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robtheplod

Galvanic Isolator - How to test?

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Hi Everyone

We've got one of these on Water Lily:

 

https://www.amazon.co.uk/gp/product/B07D9TBPPC/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

 

We got this one as it we could use it immediately without fitting and we were new to the boat. Quick question, how do we know its actually working - can we do a diode test on the earth or similar??

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54 minutes ago, rusty69 said:

I used the method outlined on this website:

 

https://www.pbase.com/mainecruising/galvanic_isolator

 

 

I test mine that way, but before connecting the DVM across the GI (in either direction) I temporarily short out the two GI terminals with a short length of wire, remove the shorting wire and then take the DVM readings.

 

 This is because many GI's have a small capacitor across the diodes to allow high frequency a.c. ripple to leak to earth, and this capacitor can charge up to the battery voltage of the DVM and give a false reading.

  • Happy 2

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Hi Everyone, I've hijacked another thread with my GI queries so thought it best to bring back here. Following up on my testing of volts from my hull to a wire attached to a mooring pin in the water. Thread gets hijacked at #76 - 

 

I'd be really interested in continuing this as I'm not sure if my GI is working as I'm getting readings connected to shoreline and completely disconnected?

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3 hours ago, robtheplod said:

Hi Everyone, I've hijacked another thread with my GI queries so thought it best to bring back here. Following up on my testing of volts from my hull to a wire attached to a mooring pin in the water. Thread gets hijacked at #76 - 

 

I'd be really interested in continuing this as I'm not sure if my GI is working as I'm getting readings connected to shoreline and completely disconnected?

Your test on the other thread has nothing to do with your GI, see my latest reply to it. 
 

Post #2 in this thread explains how to test your GI. 

  • Greenie 1

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Hi All

 

I've now had the boat a while and more familiar with the wiring etc. At present I have one of the less expensive plug in GI's and wondering I need something more substantial?  Does this look suitable, the AC current rating for 20 sec certainly looks better?   

https://www.ebay.co.uk/itm/GALVANIC-ISOLATOR-115V-AC-230V-AC-50A-CONTINUOUS-Made-in-UK-Model-E936/230627220329?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

 

or would people recommend branded such as Victron VDI-32 possibly?

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1 hour ago, robtheplod said:

Hi All

 

I've now had the boat a while and more familiar with the wiring etc. At present I have one of the less expensive plug in GI's and wondering I need something more substantial?  Does this look suitable, the AC current rating for 20 sec certainly looks better?   

https://www.ebay.co.uk/itm/GALVANIC-ISOLATOR-115V-AC-230V-AC-50A-CONTINUOUS-Made-in-UK-Model-E936/230627220329?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

 

or would people recommend branded such as Victron VDI-32 possibly?

If you want to upgrade a GI and have money to spend, don't mess about - fit an isolating transformer and forget about it for evermore.

 

Do they cost more? Yes. 

Do they do a better job? Yes. 

Can they fail like a cheap GI? Yes, but if they do the boat electric stops working, rather than them failing in a way that makes them dangerous but not obvious.

 

If you are not confident in installing one, PM me and I'll give you the phone number of the best boat electrician in the country ... but as you are self-installing LiFePo4 batteries, I guess you can wave a spanner and a crimping tool.

 

 

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7 hours ago, TheBiscuits said:

If you want to upgrade a GI and have money to spend, don't mess about - fit an isolating transformer and forget about it for evermore.

 

Do they cost more? Yes. 

Do they do a better job? Yes. 

Can they fail like a cheap GI? Yes, but if they do the boat electric stops working, rather than them failing in a way that makes them dangerous but not obvious.

 

If you are not confident in installing one, PM me and I'll give you the phone number of the best boat electrician in the country ... but as you are self-installing LiFePo4 batteries, I guess you can wave a spanner and a crimping tool.

 

 

I agree it looks best, but the cost is prohibitive at near £700 for one I've found - https://www.midlandchandlers.co.uk/products/isolation-transformer-3600w-vv-033. Others don't seem to give prices on their website so must be more??!!

 

Also 32amp is the VDI-32 https://www.batterymegastore.co.uk/victron-gdi000032000.html and is more reasonable (and up to 3200A for 20 secs), unless anyone knows a better GI for the money...  I'm assuming the ebay one I linked to is not recommended due to it being unbranded despite it *appearing* to be better specced (5000A) than the Victron VDI-32 ?   The VDI-64 isn't much more but is this overkill at 6000A for 20 secs??

 

Other GI's noted on forum are these - are these 'better' than the Victron and hence worth a look at, they seem to comply more??

 

https://www.waveinn.com/nautical-fishing/mastervolt-prosafe-fs-60/136411844/p?utm_source=google_products&utm_medium=merchant&id_producte=4793486&country=uk&gclid=CjwKCAiAu9vwBRAEEiwAzvjq-xy6oibNhPdReJ2YWpAgoA9QlF27-XqoYmzVTOtGamCjI_sfp5AyehoCRMIQAvD_BwE&gclsrc=aw.ds

 

I like that this one has a meter...

 

https://aquafax.co.uk/product/8-40991-galvanic-current-isolator-32a-with-indicator

 

 

I'm not fitting LiFePo4 batteries, I think you may have mixed me up with someone else?  Happy to install these things now I'm more familiar with the boat - I got an inline one initially as I was wary about diving into boat electrics before I was confident. 

Edited by robtheplod

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While you're testing your GI or installing a new one, have you checked that your boat is actually hull-earth bonded? It may be a stupid question but if the mains earth is not bonded to the hull (and that connection isn't sound) then the work you've done installing & testing the GI is completely pointless.

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If you have tested your GI and it’s ok and you know how to then there is no problem.   It ain’t broke don’t fix it....

 

If you are determined to change it out then one with a status monitor is the way to go (leds or meter) I would recommend the safe shore device. (You don’t even need a multimeter to test these) 
 

Personally I would pay little attention to those on here who postulate that the GI must be capable of sinking the load of Hinckley B based on some American standard. 

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3 hours ago, jonathanA said:

Personally I would pay little attention to those on here who postulate that the GI must be capable of sinking the load of Hinckley B based on some American standard. 

Why?

4 hours ago, robtheplod said:

I agree it looks best, but the cost is prohibitive at near £700 for one I've found

£300

 

https://airlinktransformers.com/post/airlink-waterproof-isolation-transformer
 

https://airlinktransformers.com/product/boating-transformer-bt3231

 

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With a GI it is the fault current rating that matters, not necessarily the continuous rating. The maximum prospective current rating is measured in kA (1kA = 1,000 amps). The one you link to shows this in kVA (kilo Volt Amps), a measure of power, not current, but may be a typo.

 

Why is the maximum propsective current rating important? It is because the maximum prospective fault current is the maximum current that can flow until such time as the fuse blows or the circuit breaker trips.

 

It is calculated by measuring the resistance between line and neutral and line and earth, and using the lower of the figures obtained.

 

For example if the line to neutral resistance is the lower of the two resistance measurements and is measured at 0.05Ω and the supply voltage measured at 230V:

 

Maximum prospective short-circuit current (line-to-neutral) = 230/0.05 = 4600 A (or 4.6 kA)

 

This current will flow until the protection operates, typically in 10 to 20 milliseconds, and will damage any components not designed to withstand it.

 

In practice the maximum prospective fault current is likely to be lower than 4.6kA, but is dependent upon the capacity of the supply transformer and lenth & size of the cables between that and your boat.

 

These days it is measured by a tester, which displays the maximum prospective fault current on a digital display. At the dawn of my career you had to measure cable lengths and look up the resistance values for the various cable sizes in tables and manually calculate the maximum prospective fault current.

  • Greenie 1

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37 minutes ago, cuthound said:

With a GI it is the fault current rating that matters, not necessarily the continuous rating.

Careful, you’re in danger of being ignored by @jonathanA ;)

  • Haha 1

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Interesting the difference in view of a Telecoms engineer and a Critical Power Systems Project Manager. I know which one I would trust re isolation transformers. Its mA V kA

  • Greenie 2

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One thing that I sometimes wonder about (because I can't be bothered to do all the maths, and I'm an ex-BT engineer) is whether, assuming the diodes have a sufficiently high peak current rating, there is actually any justification for the huge heat-sinks that many GI's are fitted with. Yes there is a lot of energy under fault conditions, but for such a short time that I can't imagine it has enough heating effect to justify anything more than quite a small heat-sink

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41 minutes ago, Keeping Up said:

One thing that I sometimes wonder about (because I can't be bothered to do all the maths, and I'm an ex-BT engineer) is whether, assuming the diodes have a sufficiently high peak current rating, there is actually any justification for the huge heat-sinks that many GI's are fitted with. Yes there is a lot of energy under fault conditions, but for such a short time that I can't imagine it has enough heating effect to justify anything more than quite a small heat-sink

It’s so that the manufacturer can state “32A Fault Current” or whatever meaningless figure their marketeers have dreamed up that week. If you have 32A flowing down the earth lead then I’d expect the RCD (or even MCB) to operate in short order but in the mean time, as the device is ‘rated’ for 32A, then it must carry that current without burning out - that means a big heat sink. 

 

What none of these manufacturers explain is why the current would be limited to only 32A. As Cuthound showed above, a direct L-E short could easily result in >4000A fault current for a cycle or two. 
 

3 minutes ago, robtheplod said:

They seem awfully big?

I believe they’re one of the smallest you can buy. 

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I've said this before, when I was working in critical power, every component failure was recorded and analysed, to help identify and eliminate weak components in the power system design.

 

Wound components (transformers, relays, inductors etc) had a much higher failure rate than solid state components (high power transistors, diodes, thyristors etc).

 

That said, isolation transformers do fail safe, which GI's may not, but consume energy and can be noisy.

Edited by cuthound
Spillung

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1 hour ago, Keeping Up said:

One thing that I sometimes wonder about (because I can't be bothered to do all the maths, and I'm an ex-BT engineer) is whether, assuming the diodes have a sufficiently high peak current rating, there is actually any justification for the huge heat-sinks that many GI's are fitted with. Yes there is a lot of energy under fault conditions, but for such a short time that I can't imagine it has enough heating effect to justify anything more than quite a small heat-sink

Because before circuits were protected by an rcd the only protection was the fuse or circuit breaker.  As an example, if the neutral wire came loose in the plug and touched onto the earth pin on your 3kW fan heater then the heater would work fine but you would have 12A running down the earth instead of the neutral. The GI would then have to dissipate 12A at 1.5V so 18W until you turn off the heater.  Which is quite a lot for a little box.  

Edited by Chewbacka

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7 hours ago, WotEver said:

Careful, you’re in danger of being ignored by @jonathanA ;)

Forgive me for having a life and not being glued to the canal world forum 24 x 7 just in case the One of the great ones may  chose to share his wisdom with us mere mortals....  ffs 

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11 hours ago, jonathanA said:

Forgive me for having a life and not being glued to the canal world forum 24 x 7 just in case the One of the great ones may  chose to share his wisdom with us mere mortals....  ffs 

So, no answer to”Why?” then...

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16 hours ago, Keeping Up said:

One thing that I sometimes wonder about (because I can't be bothered to do all the maths, and I'm an ex-BT engineer) is whether, assuming the diodes have a sufficiently high peak current rating, there is actually any justification for the huge heat-sinks that many GI's are fitted with. Yes there is a lot of energy under fault conditions, but for such a short time that I can't imagine it has enough heating effect to justify anything more than quite a small heat-sink

Putting aside your incorrect use of dimensions, I’d agree. (“There is a lot of POWER under...). If it were a lot of energy, that would equate to a lot of heat and a need for large heatsinks. Energy being the time integral of power of course.

On 25/02/2020 at 08:16, jonathanA said:

Personally I would pay little attention to those on here who postulate that the GI must be capable of sinking the load of Hinckley B based on some American standard. 

One can’t argue with your statement because it seems it is true, but it does demonstrate that you are a fool when in comes to electricity.

  • Greenie 2

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13 hours ago, nicknorman said:

 

One can’t argue with your statement because it seems it is true, but it does demonstrate that you are a fool when in comes to electricity.

The point made by many people is that yes a GI can POTENTIALLY fail following some sort of fault current and therefore should be tested after an overload trip.  This thread demonstrates how simple that is.  
 

if in your opinion then this makes me a fool well  I can live with that because I care not.

 

Educated to honours degree level and working in telecoms for 40 year I’m not merely a craftsman or technician masquerading as an ‘engineer’ but I expect that distinction would be totally beyond yours and wotevers comprehension so I shall waste no more time with you.
 

As with any post on a forum people need to weigh up the opinions expressed and make up their minds what’s appropriate for their situation. I think it’s important to debunk some of the self appointed experts when they are wrong/misleading or at least present a differing view - don’t you agree ? 

 

Edited by jonathanA

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