Galvanic Isolator

[Doorman]

With reference to galvanic isolators. I have investigated their effectiveness from articles listed on the internet and there appears to be an opinion that owing to the influx of modern equipment on board, i.e. remote control TV's etc., creating radio frequency interferance, this modern phenomina can cause a bridging

effect upon the diodes, negating their effectiveness!

 

It is also suggested, that ultimately, an isolator transformer is the only truly effective method of preventing galvanic corrosion. Although significantly more expensive, this device is claimed to deter stray a/c current flow by not having a hard wired shore line supply as per normal, but that conducted through magnetic inductance via the transformer. The extra cost should pay dividends in the long term through hull preservation.

 

I have taken an interest in this subject since blacking our own 20 year old narrowboat and noticing slight pitting of the hull surface below the waterline. I have also monitored other moorer's boats that use the boathouse on our marina, some only 3 years old and yet badly pitted. All of which had galvanic isolators fitted from new. It can be argued, that the more recent craft may have been built using steel of a lesser quality than those of older years, accounting for the earlier demise of the hull, but alarmingly, they all had anodes with effective life remaining.

 

I would welcome experienced views on this subject, as my partner and myself have just invested in a new 60' narrowboat, which we inevitably wish to live aboard and the thought of our investment slowly corroding beneath us, is less than appealing.

 

Mike

Edited November 25, 2008 by Doorman

[blackrose]

With reference to galvanic isolators. I have investigated their effectiveness from articles listed on the internet and there appears to be an opinion that owing to the influx of modern equipment on board, i.e. remote control TV's etc., creating radio frequency interfereance, this modern phenomina can cause a bridging

effect upon the diodes, negating their effectiveness!

 

It is also suggested, that ultimately, an isolator transformer is the only truly effective method of preventing galvanic corrosion. Although significantly more expensive, this device is claimed to deter stray a/c current flow by not having a hard wired shore line supply as per normal, but that conducted through magnetic inductance via the transformer. The extra cost should pay dividends in the long term through hull preservation.

 

I have taken an interest in this subject since blacking our own 20 year old narrowboat and noticing slight pitting of the hull surface below the waterline. I have also monitored other moorer's boats that use the boathouse on our marina, some only 3 years old and yet badly pitted. All of which had galvanic isolators fitted from new. It can be argued, that the more recent craft may have been built using steel of a lesser quality than those of older years, accounting for the earlier demise of the hull, but alarmingly, they all have anodes with effective life remaining.

 

I would welcome experienced views on this subject, as my partner and myself have just invested in a new 60' narrowboat, which we inevitably wish to live aboard and the thought of our investment slowly corroding beneath us is less than appealing.

 

Mike

 

Hello and welcome to the forum.

 

There are a couple of references in this thread to switch mode appliances causing the diodes in a GI to go into conduction. One contributor has directly observed the effect while two others tried to replicate the phenomenon last year without success. One of these tests was conducted on my boat with an oscilloscope using quite a lot of switch mode appliances, yet no bridging of the diodes occured.

 

Some GIs come with monitors to show whether they are conducting or not.

 

I have heard that for reasons of safety isolation transformers should be kept on the bank rather than on the boat, but I may be wrong about this.

[Keeping Up]

There are a couple of references in this thread to switch mode appliances causing the diodes in a GI to go into conduction. One contributor has directly observed the effect while two others tried to replicate the phenomenon last year without success. One of these tests was conducted on my boat with an oscilloscope using quite a lot of switch mode appliances, yet no bridging of the diodes occured.

And the contributor who did notice the effect, reported that strangely it appeared to bear no relationship to the number or type of switch mode devices on board the boat itself. That in fact seems to support the demonstrable (and explicable) observation that it is the devices on other people's boats that cause the problem.

[smileypete]

And the contributor who did notice the effect, reported that strangely it appeared to bear no relationship to the number or type of switch mode devices on board the boat itself. That in fact seems to support the demonstrable (and explicable) observation that it is the devices on other people's boats that cause the problem.

 

Agreed, might of been someone elses boat in a marina putting stray AC into the water somehow, which is then picked up on the hulls of nearby boats.

 

One thing about an IT, if there are stray voltages in the water (and via your hull) you'd be none the wiser.

 

cheers,

Pete.

Edited November 25, 2008 by smileypete

[Keeping Up]

Actually it can be a bit more devious than that. If a neighbour's boat puts stray ac current down his earth lead (either by not having a GI, or by having bypassed it with a capacitor) then that current enters the marina earth line and seeks the easiest way to earth. Often that is not by way of the long supply cable back to the substation, but via your boat's shoreline and thence into the water, driving your GI into conduction on its way through. That is what you have to block (or bypass).

[Guest]

I'll stick with me genny

[chris w]

With reference to galvanic isolators. I have investigated their effectiveness from articles listed on the internet and there appears to be an opinion that owing to the influx of modern equipment on board, i.e. remote control TV's etc., creating radio frequency interferance, this modern phenomina can cause a bridging effect upon the diodes, negating their effectiveness!

 

Mike

 

Hi Mike and welcome.

 

If you read my posts somewhere above your post you will see I go into this issue in some detail and also the theory behind why bypassing the GI with a bipolar 25,000uF capacitor will restore the diodes' effectiveness.

 

Chris

[Doorman]

Hi Mike and welcome.

 

If you read my posts somewhere above your post you will see I go into this issue in some detail and also the theory behind why bypassing the GI with a bipolar 25,000uF capacitor will restore the diodes' effectiveness.

 

Chris

 

Thanks Chris,

 

The more information the better! There seems to be various opinions on this subject, not least on the internet advisory sites.

I have decided to follow the isolation transformer system, which has been dispatched to my boatbuilder to install. It's a Victron 3.6 kva unit which is actually mounted on the boat. Only time will tell as to its effectiveness.

 

What are your thoughts on this method of detering the demon corrosion issue?

 

Mike

[mayalld]

Hi Mike and welcome.

 

If you read my posts somewhere above your post you will see I go into this issue in some detail and also the theory behind why bypassing the GI with a bipolar 25,000uF capacitor will restore the diodes' effectiveness.

 

Chris

 

OK,

 

I buy the argument.

 

Now, where do I buy the capacitor

[smileypete]

Actually it can be a bit more devious than that. If a neighbour's boat puts stray ac current down his earth lead (either by not having a GI, or by having bypassed it with a capacitor) then that current enters the marina earth line and seeks the easiest way to earth. Often that is not by way of the long supply cable back to the substation, but via your boat's shoreline and thence into the water, driving your GI into conduction on its way through. That is what you have to block (or bypass).

 

I think that's very unlikely as >30mA stray current to earth will trip any upstream RCD. Also the bonding conductor back to the substation will be pretty substantial.

 

If i was really worried about stray currents, for the price of an IT I could buy a GI with status monitoring and a pocket oscilloscope, and have change to spare for quite a few beers

 

cheers,

Pete.

Edited November 26, 2008 by smileypete

[chris w]

Thanks Chris,

 

The more information the better! There seems to be various opinions on this subject, not least on the internet advisory sites.

I have decided to follow the isolation transformer system, which has been dispatched to my boatbuilder to install. It's a Victron 3.6 kva unit which is actually mounted on the boat. Only time will tell as to its effectiveness.

 

What are your thoughts on this method of detering the demon corrosion issue?

 

Mike

An IT will stop all galvanic action PROVIDING it is wired correctly. Double-check your boatbuilder knows how to wire it up correctly. Many get the earthing incorrect which prevents the IT from doing its job, galvanically speaking.

 

The frame of the IT must NOT be connected to the steel hull or else you will get NO galvanic protection. The frame goes back to shore earth. One of the two outputs from the IT must be bonded to the hull to produce a neutral, making the other one the live. The bonded output is also your on-board mains earth. Many boatyards don't understand this so, as I say, do double-check this. If this is not done, your on-board RCD will not work either.

 

There is a great article here and the following page (with diagrams) on the Smartgauge site click here

 

The MD of Smartgauge is a regular contributor to this forum ("Gibbo"). His site is a general mine of good info.

 

Chris

Edited November 26, 2008 by chris w

[Keeping Up]

I think that's very unlikely as >30mA stray current to earth will trip any upstream RCD. Also the bonding conductor back to the substation will be pretty substantial.

 

If i was really worried about stray currents, for the price of an IT I could buy a GI with status monitoring and a pocket oscilloscope, and have change to spare for quite a few beers

 

cheers,

Pete.

No I'm not talking about currents of this size. I'm talking about the much lesser high-frequency earth currents which are generated by switch mode power supplies and the like. They will typically be less than 1mA, but they have to get to earth somewhere and because of their high-frequency nature they may see the cable back to the substation as being a relatively high impedance (especially if there is a long length of marina wiring.

 

If you get the chance, and a pocket oscilloscope, have a look at the state of the earth lead (relative to a true ground) that you were thinking of connecting to your boat. It will very probably contain enough noise - both at high frequencies and at 50 Hz - to decrease significantly the headroom of the protection offered by your GI. I was surprised when I looked at mine.

[Gibbo]

And the contributor who did notice the effect, reported that strangely it appeared to bear no relationship to the number or type of switch mode devices on board the boat itself. That in fact seems to support the demonstrable (and explicable) observation that it is the devices on other people's boats that cause the problem.

 

To me it supports the demonstrable observation that the hugely variable resistance between the hull and the real earth is behind the problem.

 

You have to remember that not all boats are bare steel hulls. Some are bare, some are partially coated, some are completely coated. Some are wooden, some are GRP.

 

I have measured DC resistances from the hull to real earth between 2 Ohms and 2000 Ohms in fresh water.

 

Gibbo

 

In the same marina?

 

Some were, some were in another marina. Some were on the open cut using the pilings as the ground measurement reference.

 

Gibbo

[smileypete]

No I'm not talking about currents of this size. I'm talking about the much lesser high-frequency earth currents which are generated by switch mode power supplies and the like. They will typically be less than 1mA, but they have to get to earth somewhere and because of their high-frequency nature they may see the cable back to the substation as being a relatively high impedance (especially if there is a long length of marina wiring.

 

If you get the chance, and a pocket oscilloscope, have a look at the state of the earth lead (relative to a true ground) that you were thinking of connecting to your boat. It will very probably contain enough noise - both at high frequencies and at 50 Hz - to decrease significantly the headroom of the protection offered by your GI. I was surprised when I looked at mine.

Maybe, but a large capacitor as mentioned would allow it to bypass the GI without the GI conducting any DC.

 

 

I have measured DC resistances from the hull to real earth between 2 Ohms and 2000 Ohms in fresh water.

What's the highest you've seen for any narrowboat?

 

Have you tried measuring AC impedance say at 50Hz and compared to DC resistance?

 

cheers,

Pete.

Edited November 26, 2008 by smileypete

[Gibbo]

What's the highest you've seen for any narrowboat?

 

IIRC it was about 130 Ohms. But that's from memory. I can't find the notes.

 

Again IIRC, I did get one reading that was much higher on a bare base plated 70 foot narrowboat but that was with it drifting in the middle of a very deep marina miles from the sides.

 

Have you tried measuring AC impedance say at 50Hz and compared to DC resistance?

 

I tried that at various frequencies and initially found some big differences. Until I sorted the test rig out properly! I then found marginal differences between AC and DC. A few percent. To be honest I put it down to measurement issues and didn't persue it any further. I drew the conslusion it was the same for both. I think the results of this test were posted on this forum but I can't find them.

 

Gibbo

[Doorman]

An IT will stop all galvanic action PROVIDING it is wired correctly. Double-check your boatbuilder knows how to wire it up correctly. Many get the earthing incorrect which prevents the IT from doing its job, galvanically speaking.

 

The frame of the IT must NOT be connected to the steel hull or else you will get NO galvanic protection. The frame goes back to shore earth. One of the two outputs from the IT must be bonded to the hull to produce a neutral, making the other one the live. The bonded output is also your on-board mains earth. Many boatyards don't understand this so, as I say, do double-check this. If this is not done, your on-board RCD will not work either.

 

There is a great article here and the following page (with diagrams) on the Smartgauge site click here

 

The MD of Smartgauge is a regular contributor to this forum ("Gibbo"). His site is a general mine of good info.

 

Chris

 

Thanks again Chris,

 

read the article you suggested. Ironically, this same article was the deciding factor for me opting for the IT rather than the GI. Also, made a note of your comment relating to the proficiency of the boatbuilder. Fortunately, he is amongst some of the most respected builders in the UK, yet, didn't know anything about Isolation Transformers! Will send him a copy of the article from Smartgauge together with checking the wiring myself!

[Liam]

I also think I remember seeing at least one diagram of some electrical system or other. I will check the paperwork tomorrow to double check and will scan it in if this will help me.

 

That would be great if you could post it on here.

 

Chris

 

Well I have finally had a look through all the paperwork which came with the boat and unfortunately I can only find 12v diagrams, no 240v stuff at all. However, I have scanned the main one in anyway for you to check over in case this helps?

 

12V Schematic Electrical Diagram

 

One thing is that in one of the letters sent between the previous owner and Adverc, it states that when they bought the boat it had a single starter battery and a single leisure battery off the one alternator but then a few letters down the line it says that he's in the progress of changing it to 1 starter and 4 leisure batteries, each bank with it's own alternator. Can anybody tell whether the alternators are completely seperate or whether the starter alternator is helping out the leisure alternator ?

 

Thanks in advance.

[Keeping Up]

On that diagram both alternators are working together to charge both sets of batteries.

[Liam]

Hi Allan,

 

Thanks for that!

 

Something which I have been wondering from day one but as I have mentioned earlier, my electronics knowledge is non-existent and as it's working like a dream then there's no reason to change anything

[chris w]

Hi Allan,

 

Thanks for that!

 

Something which I have been wondering from day one but as I have mentioned earlier, my electronics knowledge is non-existent and as it's working like a dream then there's no reason to change anything

Just to be pedantic, if you have no electronics knowledge Liam, how are you able to judge that "it's working like a dream"?

 

Chris

[Liam]

Well I suppose that was a bit of an over statement... it's working exactly like I suppose it should.

 

We've had the boat for four years this April and even though we use it every weekend, we don't go out every weekend and we've been able to run a TV, fridge, CD player etc for weekends on ends some times without a recharge without any effects.