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Plug in Galvanic Isolator.. which one


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1 minute ago, blackrose said:

As you say, AYBC are guidelines not regs. I'm not quite sure why we're using American recommendations in this country anyway? Why don't we have our own?

 

 

Probably because the market is not big enough to support the testing and certification which was the reason why some paint is no longer suitable for potable water tanks.

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On 16/01/2019 at 17:59, Flyboy said:

So none of these 70 to 100amp GI 's are any good then, please explain why?     https://safeshoremarine.com/products/

 

On 16/01/2019 at 18:14, WotEver said:

The explanation has already been given and yes you are correct, none of those are any good. 

I would disagree with this, as I think the explanations that were given are based on false assumptions about the diodes in the GI.

I don't disagree in any with the comments about prospective fault currents. However I did discuss thus with Safesure and other suppliers some years ago, the point was that in for example a so-called 70 amp isolator the diodes are rated at 70 amps; this is in fact their manufacturer's CONTINUOUS rating when connected to a perfect heatsink. Of course such a heatsink doesn't exist, but it means that at their rated 50 amp current the diodes will be perfectly happy until they warm up to an excessive temperature. On a typical GI unit the heat sink will be good enough to ensure that this will take several seconds (up to a minute on the better ones) which is plenty more than enough to ensure that the breaker will have tripped. The peak current rating if the diodes  is typically up to 50-100 times greater, but then begins the debate on how long is a Peak? I was told, but I have seen no evidence, that the peak rating/duration relationship has been compared against the operating current / trip time characteristics of a breaker to ensure that there is always a good margin so that the breaker will always trip before the diodes can be damaged. This may or may not be true, as I say I have seen no evidence, but I do feel that it is an over-simplification to say that all such GIs are unfit for purpose.

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1 minute ago, blackrose said:

I've just spoken to them. 70amp is continuous and 500A peak. I don't have a link sorry. I never said 5000A?

Ok. I wonder why they don't publish that 500A figure in their sales information. 

 

I know you didn't claim 5000A for the Safeshore, but that is the standard we have been discussing on the thread.  The calculations this is based on were posted early in the thread. 

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4 minutes ago, Keeping Up said:

 

I would disagree with this, as I think the explanations that were given are based on false assumptions about the diodes in the GI.

I don't disagree in any with the comments about prospective fault currents. However I did discuss thus with Safesure and other suppliers some years ago, the point was that in for example a so-called 70 amp isolator the diodes are rated at 70 amps; this is in fact their manufacturer's CONTINUOUS rating when connected to a perfect heatsink. Of course such a heatsink doesn't exist, but it means that at their rated 50 amp current the diodes will be perfectly happy until they warm up to an excessive temperature. On a typical GI unit the heat sink will be good enough to ensure that this will take several seconds (up to a minute on the better ones) which is plenty more than enough to ensure that the breaker will have tripped. The peak current rating if the diodes  is typically up to 50-100 times greater, but then begins the debate on how long is a Peak? I was told, but I have seen no evidence, that the peak rating/duration relationship has been compared against the operating current / trip time characteristics of a breaker to ensure that there is always a good margin so that the breaker will always trip before the diodes can be damaged. This may or may not be true, as I say I have seen no evidence, but I do feel that it is an over-simplification to say that all such GIs are unfit for purpose.

 

Thank you Allan

Just now, TheBiscuits said:

Ok. I wonder why they don't publish that 500A figure in their sales information. 

 

I know you didn't claim 5000A for the Safeshore, but that is the standard we have been discussing on the thread.  The calculations this is based on were posted early in the thread. 

I've no idea, but they are 70amp (some are 100amp) continuous rated, so we shouldn't be comparing that to peak ratings as some were doing earlier in this thread.

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2 minutes ago, blackrose said:

I've just spoken to them. 70amp is continuous and 500A peak. I don't have a link sorry. I never said 5000A?

Go back to post 13 where I asked if the Safeshore 70-100 amp Gi's were ok. The answer from Wotever was NO.  The bottom line it would appear is that any Gi with a fault current rating of under 4600 amps is not fit for purpose.

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20 minutes ago, blackrose said:

You're still saying 5000A for the Sterling GIs but you said 70A for the Safeshore

I was quoting what others, including you, had said about the Safeshore. I hadn’t looked it up. Regardless, 500A vs 5000A is a pretty big difference. 

10 minutes ago, Flyboy said:

Go back to post 13 where I asked if the Safeshore 70-100 amp Gi's were ok. The answer from Wotever was NO.  The bottom line it would appear is that any Gi with a fault current rating of under 4600 amps is not fit for purpose.

Yes, that is an accurate summation of what I wrote. If there is a ‘gold standard’ why would you not want to adhere to it?

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45 minutes ago, Keeping Up said:

The peak current rating if the diodes  is typically up to 50-100 times greater, but then begins the debate on how long is a Peak?

It’s a while since I read the AYBC specs but I seem to recall it is half a cycle. There is a bench test available which determines that the diodes can survive. You require very expensive diodes to achieve this which is why the Victron GI, the Sterling FS GIs and all the others that meet the AYBC spec are more expensive than ones which don’t (plus the cost of the certification). 

41 minutes ago, TheBiscuits said:

Ok. I wonder why they don't publish that 500A figure in their sales information. 

My guess would be for 2 reasons.

1) It’s an estimation because they’ve not actually tested them and

2) It’s a factor of 10 below the only extant standard so is nothing really to shout about. 

 

Eta: looks from Mike’s post below that they do, so presumably my guesses above are redundant. 

Edited by WotEver
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26 minutes ago, WotEver said:

I was quoting what others, including you, had said about the Safeshore. I hadn’t looked it up. Regardless, 500A vs 5000A is a pretty big difference. 

 

Yes, I already accepted the difference.

 

I'm not sure who said what first now, but I didn't join the thread until after you'd slated the Safeshore and I intuitively thought that you must comparing two different things above. Despite my lack of electrical knowledge it turns out I was right.

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2 minutes ago, blackrose said:

Yes, I already accepted the difference.

 

I'm not sure who said what first now, but I didn't join the thread until after you'd slated the Safeshore and I intuitively thought that you must comparing two different things above. Despite my lack of electrical knowledge it turns out I was right.

Elecosol batteries had good marketing too. 

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These Sterling AYBC compliant GIs look good and I might upgrade but they are f@cking expensive and they have no status monitor. They say you don't need one but then how would you know about or be able to investigate any fault condition?

 

https://www.sp-shop.co.uk/Pro-Save-FS/

 

Surely if the diodes go into circuit then you want to know about it? 

 

 

Edited by blackrose
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2 minutes ago, blackrose said:

These Sterling AYBC compliant GIs look good and I might upgrade but they are f@cking expensive and they have no status monitor. They say you don't need one but then how would you know about or be able to investigate any fault condition?

 

https://www.sp-shop.co.uk/Pro-Save-FS/

 

Surely if the diodes go into circuit then you want to know about it? 

The whole point of the AYBC spec is that they won’t go open circuit. However, you can check them in seconds with a multimeter on resistance range. 

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55 minutes ago, Flyboy said:

  The bottom line it would appear is that any Gi with a fault current rating of under 4600 amps is not fit for purpose.

No, that's not the bottom line. As Allan and another qualified person I have spoken to have said, that's an over-simplification.

 

There's obviously some difference of opinion on the subject of fitness for purpose, even amongst people in the know.

4 minutes ago, WotEver said:

The whole point of the AYBC spec is that they won’t go open circuit. However, you can check them in seconds with a multimeter on resistance range. 

Even in the event of a fault? So what's the point of the diodes or are there no diodes? Is there a large capacitor built in to divert stray AC leaks to earth? 

Edited by blackrose
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As I see it if you have an ABYC rated unit and have a live to earth short on your boat the unit will survive and be ok after the fault had been fixed.  With the non rated unit, after an on-board dead short the supply fuse/breaker will ‘open’ cutting off the supply, BUT the diodes in the GI may not have survived, so upon fixing the fault and reconnecting the supply you may no longer have a safe and effective connection to the supply earth. The chance of the diodes blowing depends on the diode capability and the supply fault capability.   So if you go for the non ABYC unit you must check the unit is working after every time the supply ‘trips’, and if no longer working, replace or remove the gi before reconnecting the supply.

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27 minutes ago, ditchcrawler said:

On the ones with indication, what does the indication indicate, does it show both diodes are healthy of that you have earth leakage taking place or both.

Think from memory if one light is showing it’s a DC leak. Both lights AC leak. 

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This is what Vic at Safeshore has to say (I'm sure he won't mind me posting this):

 

Hi , further to our brief conversation this morning re your forum just to clarify. The nominal current rating (rms) is the important rating. It is the continuous current the isolators ) can handle under severe fault current. Legislation requires the isolator to be able to handle the total available current (from the shore power supply) plus a minimum of 20% more. All Safeshore isolators are rated at 70 or 100 amps making them more than compliant for both 16 and 32 amp supplies. The transient current rating even for the most basic of diodes is at least 400 amps. This is really almost irrelevant as this alone does not make them reliable. The more important consideration is actually the heat sinking of the isolator. Any diode under fault condition will generate heat. If the isolator cannot dissipate this heat the diodes will fail. There are many isolators on eBay built into plastic boxes and tubes. Without heat sinks they simply fail under load leaving you with zero protection and probably no safe earth connection. The whole structure of Safeshore isolators is one large efficient heat sink ensuring total reliability. Hope this clarifies . We are very proud of our reliability record of 24000 sales with no reported failures hence our lifetime warranty. You bought wisely! Kind regards, Vic Safeshore UK

 

My safeshore GI is now nearly 14 years old, regularly tested and still working fine, so I think he's right...

 

 

And this was the response from someone else I know: 

 

Let’s use the scientific method: A diode carrying 5000A at around 1.2 volts (the forward voltage rises with current) will therefore dissipate 5000 x 1.2 watts; ie: 6KW !!!!!!!!

 

With a huge block of fan-cooled metal bonded to the diode, the temperature rise would still be 600 degC (0.1degC/W). Thus the diode would first fail at above 150degC and then melt as it reached several hundred degrees. With 500A diode, the temperature rise would be about 50degC. 

 

It’s also worth asking which power supply could actually deliver 5000A? 

Edited by blackrose
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1 hour ago, blackrose said:

.........lol

 

My safeshore GI is now nearly 14 years old, regularly tested and still working fine, so I think he's right...

........

trimmed

.........

 

 

 

It’s also worth asking which power supply could actually deliver 5000A? 

Have you ever had a direct live to earth fault on you boat??  If not then it’s to be expected that it’s still ok.

 

Probably not 5000A as that is why ABYC chose that figure, but a bollard fed by decent cables not too far from the local distribution station will be capable of fault currents peaking at a few thousand amps before the fuses/breakers disconnect the supply.

 

 

added - your temperature rise calculation does not seem to include the fact that the fault current only flows until the circuit protection devices operate, so just a few mS. 

Edited by Chewbacka
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1 hour ago, blackrose said:

And this was the response from someone else I know

 

Let’s use the scientific method: A diode carrying 5000A at around 1.2 volts (the forward voltage rises with current) will therefore dissipate 5000 x 1.2 watts; ie: 6KW !!!!!!!!

 

With a huge block of fan-cooled metal bonded to the diode, the temperature rise would still be 600 degC (0.1degC/W). Thus the diode would first fail at above 150degC and then melt as it reached several hundred degrees. With 500A diode, the temperature rise would be about 50degC. 

He obviously hasn’t read the spec, or he would understand that that’s nonsense. 

2 minutes ago, Chewbacka said:

Probably not 5000A as that is why ABYC chose that figure, but a bollard fed by decent cables not too far from the local distribution station will be capable of fault currents peaking at a few thousand amps before the fuses/breakers disconnect the supply.

Exactly so. 

1 hour ago, blackrose said:

... the temperature rise would still be 600 degC (0.1degC/W).

Now get him to tell you how long that temperature rise would take. 

Edited by WotEver
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The two extreme fault conditions that a GI needs to survive are -

1.  The live to earth short on the boat that will put a brief (a few mS) current of hundreds if not thousands of amps through the gi before the protection devices fail.  This is where the ABYC 5000A requirement comes from.

 

2.  A fault condition where by the over current protection devices do not operate.  This could happen where (example) the electric kettle neutral wire came loose in it’s plug and touched the earth pin.  The circuit would now have current returned via the earth rather than the neutral, so the kettle would work just fine, and the current would be at normal levels, but you would have all the kettle current passing through the gi.  This would of course operate the rcd protection device but not the over-current protection device,  but it is feasible that the rcd device was faulty or even missing etc etc.   Hence the requirement for the gi to be able to sustain 120% of the supply capability.

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Exactly.

 

As for Safeshore - remember when their adds implied that ALL boats would suffer corrosion damage if no GI was used? I challenged him and mentioned the ASA. He admitted that was not so  & the adds changed. Not a company who's technical and ethical competence I would accept without question.

 

In my view it is vital to remember that diodes can fail open OR closed (short) circuit. In one you may have no RCD protection so making electrocution more likely while the other leaves you with the potential for a corroding hull etc. This is why I would go for an isolation transformer or better still a 12/24 volt boat.

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As to GI or IT, it’s about risk - both personal safety and boat corrosion.  If I was on a shore supply for long periods of time I would have the transformer, but for occasional shore line connections I would use a GI from a reasonable manufacturer as diode failure in normal use is pretty unlikely, and it’s smaller and cheaper and silent.

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