Galvanic Isolators

[blackrose]

I'd be interested to see it. Just out of curiosity. And of course I can let you know whether what they say is correct (which I can only assume it is in these days of litigation for everything).

 

But I doubt they'll give it to you! Thing is, unless the monitoring circuit has an external power supply, it's actually far from a trivial task to light an led from 0.8 volts. It can be done, obviously, but it's nowhere near as simple as it at first appears.

 

Gibbo

 

I'm getting a tad confused but it's supposed to take 2.4v to activate the diodes. Anyway, thanks I'll PM you if he decides to send it to me.

[Gibbo]

I'm getting a tad confused but it's supposed to take 2.4v to activate the diodes. Anyway, thanks I'll PM you if he decides to send it to me.

 

Indeed. It can get confusing. For everyone.

 

On the one hand there is the natural logic saying "we want the GI to conduct at the lowest voltage possible without causing galvanic corrosion". Hence *most* GI's start to conduct at 1.2 volts. But why? What is the *real* reason behind this? Is it just how they evolved? A lot of later ones are not starting to conduct until (as you state) 2.4 volts. It certainly makes monitoring a lot easier.

 

What is wrong with a GI that conducts at say 10 volts? or even 20 volts? Perhaps even higher?

 

Like almost everything to do with electrickery, what starts out out as a really simply problem to be cracked ends up incredibly compicated when *everything* is taken into consideration.

 

It's a minefield.

 

Gibbo

[blackrose]

Indeed. It can get confusing. For everyone.

 

On the one hand there is the natural logic saying "we want the GI to conduct at the lowest voltage possible without causing galvanic corrosion". Hence *most* GI's start to conduct at 1.2 volts. But why? What is the *real* reason behind this? Is it just how they evolved? A lot of later ones are not starting to conduct until (as you state) 2.4 volts. It certainly makes monitoring a lot easier.

 

What is wrong with a GI that conducts at say 10 volts? or even 20 volts? Perhaps even higher?

 

Like almost everything to do with electrickery, what starts out out as a really simply problem to be cracked ends up incredibly compicated when *everything* is taken into consideration.

 

It's a minefield.

 

Gibbo

 

I take it there's no simple test I can do to check AC conductivity?

[Yoda]

Indeed. It can get confusing. For everyone.

 

Like almost everything to do with electrickery, what starts out out as a really simply problem to be cracked ends up incredibly compicated when *everything* is taken into consideration.

 

It's a minefield.

 

Gibbo

 

 

I think that it is time that we put best practice first and stanardise electrical installations on steel hulled boats. There would be three or four basic types of installation dependant on voltages required. This will deal with the problems of galvanic isolation, sacrificial anodes, salt water, painting bottoms and isolation transformers.

[ChrisPy]

I think that it is time that we put best practice first and stanardise electrical installations on steel hulled boats. There would be three or four basic types of installation dependant on voltages required. This will deal with the problems of galvanic isolation, sacrificial anodes, salt water, painting bottoms and isolation transformers.

who is 'we' ?

 

......... or should I assume that you will shortly issue a standard design that somehow suits everybody's requirements and complies with all the appropriate BSC and more importantly RCD standards, and that my well thought out design should now be scrapped, along with all the equipment, cabling, etc ? And that you will take full responsibility for its safety and efficacy ?

 

..... life don't work like that I'm afraid

Edited November 8, 2006 by chris polley

[Gibbo]

I think that it is time that we put best practice first and stanardise electrical installations on steel hulled boats. There would be three or four basic types of installation dependant on voltages required. This will deal with the problems of galvanic isolation, sacrificial anodes, salt water, painting bottoms and isolation transformers.

 

I think that's probably not a bad idea. If it was thought out properly (i.e. with input from lots of industry insiders, qualified engineers, the BMEA, BMIF, ABYC etc etc etc) and I mean properly, not like the half baked ideas that came as a result of the RCD and CE then it could well work.

 

Gibbo

 

who is 'we' ?

 

......... or should I assume that you will shortly issue a standard design that somehow suits everybody's requirements and complies with all the appropriate BSC and more importantly RCD standards, and that my well thought out design should now be scrapped, along with all the equipment, cabling, etc ? And that you will take full responsibility for its safety and efficacy ?

 

..... life don't work like that I'm afraid

 

I think if it was done properly, and your installation is as good as your posts seem to indicate it is, then you have nothing to fear. It would probably fall perfectly within one of the "categories" of different installations.

 

Gibbo

[Yoda]

I think that's probably not a bad idea. If it was thought out properly (i.e. with input from lots of industry insiders, qualified engineers, the BMEA, BMIF, ABYC etc etc etc) and I mean properly, not like the half baked ideas that came as a result of the RCD and CE then it could well work.

 

Gibbo

 

This is the 'we' Chris, people who understand how to achive engineering standards that will help everyone.

[tomsk]

who is 'we' ?

 

......... or should I assume that you will shortly issue a standard design that somehow suits everybody's requirements and complies with all the appropriate BSC and more importantly RCD standards, and that my well thought out design should now be scrapped, along with all the equipment, cabling, etc ? And that you will take full responsibility for its safety and efficacy ?

 

..... life don't work like that I'm afraid

 

 

Now, now girls, little weird alien dudes contentious always are

[Gibbo]

I take it there's no simple test I can do to check AC conductivity?

 

Depends what equipment you have a available. An AC signal generator and 'scope would allow you to see what happens at various frequencies and amplitudes and see what the LEDs do.

 

If that isn't considered a "simple test" then no there isn't one

 

Gibbo

[smileypete]

Blatent plug time

Declares finacial interest

 

There will be an insulated one on the market soon for about £275 cant say more than that.

 

Is there a spec?

 

Out of interest, what is the power rating, efficiency, and power consumption when idle?

 

cheers,

Pete.

Edited November 8, 2006 by smileypete

[blackrose]

I'd be interested to see it. Just out of curiosity. And of course I can let you know whether what they say is correct (which I can only assume it is in these days of litigation for everything).

 

But I doubt they'll give it to you! Thing is, unless the monitoring circuit has an external power supply, it's actually far from a trivial task to light an led from 0.8 volts. It can be done, obviously, but it's nowhere near as simple as it at first appears.

 

Gibbo

 

Just had an email from the guy - no schematic is forthcoming but that's understandable, most manufacturer's won't divulge research & development.

 

Anyway, here's what he had to say:

 

Further to your email re galvanic isolators & the ability of switched mode appliances to "turn on " the isolator rendering it "useless". I have reconducted the test on Safeshore Gi70sm isolator (which is purposely designed to give a higher turn on threshold than a conventional isolator), by loading 5 switched mode appliances in close proximity to the galvanic protected earth line. With all appliances running the isolator continued to block all d.c. galvanic currents as previously stated. The accumulated ac voltage present across the isolator with all appliances running was 1.83 volts. We found that the switched mode appliances varied significantly with pulse output but even the combined pulse voltages of 5 appliances failed to turn on the GI70sm. Please note : many switched mode appliances do not have an earth wire connected & run on a two core cable from the mains. These appliances offer no threat to galvanic isolators as they do not have an earth connection & are double insulated for safety. I also confirm that with the 5 appliances running the status monitor leds remain extinguished. Safeshore isolators require a minimum of 2 volts before conduction of the isolator begins at which point the leds will begin to illuminate requiring a "fault" voltage of around 2.4 volts to fully illuminate the status monitor. With reference to your concerns re failure of diodes: Safeshore use 70 amp diodes rated at 800 volts to obtain maximum reliability. We have pulse tested these isolators @ 2kv & they are compliant to the 5 kva requirements making them perhaps the most robust units on the market today.

I do hope this information is of help to you & remain

Yours Faithfully,

Vic Wilkinson

Safeshore Marine UK

 

As Gibbo says, nothing is simple, and there seems to be no consensus on this subject even amongst marine electricians. Personally, from what I've heard I think a high spec galvanic isolator can do the job as well as an isolation transformer.

Edited November 8, 2006 by blackrose

[Yoda]

All that has brought up is something I was questioning earlier. With the use of double insulation therefore the use of to wire power connection there is no use for a galvanic isolater is the hull is not used for grounding.

[tomsk]

All that has brought up is something I was questioning earlier. With the use of double insulation therefore the use of to wire power connection there is no use for a galvanic isolater is the hull is not used for grounding.

 

 

Man alive you little alien freak, do you want to do that again in English?

[blackrose]

All that has brought up is something I was questioning earlier. With the use of double insulation therefore the use of to wire power connection there is no use for a galvanic isolater is the hull is not used for grounding.

 

I thought it was the fact that your hull is connected to earth and to every other boat via the shorline that allows galvanic action to occur, not the use of appliances. The argument is that the use of the switch mode appliances (with 3 core cable) stops your galvanic isolator from working properly, but the use of double insulated non-earthed appliances doesn't mean your hull isn't connected to earth, just that those particular appliances aren't.

Edited November 8, 2006 by blackrose

[Yoda]

I thought it was the fact that your hull is connected to earth and to every other boat via the shorline that allows galvanic action to occur, not the use of appliances. The argument is that the use of the switch mode appliances (with 3 core cable) stops your galvanic isolator from working properly, but the use of double insulated non-earthed appliances doesn't mean your hull isn't connected to earth, just that those particular appliances aren't.

 

I see it as if you have double insulation and therefore two wire electrics you don't have an earth.

[blackrose]

I see it as if you have double insulation and therefore two wire electrics you don't have an earth.

 

You'd still have an earth on the rest of your boat's 240v system. Nobody's going to wire up a whole boat in twin core.

 

As I said, it's the fact that your hull is connected to earth and other boats via the shore power cable that allows galvanic corrosion.

 

http://www.waterwaysinteractive.com/docs/f...;PagePosition=1

Edited November 8, 2006 by blackrose

[GUMPY]

Is there a spec?

 

Out of interest, what is the power rating, efficiency, and power consumption when idle?

 

I am still awaiting a sample and figures from the manufacturer.......

All I can say at the moment is 3.6kVa (16amps cont)

 

When I have more details they will be forthcoming.

 

Julian

[jollyroger]

I see it as if you have double insulation and therefore two wire electrics you don't have an earth.

Why an earth? If the metal hull was not earthed to the shore when connected to o shore line the following could easily occur.

1, Live cable comes into contact with hull. Hull is now at 240v to earth.

2) No earth wire so breaker does not trip.

3) You are moored next to a boat that does have an earth to shore.

4) Your boat is at 240v, the other at earth potential, 0v.

5) You stand on your boat and touch the other one, ZAP, 240v from your hull through you to the other boat.

Thats why you have an earth

[Audi Fan]

If you have an insulation transformer, do you use the boats hull as earth?

 

or is it similar to using an inverter which combines earth with the neutral?

[ChrisPy]

If you have an insulation transformer, do you use the boats hull as earth?

 

or is it similar to using an inverter which combines earth with the neutral?

why so much confusion?

 

ignore insulating (actually isolating) transformers and galvanic isolators for the moment.

 

ALL AC EARTHS MUST BE BONDED TO A COMMON POINT ON THE HULL SHELL, ADJACENT TO THE POINT WHERE ALL THE DC NEGATIVES ARE ALSO BONDED TO THE HULL.

 

having done that, the choice of isolating method is for you to decide.

[blackrose]

why so much confusion?

 

ignore insulating (actually isolating) transformers and galvanic isolators for the moment.

 

ALL AC EARTHS MUST BE BONDED TO A COMMON POINT ON THE HULL SHELL, ADJACENT TO THE POINT WHERE ALL THE DC NEGATIVES ARE ALSO BONDED TO THE HULL.

 

having done that, the choice of isolating method is for you to decide.

 

A voice of reason & logic.

[chris w]

Hi Guys

 

My first post on this forum. As I have an electronics background, I thought I would throw in my thoughts in case it helps. (If I can help answer any other electronic/electrical queries, please ask me)

 

In my humble view:

 

1. A GI is definitely a good thing to have installed. It consists internally of 2 diodes in series connected in parallel with 2 more series diodes wired the opposite way round. So DC is blocked either way, but AC can still flow either way. A diode needs about 0.6v across it in order to conduct so, for 2 diodes in series to be able to conduct, they would have to have about 1.2v across them. An AC mains fault (240v AC) reaches over 330v at its peak so the diodes effectively don't exist for AC faults.

 

2. You should definitely have a GI if you connect to shore power because you, the metal jetty posts, and all the other boats around you are connected together through the shore earth. This means that (depending on the exact steel constituents) your boat is at real risk of corroding fast.

 

3. A GI will stop galvanic currents providing they are below about 1.2 volts (most are) and effectively isolate you from interaction with other boats.

 

4. They will not affect the protection level of the earth bonding in terms of normal AC faults. GI's can handle 30A or more of current if a true earth fault develops. Your circuit breakers will blow before this happens. (or should do if they are of the correct current rating for protection).

 

5. AC leakage can override a GI, if of sufficient magnitude, because it forward biases the blocking diodes into conduction and thereby switches then ON. In effect, this means they would then offer little or no protection. Switched mode battery chargers et al divert some high frequency AC to earth to enable a smoother dc at their output. However, the amount is very tiny indeed and should not be sufficient to affect the GI. (It would need to be greater than 1.2v)

 

6. To prevent any risk of this happening however, there are variants of GI's that contain a capacitor in parallel with the GI's internal diodes. Capacitors pass AC but block DC so any stray AC currents will not override the DC blocking capability of the GI because they are diverted around it. (The American Yacht & Boat Council [ABYC] insist on their being a capacitor in parallel for this reason in order to pass that part of their safety spec). GI's so constructed tend to be much more expensive unfortunately. The capacitors are large because they need to be able to pass low frequency mains AC too.

 

7. As someone above correctly pointed out, a simple dc meter across the GI will not give any indication of AC leakage through the GI but it will give you an indication of the DC activity you are blocking and so make you feel it's money well spent. (Typical cost is about £70-£80)

 

Hope this helps

 

Chris

[jollyroger]

Hi Guys

 

My first post on this forum. As I have an electronics background, I thought I would throw in my thoughts in case it helps. (If I can help answer any other electronic/electrical queries, please ask me)

 

In my humble view:

 

1. A GI is definitely a good thing to have installed. It consists internally of 2 diodes in series connected in parallel with 2 more series diodes wired the opposite way round. So DC is blocked either way, but AC can still flow either way. A diode needs about 0.6v across it in order to conduct so, for 2 diodes in series to be able to conduct, they would have to have about 1.2v across them. An AC mains fault (240v AC) reaches over 330v at its peak so the diodes effectively don't exist for AC faults.

 

2. You should definitely have a GI if you connect to shore power because you, the metal jetty posts, and all the other boats around you are connected together through the shore earth. This means that (depending on the exact steel constituents) your boat is at real risk of corroding fast.

 

3. A GI will stop galvanic currents providing they are below about 1.2 volts (most are) and effectively isolate you from interaction with other boats.

 

4. They will not affect the protection level of the earth bonding in terms of normal AC faults. GI's can handle 30A or more of current if a true earth fault develops. Your circuit breakers will blow before this happens. (or should do if they are of the correct current rating for protection).

 

5. AC leakage can override a GI, if of sufficient magnitude, because it forward biases the blocking diodes into conduction and thereby switches then ON. In effect, this means they would then offer little or no protection. Switched mode battery chargers et al divert some high frequency AC to earth to enable a smoother dc at their output. However, the amount is very tiny indeed and should not be sufficient to affect the GI. (It would need to be greater than 1.2v)

 

6. To prevent any risk of this happening however, there are variants of GI's that contain a capacitor in parallel with the GI's internal diodes. Capacitors pass AC but block DC so any stray AC currents will not override the DC blocking capability of the GI because they are diverted around it. (The American Yacht & Boat Council [ABYC] insist on their being a capacitor in parallel for this reason in order to pass that part of their safety spec). GI's so constructed tend to be much more expensive unfortunately. The capacitors are large because they need to be able to pass low frequency mains AC too.

 

7. As someone above correctly pointed out, a simple dc meter across the GI will not give any indication of AC leakage through the GI but it will give you an indication of the DC activity you are blocking and so make you feel it's money well spent. (Typical cost is about £70-£80)

 

Hope this helps

 

Chris

 

I fully agree with you.

Regarding the cost I made my own, total cost, less than £15.

[William Martin]

Thanks Chris W - that would appear to be the most clear and comprehensive answer yet!

 

 

Just in case you though it was over (!) - with an Isolation Transformer, surely the hull still needs to be connected to the earth, even if the incoming supply is totally separate from the boat's circuit? - and if it does, then where is the protection from galvanic action?

[GUMPY]

Thanks Chris W - that would appear to be the most clear and comprehensive answer yet!

Just in case you though it was over (!) - with an Isolation Transformer, surely the hull still needs to be connected to the earth, even if the incoming supply is totally separate from the boat's circuit? - and if it does, then where is the protection from galvanic action?

 

Afraid that Chris W whilst correct in theory is wrong in practice.

As has been said if you remove the earth wire from the plug of a switch mode unit like a PC and measure between that wire and earth you will get 120volts, more than enough to cause any GI to conduct.

I do not recomend doing this unless you are suitably qualified.

 

As for the earth on an isolating transformer system it is connected to the "neutral " on the secondary of the transformer and not to the supply earth

 

Have a look here

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

 

If GIs were still such a good idea why would I remove one from my boat and fit an iso transformer.

Oh if anyone wants a GI its for sale cheap but there is no comeback if your boat disolves.

 

J