Galvanic Isolator

[Keeping Up]

The diodes in any GI will totally block any galvanic currents. No question - job done. However, there can be other (stray) currents on the earth lead notably from any piece of equipment that uses "switched-mode" technology like modern chargers and inverters. An amount of low and high frequency ac current can flow down the earth lead and through the GI.

 

If the voltage of these stray currents exceeds 1.2v (or 2.4v for your Safeshore model) then the diodes in the GI will switch on and act like a piece of wire...ie: they will block nothing, not even the galvanic currents, rendering your GI totally useless (but not damaging it).

The missing factor from this is the fact that any stray ac current on the earth lead from your on-board equipment, will take a route to earth through the hull and the water, bypassing the GI and never coming even remotely close to producing enough voltage across it to drive the diodes into conduction (you and I proved this in our tests last year Chris).

 

If it weren't for that fact, a GI would never work because the stray currents are usually produced from a 120v high-impedance source. It is also the reason why I remain unconvinced about the need for a capacitor.

 

As far as anyone has been able to tell, an ac current from the hull to the water does not cause galvanic corrosion in the way that a dc current would do.

[Gibbo]

Here we go again. I'm making this one post then not posting again in this thread.

 

The missing factor from this is the fact that any stray ac current on the earth lead from your on-board equipment, will take a route to earth through the hull and the water, bypassing the GI and never coming even remotely close to producing enough voltage across it to drive the diodes into conduction (you and I proved this in our tests last year Chris).

 

No. You did not.

 

What you proved was that it didn't happen in THAT boat. That, by pure accepted logic, does NOT prove that it cannot happen. However, if it happens on just ONE boat that DOES prove that it CAN and does happen. Theory also shows that it can happen.

 

I have seen it, measured it and logged it happening on SEVERAL boats.

 

As far as anyone has been able to tell, an ac current from the hull to the water does not cause galvanic corrosion in the way that a dc current would do.

 

That is (or rather was) indeed the general concensus. However this idea seems to be losing favour..... http://www.proboat-digital.com/proboat/200...g=105%20target=

 

I haven't made up my mind on this point.

 

Gibbo

[blackrose]

What you proved was that it didn't happen in THAT boat. That, by pure accepted logic, does NOT prove that it cannot happen. However, if it happens on just ONE boat that DOES prove that it CAN and does happen. Theory also shows that it can happen.

 

I have seen it, measured it and logged it happening on SEVERAL boats.

 

Gibbo

 

Chris W showed that it didn't happen on my boat too. We had quite a lot of switch mode stuff on at the time including 2 x 30amp battery chargers, a 20 amp battery charger and some other household AC appliances.

 

I wonder what causes the phenomenon to happen on one boat but not others?

[Keeping Up]

We proved that on two typical boats the path to earth is 2 or 3 orders of magnitude within the conditions necessary to cause any significant voltage to appear on the GI, which also explained why your own tests appeared not to correlate with the equipment on board the boat you were testing. That's good enough for me on two counts, it will take some strong evisdence now to make me change it.

 

Similarly I'm not going to debate it at length on this thread. The facts are clear, the question of how to interpret them may never be resolved.

[chris w]

Allan.... we're getting old rapidly. It seems like a few weeks ago since we carried out those tests but it was November 2006!!!! 2 years ago. Where does the time go to?

 

Chris

[Robbo]

I have heard that some modern devices like washing machines and computers will negate the protection of a GI and you then need to consider an isolation transformer. If you are not a live-aboard, then the best strategy is to only plug the landline in when you need it, then transformers and isolators aside, the boat just does not have much opportunity to dissolve.

 

Most modern electronic devices don't use a earth! My laptop, radio or TV don't!

[Liam]

I've eventually managed to find the pictures I randomly took when I was down at the boat, hoping that they'd come in use in the future... Hopefully from these pics the people in the know can get a better understanding of what I have... an explanation would be excellent if some kind soul could spare me a few minutes!

 

Picture 1

The first pic is of the top of the inverter, showing the connection from that to the fused switch which you can just see the top of below the extinguisher. The cable coming from this then goes to the various power sockets around the boat.

 

Picture 2

This shows the Black and Red cables which runs from the other side of the inverter, going down towards the engine and the other bits and peices. Does anybody know that the square dial is fixed to the bulkhead to the left of the Adverc system? We have a round ammeter further down which you can just see the top of so I don't think it's another one? Anyway as you can see, the red cable goes to one "junction box" and the black cable goes to another which both then run further down between the engine fixed to the bulkhead.

 

Picture 3

Shows the junction boxes again, with the ammeter and two what look like heatsinks with "Battery Isolator" on them both.

 

Picture 4

Shows all of the cables dissapearing down the back of the engine. Below the alternators is the starter motor and batteries to where some of the cables run to... some other cables go through the bulkhead, and into the cupboard behind in the main cabin where the fuses are for the water pump, internal lights, headlight, horn etc.

[ditchcrawler]

I've eventually managed to find the pictures I randomly took when I was down at the boat, hoping that they'd come in use in the future... Hopefully from these pics the people in the know can get a better understanding of what I have... an explanation would be excellent if some kind soul could spare me a few minutes!

 

Picture 1

The first pic is of the top of the inverter, showing the connection from that to the fused switch which you can just see the top of below the extinguisher. The cable coming from this then goes to the various power sockets around the boat.

 

Picture 2

This shows the Black and Red cables which runs from the other side of the inverter, going down towards the engine and the other bits and peices. Does anybody know that the square dial is fixed to the bulkhead to the left of the Adverc system? We have a round ammeter further down which you can just see the top of so I don't think it's another one? Anyway as you can see, the red cable goes to one "junction box" and the black cable goes to another which both then run further down between the engine fixed to the bulkhead.

 

Picture 3

Shows the junction boxes again, with the ammeter and two what look like heatsinks with "Battery Isolator" on them both.

 

Picture 4

Shows all of the cables dissapearing down the back of the engine. Below the alternators is the starter motor and batteries to where some of the cables run to... some other cables go through the bulkhead, and into the cupboard behind in the main cabin where the fuses are for the water pump, internal lights, headlight, horn etc.

Well picture 1 shows an earth wire from the case of the inverter to the socket, so it looks as if the inverter case is bonded to something but if its all bonded to earth is another question.

[chris w]

I've eventually managed to find the pictures I randomly took when I was down at the boat, hoping that they'd come in use in the future... Hopefully from these pics the people in the know can get a better understanding of what I have... an explanation would be excellent if some kind soul could spare me a few minutes!

 

Picture 1

The first pic is of the top of the inverter, showing the connection from that to the fused switch which you can just see the top of below the extinguisher. The cable coming from this then goes to the various power sockets around the boat.

 

Picture 2

This shows the Black and Red cables which runs from the other side of the inverter, going down towards the engine and the other bits and peices. Does anybody know that the square dial is fixed to the bulkhead to the left of the Adverc system? We have a round ammeter further down which you can just see the top of so I don't think it's another one? Anyway as you can see, the red cable goes to one "junction box" and the black cable goes to another which both then run further down between the engine fixed to the bulkhead.

 

Picture 3

Shows the junction boxes again, with the ammeter and two what look like heatsinks with "Battery Isolator" on them both.

 

Picture 4

Shows all of the cables dissapearing down the back of the engine. Below the alternators is the starter motor and batteries to where some of the cables run to... some other cables go through the bulkhead, and into the cupboard behind in the main cabin where the fuses are for the water pump, internal lights, headlight, horn etc.

First Liam, can I suggest you put up photos in web size rather than full size. They would download instantly then and would lose no resolution on a PC screen.

 

OK......

Photo 1.... shows an earth going to the socket from the inverter case. However, unless one of the two live outputs is also connected to earth (in order to create a neutral), any RCD won't function. Also, we don't yet know whether the mains earth is actually connected to the hull. There is no evidence of any RCD nor of any circuit breakers in any photos.

 

Photo 2..... the square box is another ammeter. It has very tiny wires going to it so I believe it is monitoring an ammeter shunt. Likewise with the round ammeter. Because of the small wires it is probably monitoring another ammeter shunt. The ammeter shunts will look like thick bars of metal, about a 1/4" thick and a few inches long with thick battery cables going to either end. Follow the ammeter thin wires to find them.

 

Photo 3........ the "battery isolators" are diode splitters so that your one alternator can charge different banks of batteries. The heatsinks are to keep them cool because the diodes dissipate a lot of heat. There are 2 isolators which means you presumably have a bow thruster as well?

 

Photo 4.... does not add anything new, other than one can see the cable coming from the alternator output. This is seen as the input to one of the isolators in photo 3

 

Chris

Edited November 24, 2008 by chris w

[blackrose]

The astute amongst you may have thought, "well, why have a GI at all, why not just have a big capacitor?" The reason is that the capacitor in the GI needs to be an extremely large value (electronically speaking) in order to allow low frequency (50Hz) currents to pass but it only ever has to operate at 1.2v (the forward voltage drop of the two diodes). If the voltage exceeds that, the diodes switch on and act like a piece of wire carrying any dangerous currents to earth. If we only had a capacitor and no GI, the capacitor would need to be able to handle the full PEAK mains voltage of 339v plus some reserve (say 400V). That together with the large value needed, would mean that the capapcitor would be enormous and would need to be towed behind on a butty!!!!

 

Chris

 

In that case, even in the example of a GI with a capacitor, if the fault exceeds 1.2v (or in my case 2.4v), then I can't help thinking "aren't we back to square one" ? i.e. diodes go into circuit and the possibility of galvanic corrosion occurs.

 

As WJM said, for people who don't live aboard there's a simple solution - just unplug the shore power cable when you're not onboard. If on the other hand, you don't want to do that or you live aboard and have a non-capacitor type GI with some sort of status monitor (which lets you know if the diodes have gone into circuit indicating a fault), then surely there's no need to panic. As you said Chris, that's just like having no GI at all, and there are plenty of people in my marina who don't! Galvanic corrosion doesn't happen overnight, so doesn't it just mean you need to find the source of the fault as soon a possible and reset your GI?

 

Of course I could be talking out of my arse, but it seems to me that a status monitor is essential on a GI - whether it's a capacitor type or not.

Edited November 25, 2008 by blackrose

[chris w]

In that case, even in the example of a GI with a capacitor, if the fault exceeds 1.2v (or in my case 2.4v), then I can't help thinking "aren't we back to square one" ? i.e. diodes go into circuit and the possibility of galvanic corrosion occurs.

No Mike.

 

The stray currents are AC currents which will flow easily through the capacitor and will NOT affect the GI at all. Only DC currents will go through the GI (DC can't get through the capacitor) and the only DC currents are galvanic currents which the GI will block totally.

 

Chris

Edited November 24, 2008 by chris w

[steelaway]

I've eventually managed to find the pictures I randomly took when I was down at the boat, hoping that they'd come in use in the future... Hopefully from these pics the people in the know can get a better understanding of what I have... an explanation would be excellent if some kind soul could spare me a few minutes!

 

Picture 1

The first pic is of the top of the inverter, showing the connection from that to the fused switch which you can just see the top of below the extinguisher. The cable coming from this then goes to the various power sockets around the boat.

 

Picture 2

This shows the Black and Red cables which runs from the other side of the inverter, going down towards the engine and the other bits and peices. Does anybody know that the square dial is fixed to the bulkhead to the left of the Adverc system? We have a round ammeter further down which you can just see the top of so I don't think it's another one? Anyway as you can see, the red cable goes to one "junction box" and the black cable goes to another which both then run further down between the engine fixed to the bulkhead.

 

Picture 3

Shows the junction boxes again, with the ammeter and two what look like heatsinks with "Battery Isolator" on them both.

 

Picture 4

Shows all of the cables dissapearing down the back of the engine. Below the alternators is the starter motor and batteries to where some of the cables run to... some other cables go through the bulkhead, and into the cupboard behind in the main cabin where the fuses are for the water pump, internal lights, headlight, horn etc.

 

 

Have a look here - Standard 240 panel - meets all requirements --- http://www.gscontrols.com/standard-ac-panels-9-c.asp

 

 

Alex

[smileypete]

I have seen it, measured it and logged it happening on SEVERAL boats.

 

In the same marina?

 

cheers,

Pete.

[Liam]

Hi Chris,

 

I normally do resize pictures before posting them but in this case I thought full size was better in case any detail would be missed when making them smaller... anyway note taken.

 

Picture 1

When you say that unless one of the two live outputs is connected to earth an RCD won't function? Could you please explain in laymans terms?

 

Picture 2

Will the ammeter shunt be what the two black plastic things are covering? I have no idea whats underneath there as I have never looked or I could simply trace the thin wires as you suggest on Saturday when I'm at the boat as looking at pic 4 it looks to carry on with the other wires.

 

Picture 3

There's no bow thruster on the boat... but we do have 1 x 70Ah starter battery and 4 x 135Ah domestic batteries which are hooked up to a 70Ah and 90Ah alternators respectively. Would this explain it?

 

Thinking on, when we bought the boat, the bloke at the brokerage who was selling the boat for the owner said that he knew the boat well after doing alot of work on it over the years the owners had it and said that the owners father was pretty well up in electrics and pretty much re-installed the system... whether this was just a sales pitch or not I don't know but there are lots of letters from her father to various companies (Advert, Stirling etc) and 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.

 

Once again thanks for the info, much appreciated!

[chris w]

Hi Chris,

 

I normally do resize pictures before posting them but in this case I thought full size was better in case any detail would be missed when making them smaller... anyway note taken.

 

Picture 1

When you say that unless one of the two live outputs is connected to earth an RCD won't function? Could you please explain in laymans terms?

 

At the inverter output there is not a live and a neutral. There is just 240v between the output wires. Strange as it may sound, in the absence of any earthing and bonding, you could safely touch each output wire in turn (but NOT both at the same time) and you would not get a shock. DO NOT TRY THIS YOURSELF. To establish a neutral and a live, one of the two output wires must be connected to earth; that becomes the neutral. Then one wire is at zero and the other one at 240v. This time you would definitely get a shock if you touched the live wire. An RCD needs an earth AND a neutral in order to function and to be able to detect an earth fault (ie: current running to earth instead of through the normal circuit).

 

Even in your home mains electricity, the power station sends out the power as just live cables with no neutral. At the substation, one of the wires is bonded to earth and that becomes the neutral so that into your house comes a live and a neutral. (I have simplified the process slightly so, please, no experts jumping on me). Wiring up your inverter correctly means that we have to simulate exactly what the power company does. Afterall, the inverter is a power station effectively.

 

Picture 2

Will the ammeter shunt be what the two black plastic things are covering? I have no idea whats underneath there as I have never looked or I could simply trace the thin wires as you suggest on Saturday when I'm at the boat as looking at pic 4 it looks to carry on with the other wires.

They may be, if the tiny wires end there or they may just be junction boxes. I can't tell from the photos.

 

Picture 3

There's no bow thruster on the boat... but we do have 1 x 70Ah starter battery and 4 x 135Ah domestic batteries which are hooked up to a 70Ah and 90Ah alternators respectively. Would this explain it?

Ah... you have 2 alternators, that explains the 2 sets of isolation diodes. I thought you just had the one alternator. It would be interesting to see if they are what is known as "battery sensing" as otherwise your charging will not be very effective. pity you are so far away, your boat electrics would make a great "adoptive" project.

 

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

[blackrose]

No Mike.

 

The stray currents are AC currents which will flow easily through the capacitor and will NOT affect the GI at all. Only DC currents will go through the GI (DC can't get through the capacitor) and the only DC currents are galvanic currents which the GI will block totally.

 

Chris

 

In that case I haven't understood the difference between AC and DC leakages.

 

I thought it was because in a marina we all share a common earth that led to the possibility of galvanic corrosion - so in an example of boats which are not isolated it's just the fact that dissimilar metals are connected up and sitting in an electrolyte that creates a potential difference causing galvanic corrosion and nothing to do with leakages from the boat's AC or DC electrical systems down that earth path.

 

So your capacitor ensures that there is no direct connection to earth unless there is an AC leak, but if there is an AC leak across the cap (of which you are unaware), then would this mean your boat has a direct connection to earth - a galvanic condition?

[WJM]

Blackrose is on the right track here. Short bursts of connected-earth conditions will not do any significant harm. It is the long ongoing periods of earth connection that does the harm. If your washing machine forces the earth connection open for an hour or two a week that is not a big problem. Your switched-mode devices are not running all day when you are at work or all night, so a totally ordinary GI will protect you all through these times.

 

We are obsessing with minutia here, time to take an overview. Unless you have a boat made of magnesium or you run an onboard computer server, a standard GI is enough protection for anyone. It closes the door on galvanic currents for 95% of the time, that is all you need.

[chris w]

Your switched-mode devices are not running all day when you are at work or all night, so a totally ordinary GI will protect you all through these times.

Well my switched-mode chargers are on 24/7, unless I'm cruising which is rare at this time of year. My neighbours' chargers are also on 24/7 as many people are resident here (but not me).

[chris w]

In that case I haven't understood the difference between AC and DC leakages.

I'm afraid that's right Mike. I'll try to explain.

 

I thought it was because in a marina we all share a common earth that led to the possibility of galvanic corrosion - so in an example of boats which are not isolated it's just the fact that dissimilar metals are connected up and sitting in an electrolyte that creates a potential difference causing galvanic corrosion and nothing to do with leakages from the boat's AC or DC electrical systems down that earth path.

 

That bit is absolutely spot on, and it's ONLY the DC current about which we need to worry. Galvanic corrosion is a DC current flowing between your boat and neighboring boats because of the common earth lead and the fact that even 2 steel boats will have slightly different steel compositions and thus are dissimilar metals.

 

So your capacitor ensures that there is no direct connection to earth unless there is an AC leak, but if there is an AC leak across the cap (of which you are unaware), then would this mean your boat has a direct connection to earth - a galvanic condition?

AC leakage currents don't cause galvanic corrosion. Without a capacitor, an AC leakage current, from a charger etc, will flow through the GI directly. However, one of the properties of diodes is to convert AC into DC (a process known as "rectification"). So the diodes in the GI will rectify any AC leakage currents and in so doing will produce a DC voltage across themselves which will switch the diodes either partially ON or fully ON depending on the magnitude of the original AC current.

 

If they are fully ON then they will act like a piece of wire to any DC galvanic currents and the GI is totally ineffective. If the diodes are switched partially ON then they will only block tiny galvanic currents and not larger ones (the largest will still only be 0.8v).

 

Think of the GI as a wall, over the top of which any galvanic currents have to get to cause a problem. That wall is 1.2 volts high (2 diodes in series). The highest galvanic voltage (from the physics of dissimilar metals) is 0.8 volts high so these voltages cannot get past the diode "wall".

 

However, any stray AC currents are rectified by the diodes (that's just what diodes do) and cause a small voltage which reduces the height of the wall. So. let's say that some stray AC leakage causes a 0.7 volt reduction in the wall height to 0.5 volts (1.2 - 0.7). Now, any galvanic voltages under 0.5 volts can still not cross the wall, but any over 0.5 volts will pass straight over due to the "leg up" caused by the AC voltage.

 

If the AC voltage is larger and caused a 1.5 volt drop in wall height, the wall disappears and all galvanic voltages can get through so the GI is doing nothing.

 

The capacitor acts as a bypass to the AC curents ONLY and so prevents any AC going through the GI and causing a reduction in wall height, leaving the GI totally effective.

 

Chris

Edited November 25, 2008 by chris w

[WJM]

"Well my switched-mode chargers are on 24/7"

 

Why charge 24/7? or Change your charger?

[chris w]

"Well my switched-mode chargers are on 24/7"

 

Why charge 24/7? or Change your charger?

The charger is in float mode and acting as the 12v power pack for the boat. Therefore, my batteries are not being used to run the boat (although they're still of course connected to the charger). This then means that they are not being run down and charged up over and over again which would shorten their life. Although I don't live on the boat, I leave the 12v fridge running 24/7 all year (it does have provisions in it as we spend many weekends on the boat) so the charger is effectively running the fridge rather than the bateries running the fridge.

 

Chris

Edited November 25, 2008 by chris w

[WJM]

I have the same boat usage. Try powdered milk!

[Keeping Up]

Somebody once brought two mugs of coffee to the back of the boat while we were going through a tunnel. One was black coffee, one had been made with powdered milk. In the darkness we couldn't see which was which, so we both tasted both cups. Neither of us guessed correctly.

[blackrose]

Somebody once brought two mugs of coffee to the back of the boat while we were going through a tunnel. One was black coffee, one had been made with powdered milk. In the darkness we couldn't see which was which, so we both tasted both cups. Neither of us guessed correctly.

 

I had the same experience with powdered eggs during the war.

[Keeping Up]

I had the same experience with powdered eggs during the war.

What, someone brought you powdered eggs on the back of the boat during the war? Were you an Idle Woman?