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Galvanic Isolators

William Martin

By William Martin
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Osprey Sprinkler Explorer

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I've no idea but a guesstimate would be several MegOhms. I built a level indicator which used normal tap water and that was about 300K resistance over a couple of inches. canal water would probably be much more conductive than tap water though.

 

Chris

 

 

Yes, I guessed it would be pretty high in fresh water, even dirty canal fresh water. But you've got a good few square meters of unpainted baseplate contact area.

 

Shame so much of the information out there relates to sea water.

 

Ashley

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Gibbo Veteran II

Gibbo

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Yes, I guessed it would be pretty high in fresh water, even dirty canal fresh water. But you've got a good few square meters of unpainted baseplate contact area.

 

Shame so much of the information out there relates to sea water.

 

Ashley

 

I think the resistance between a hull and another boat or quayside is actually *substantially* lower than this. Like a few orders of magnitude lower.

 

The problem is that it's such a tw*t to measure. :closedeyes:

 

Isn't it Pete :mellow:

 

Gibbo

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Keeping Up Veteran II

Keeping UpDonate to Canal World

Posted
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I think the resistance between a hull and another boat or quayside is actually *substantially* lower than this. Like a few orders of magnitude lower.

 

The problem is that it's such a tw*t to measure. :closedeyes:

 

Isn't it Pete :mellow:

 

Gibbo

I think the resistance must be quite low, for the reason I proposed before: there will inevitably be a small leakage to earth of 50Hz current from the mains filters on any equipment that is connected. If you were to disconnect the earth lead a high-impedance meter show 120v ac (fact). But a working GI (and let's suppose there are no RF currents or switched-mode devices around) looks just like an open-circuit, of almost infinite impedance at 50Hz. The only thing that stops us from getting a voltage across it, is that the impedance of the parallel path to earth via the water is low enoiugh to prevent this.

 

Allan

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PeterF Rising Star

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I think the resistance must be quite low, for the reason I proposed before: there will inevitably be a small leakage to earth of 50Hz current from the mains filters on any equipment that is connected. If you were to disconnect the earth lead a high-impedance meter show 120v ac (fact). But a working GI (and let's suppose there are no RF currents or switched-mode devices around) looks just like an open-circuit, of almost infinite impedance at 50Hz. The only thing that stops us from getting a voltage across it, is that the impedance of the parallel path to earth via the water is low enoiugh to prevent this.

 

Allan

 

Some figures I have for water conductivity are 100 µS/cm for tap water, 200 for river water and 500 for mineral water. Heaven knows what some of the canal water is because it has a long residence time to collect agricultural run off / grey water from boats etc. Taking 200µS/cm as a good value, this should be through of a 200µS.cm/cm², i.e. it depends upon both distance through which the current flows and area through which it flows. If we correct this to a length scale of metres then it becomes 200mS.m/m². If we assume the narrowboats have unpainted baseplates of 2m x 15m (area = 30m²) and the boats are 4m apart then the conductance is 0.75S which is 1.33 Ohms. This is simplified as the two baseplates are not parallel plates and there will need to be some geometry in there to account for this and if the canal is shallower than 4m then the point of maximum resistance will be dictated by that. However, as an order of magnitude calculation it illustrates that the resistance values should be quite low, mainly because of the large areas involved.

 

Peter

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chris w Veteran II

chris w

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Pete

 

I agree with the estimation and the average figure for the conductance of water, but I think you may have made a small arithmetic error as I make the conductance 0.15S (rather than 0.75S) which equates to about 7 ohms. Doesn't affect the order of magnitude result of course.

 

Chris

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PeterFDonate to Canal World

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Pete

 

I agree with the estimation and the average figure for the conductance of water, but I think you may have made a small arithmetic error as I make the conductance 0.15S (rather than 0.75S) which equates to about 7 ohms. Doesn't affect the order of magnitude result of course.

 

Chris

 

 

Chris,

 

ooooh - its back to school for the both of us I am afraid. I scribbled some sums on a piece of paper based on tapwater giving 0.75S (100µS/cm) and then wrote the note on river water.

 

The correct answer for river water is 0.2S/m (200mS/m or 200µS/cm) x 30m² / 4m = 1.5S which is 0.67Ohms.

 

For this reason I suggest safety regs are ammended to suggest you do not bath in Perrier water whilst carrying out DIY electrics!!!

 

Peter.

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Gibbo Veteran II

Gibbo

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Chris,

 

ooooh - its back to school for the both of us I am afraid. I scribbled some sums on a piece of paper based on tapwater giving 0.75S (100µS/cm) and then wrote the note on river water.

 

The correct answer for river water is 0.2S/m (200mS/m or 200µS/cm) x 30m² / 4m = 1.5S which is 0.67Ohms.

 

For this reason I suggest safety regs are ammended to suggest you do not bath in Perrier water whilst carrying out DIY electrics!!!

 

Peter.

 

0.2 S/m = 2mS/cm not 200uS/cm

 

Gibbo

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PeterF Rising Star

PeterFDonate to Canal World

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0.2 S/m = 2mS/cm not 200uS/cm

 

Gibbo

 

 

Ohhh B****r I see my error now. I am now eating a large portion of humble pie and aplogise to Chris, it is only me that needs to go back to school for getting cm conversions mixed up.

 

So 7 ohms is the answer.

 

Thus with low resistance currents can readily flow between boats even with small voltage differences.

 

Peter.

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chris w Veteran II

chris w

Posted
Posted (edited)

Yep Pete , you're still a factor of 10 out. You're in detention for putting a 1000cm in a metre. :closedeyes:

 

I still make it 6.7 ohms based on your other figures

 

Sorry Peter, our posts crossed - I wasn't trying to rub it in

Edited by chris w
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Gibbo Veteran II

Gibbo

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Posted (edited)

Yep Pete , you're still a factor of 10 out. You're in detention for putting a 1000cm in a metre. :closedeyes:

 

I still make it 6.7 ohms based on your other figures

 

Sorry Peter, our posts crossed - I wasn't trying to rub it in

 

Right now you've got that cleared up.........

 

If we assume that the sides of the boats are completely covered with paint and only the base plates are in contact with the water then we have a problem.

 

Assume that both base plates are level with each other. In a conductive fluid the current flows in directly straight lines between the potential differences. It doesn't run around in arcs or circles or go round corners. This means that the current will be travelling in a *very* narrow strip of water.

 

Now calculate the resistance between the two boats :mellow:

 

Gibbo

Edited by Gibbo
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chris w Veteran II

chris w

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One could simulate this, in order to get an idea of current flow, by using 2 pieces of single-sided copper plated PCB board of known area attached to a power source in the bath. Throw in some chemicals (eg: salt) to make the water conductive and then calculate the effective current path from board to board knowing the conductivity, the board area and the applied voltage.

 

Rotating the boards vertically first would enable one to calculate the conductivity of the water. Using this figure then rotate the boards horizontally and re-measure the current and from this calculate effective area. If the boards are small and the bath long, the actual size of the boards themselves compared to the water available shouldn't be a factor.

 

Guess what, I'm gonna try it :closedeyes:

 

"Mummy, Mummy I need some salt and a mains plug" :mellow:

 

Chris

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chris w Veteran II

chris w

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I just checked with Adverc by phone (as they didn't reply to my email) and their galvanic isolator will only withstand 7 amps before failing. They rely on there being a functioning RCD.

 

I mentioned the issue to the "technical" guy at Adverc about AC leakage biasing the GI diodes ON and he had never heard of it and only just understood it!

 

He referred me to Mastervolt who supply Adverc's chargers and Mastervolt's response was (understandably 'cos they sell 'em) use an isolation transformer.

 

Chris

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Gibbo Veteran II

Gibbo

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Latest phone update: Aquafax's GI (the one with the built-in voltmeter) will withstand 16A continuous. The meter is a moving coil version and so won't detect any AC on the earth lead.

 

Chris

 

It's in a pastic box about 3 inches long. No way on this planet will it withstand 16 amps continuous. I think they're assuming because it has >16amp diodes it can handle 16amps. Which simply isn't the case.

 

In reality, as long as there is an RCD or MCB with a lower rating it will never have to carry this current. But the regs for GIs say they do, just incase.

 

Gibbo

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chris w Veteran II

chris w

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Posted (edited)

So what - it's the heat sink to which the diodes are connected which give them their ultimate current carrying capacity within their rating. They claim categorically that it is 16Amps continuous and they are mailing me the full installation manual.

 

4e.jpg

Edited by chris w
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Gibbo Veteran II

Gibbo

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So what - it's the heat sink to which the diodes are connected which give them their ultimate current carrying capacity within their rating. They claim categorically that it is 16Amps continuous and they are mailing me the full installation manual.

 

4e.jpg

 

The diodes aren't attatched to that metal thing on the front. The actual *manufacturer* of that unit sell it without that metal plate on it.

 

Gibbo

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Gibbo Veteran II

Gibbo

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The photo above is from an installation on a boat I visited NOT from a product catalogue - although the catalogue does indeed show the heat sink too.

 

Here's a photo of the Sterling 30A beefy one.

 

4a.jpg

 

It's not a heatsink. It's nothing really.

 

Here's how the unit comes from the manufacturer....

 

http://www.leisure-electrics.co.uk/acatalo...d_cable_25.html

 

6th item down.

 

Gibbo

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