When I am using shore power I use an EU with 3 sockets (output) with a built in RCB, the ones you see in caravan shops. I may connect a small oil heater, light and battery charger. The battery's are disconnected before I put them on charge. My thinking is that in doing so the boat is not a part of the electrical circuit, could be wrong of course.

 

[laika]

IIRC from his book, Nigel Calder is rather fond of isolation transformers

Downside is the weight of the transformer.

..and cost is in a different league entirely

 

[rogerthebodger]

" unless you don't carry the earth connection through to the internal boat wiring "

That is exactly how the isolation transformer prevents galvanic corrosion.

http://www.smartgauge.co.uk/galv_tran.html

A while ago VicS posted alternate ways of connecting GI into a boat circuit.

IMHO both ways will block the DC component of the galvanic corrosion current between the metal fittings of a boat and external structures.

In one of the methods the GI would not carry any short circuit current but still stop the galvanic corrosion current. the other method,which is the most common , would carry the short circuit current.

Having a steel hull boat I have a safeshore GI currently connected in the conventional way but plan to change it. Also if a RCD is fitted which trips at 20/30 mA this would reduce the instance current flow through the GI greatly as the RCD trip speed is designed to prevent death in the case of electric shocks due to faulty wiring.

 

[maby]

IIRC from his book, Nigel Calder is rather fond of isolation transformers

..and cost is in a different league entirely

the price is very dependent on the power rating. Something capable of carrying multiple KW will cost many hundreds of pounds if not thousands, but there is a 250W isolation transformer on Amazon at the moment for less than £100. If you are just going to leave the battery charger and a couple of small domestic appliances running on a small boat, then the transformer can be reasonably priced - if you want to run a fan heater or dehumidifier, it may not be a reasonable option.

 

[maby]

I am slightly confused about G.I's, so hope I can explain this properly. When I am using shore power I use an EU with 3 sockets (output) with a built in RCB, the ones you see in caravan shops. I may connect a small oil heater, light and battery charger. The battery's are disconnected before I put them on charge. My thinking is that in doing so the boat is not a part of the electrical circuit, could be wrong of course.

It all boils down to the question of whether or not the shorepower earth connection is linked to any of the metalwork that is in the water under the boat. We have had older boats where there was no such connection, but I think that the current EU design rules require one (don't quote me on that). Our saildrive appears to be isolated, but I believe that the hull anode is linked to the earth.

 

[VicS]

I am slightly confused about G.I's, so hope I can explain this properly. When I am using shore power I use an EU with 3 sockets (output) with a built in RCB, the ones you see in caravan shops. I may connect a small oil heater, light and battery charger. The battery's are disconnected before I put them on charge. My thinking is that in doing so the boat is not a part of the electrical circuit, could be wrong of course.

It will be "part of the electrical circuit" if the shorepower earth is bonded to the DC negative. This is a requirement of the current edition of the International standard ISO 13297. The previous edition allowed this not to be so, as I think I have already said earlier in the thread, provided the boat is protected by a "whole craft RCD".

( Different consideration apply if the DC system is fully isolated, like maby's, and to metal hulls)

If you are using one of the Campsite units then very probably you wont have the earth bonding in place. If you fit a permanent fixed installation it should all comply with ISO 13297

 

[laika]

if you want to run a fan heater or dehumidifier[...]

Living in the UK I think that's a given . Then obviously there's power tools...

 

[maby]

Living in the UK I think that's a given . Then obviously there's power tools...

There are a lot of people who haul the boat out sometime in October and don't put it back till April - they may get away with a low power transformer

 

[Barbican]

An isolation transformer isolates you totally from the from the shoreside power supply, including the earth as the boat wiring is not connected to it.

Since there is no connection to the shoreside earth the problems that a GI might be fitted to solve should not exist.

Electrical safety is re-established by creating a "neutral" by earthing one side of the secondary to the boats own internal earth system or hull in the case of a metal hulled boat. RCDs will then operate in the same way as they do normally as of course will overcurrent protection devices.

If the boat is wired to US standards maybe the isolation transformer secondary is centre tapped and earthed giving 55-0-55 volts on a 110volt supply. I dont know about that .. I would have to look into how an isolation transformer would be connected in the US.

Downside is the weight of the transformer. Also you have to remember that a mains voltage power supply in the boat even from an isolation transformer can be just as dangerous as it is on shore. It solves the galvanic isolation issue but IMO does not make a huge contribution to safety.

Thanks for the explanation - Needs more research before I take that decision I think.

 

[tonygibbs]

Thanks Maby & VicS