Galvanic Isolators and Isolation Transformers

[dianamay]

Could someone please explain in layman's terms the advantages of a isolation transformer over a galvanic isolator in relationship to a inverter and a isolator. Thanks in anticipation.

[Phil Ambrose]

My understanding is that the isolation transformer does a better job but the GI is less expensive and a lot lighter and less bulky. I have a GI which over the last 5 years does seem to have done a good job. More advice from others to follow

Phil

Edited March 8, 2016 by Phil Ambrose

[ditchcrawler]

You only need them if you have a shore supply not for an inverter.

[Keeping Up]

A transformer WILL work perfectly if correctly wired, but will be very big and very expensive. If it breaks it will usually just switch off.

 

A GI will work perfectly in 99.9% of installations and will be small and cheap. If you get one with an indicator light it will tell you 99 times out of 100 if it has fallen into the other 0.1% provided you look at the indicator light. The remaining 0.001% of cases will usually be noticed but only if you regularly test it and understand the results of that test (on most the test is simply that you put a 9v battery across it and see if it lights up). The few remaining examples (a small fraction of the 0.001%) could possibly be dangerous or potentially even lethal if your on-board equipment develops a fault.

 

The percentages I have given are just my guess as an experienced electrical engineer.You pay your money and you take your choice. Personally I have a GI.

[Loafer]

A transformer WILL work perfectly if correctly wired, but will be very big and very expensive. If it breaks it will usually just switch off.

 

A GI will work perfectly in 99.9% of installations and will be small and cheap. If you get one with an indicator light it will tell you 99 times out of 100 if it has fallen into the other 0.1% provided you look at the indicator light. The remaining 0.001% of cases will usually be noticed but only if you regularly test it and understand the results of that test (on most the test is simply that you put a 9v battery across it and see if it lights up). The few remaining examples (a small fraction of the 0.001%) could possibly be dangerous or potentially even lethal if your on-board equipment develops a fault.

 

The percentages I have given are just my guess as an experienced electrical engineer.You pay your money and you take your choice. Personally I have a GI.

 

What's quick way of testing one that doesn't have a light? (We're never connected to shore power, but have a GI)

[nicknorman]

 

What's quick way of testing one that doesn't have a light? (We're never connected to shore power, but have a GI)

Why bother to test it then?

 

Anyway you test them with a PP3 battery and bicycle bulb, plus a meter. With the shore lead unplugged you put the bulb/battery circuit across the GI's terminals. The bulb should light up. You check the voltage across the GI, should be around 1v depending on whether 1 or 2 diodes. Then reverse the connections to the bulb/battery and repeat. If the bulb doesn't light up it means the diodes are open circuit. If it does light up but the voltage across the GI terminals is virtually zero it means the diodes are short circuit.

[Loafer]

Why bother to test it then?

 

Anyway you test them with a PP3 battery and bicycle bulb, plus a meter. With the shore lead unplugged you put the bulb/battery circuit across the GI's terminals. The bulb should light up. You check the voltage across the GI, should be around 1v depending on whether 1 or 2 diodes. Then reverse the connections to the bulb/battery and repeat. If the bulb doesn't light up it means the diodes are open circuit. If it does light up but the voltage across the GI terminals is virtually zero it means the diodes are short circuit.

 

Because it's THERE!

 

Anyway, thanks for the tip. I've cut and pasted it for when age forces us into a marina!

[arcsyst]

Personal experience:

 

We had a new boat with a GI, being a new boat we blacked it every year for the first 4 years, for the first 3 years there was quite severe pitting to the hull until I replaced the GI with an isolating transformer that actually wasn't that expensive. Now I see virtually no pitting. We cruise about 3 months and tie up in a marina on a shore line the rest.

[jonesthenuke]

GI = Cheap but small risk of failure with consequent risk of hull corrosion if on a shoreline. Risk mitigated by testing, which needs to be frequent enough.

 

IT = Dearer, bigger etc. Risk of failure lower and more likely to be self revealing. Residual risk mitigated by testing which can be less frequent than with a GI.

 

Our boat spends much of ts time on a shore line. The increased cost of an isolating transformer is inconsequential in comparison to the potential cost of hull repairs, so a judgement has to be made dependent on your assessment of the risk reduction that you get by using an IT. This will depend on time spent on a shore line and ability and willingness to test a GI.

 

I fitted an IT, but its a personal judgement.

[nicknorman]

Personal experience:

We had a new boat with a GI, being a new boat we blacked it every year for the first 4 years, for the first 3 years there was quite severe pitting to the hull until I replaced the GI with an isolating transformer that actually wasn't that expensive. Now I see virtually no pitting. We cruise about 3 months and tie up in a marina on a shore line the rest.

I know of several new boats whose GI was incorrectly installed such that it was bypassed. Including ours! It was installed in accordance with the instructions, which were wrong. A correctly installed GI should protect the boat same as an IT. Well maybe except in the most severe cases of a problem with the shore earth, but if you get one with a meter or LEDs you can see whether or not it's being overwhelmed.

[arcsyst]

I know of several new boats whose GI was incorrectly installed such that it was bypassed. Including ours! It was installed in accordance with the instructions, which were wrong. A correctly installed GI should protect the boat same as an IT. Well maybe except in the most severe cases of a problem with the shore earth, but if you get one with a meter or LEDs you can see whether or not it's being overwhelmed.

 

Mine was correctly wired and non-faulty. I can't explain why I just observe empirical evidence.!

 

GI = Cheap but small risk of failure with consequent risk of hull corrosion if on a shoreline. Risk mitigated by testing, which needs to be frequent enough.

 

IT = Dearer, bigger etc. Risk of failure lower and more likely to be self revealing. Residual risk mitigated by testing which can be less frequent than with a GI.

 

Our boat spends much of ts time on a shore line. The increased cost of an isolating transformer is inconsequential in comparison to the potential cost of hull repairs, so a judgement has to be made dependent on your assessment of the risk reduction that you get by using an IT. This will depend on time spent on a shore line and ability and willingness to test a GI.

 

I fitted an IT, but its a personal judgement.

I agree with jonesthenuke an IT is a guaranteed solution for what? Twice the cost? Another £120? I really can't see why GI's are so popular.

 

http://www.airlinktransformers.com/boating_transformers/BT3231/

Edited March 9, 2016 by arcsyst

[nicknorman]

Mine was correctly wired and non-faulty. I can't explain why I just observe empirical evidence.!

 

 

I agree with jonesthenuke an IT is a guaranteed solution for what? Twice the cost? Another £120? I really can't see why GI's are so popular.

 

http://www.airlinktransformers.com/boating_transformers/BT3231/

It's not just cost, it is also the size and (to a lesser extent on a narrowboat) the weight. Plus slight loss of efficiency and possible humming.

[arcsyst]

It's not just cost, it is also the size and (to a lesser extent on a narrowboat) the weight. Plus slight loss of efficiency and possible humming.

 

IMHO they are slightly spurious reasons,

 

• Size, whilst larger than a GI it is still contained in a space slightly over a foot cubed
• Weight, think of it as ballast!
• Efficiency, 97.5% = 33p/year at 1KVA and 15p/KWh
• Noise, OK I'll concede slightly on this one, on rare occasions mine will hum a little so I put it where it wont be heard, I never hear mine.

 

For me the argument is solely about how do I protect my significant investment in underwater ferrous metal, a GI may be OK, an IT is guaranteed to be OK. Set against the cost of replating.....

Edited March 9, 2016 by arcsyst

[nicknorman]

IMHO they are slightly spurious reasons,

 

 

• Size, whilst larger than a GI it is still contained in a space slightly over a foot cubed
• Weight, think of it as ballast!
• Efficiency, 97.5% = 33p/year at 1KVA and 15p/KWh
• Noise, OK I'll concede slightly on this one, on rare occasions mine will hum a little so I put it where it wont be heard, I never hear mine.

For me the argument is solely about how do I protect my significant investment in underwater ferrous metal, a GI may be OK, an IT is guaranteed to be OK. Set against the cost of replating.....

I'm just arguing in order to provide balance but for example, thinking about our boat with trad stern/rear engine room I can't think of a location where an IT could go. The engine bay is full of engine, hospital silencer, batteries, calorifier, Mikuni, Combi inverter etc. There is no spare "floor space". The GI is mounted on the rear bulkhead and as such doesn't take up any floor space.

[arcsyst]

I'm just arguing in order to provide balance but for example, thinking about our boat with trad stern/rear engine room I can't think of a location where an IT could go. The engine bay is full of engine, hospital silencer, batteries, calorifier, Mikuni, Combi inverter etc. There is no spare "floor space". The GI is mounted on the rear bulkhead and as such doesn't take up any floor space.

 

I take your point, my boat is a modern design and as such has some cupboard bottoms that are underutilised, the 'electrics cupboard' is first on the left through the stern doors and has a handy well in the bottom which is where mine is kept.

 

I suppose what I'm saying is that given my experience with GI's I'd probably take out the bed to make room for an IT!!

[Paul C]

>1 cu ft. is a big box to find a space for if the boat wasn't intended to have an IT. Maybe we'll see new boats being built with elecric systems including an IT as standard, or more people specifying them when commissioning a new build. I dare say, more boats don't have a GI or IT, than do, because people don't really appreciate the science behind the requirement for these devices.

[arcsyst]

>1 cu ft. is a big box to find a space for if the boat wasn't intended to have an IT. Maybe we'll see new boats being built with elecric systems including an IT as standard, or more people specifying them when commissioning a new build. I dare say, more boats don't have a GI or IT, than do, because people don't really appreciate the science behind the requirement for these devices.

 

Again using my experience, boats seem to be built by people who have poor knowledge of electrics. My own catalogue also includes undersized poorly terminated cables and inadequate chargers. Starting from a shell it would be comparatively easy to do a 'Proper Job' instead of trying to patch it all up after the event. Now if only my carpentry skills matched my electronic skills ............

[George94]

 

Again using my experience, boats seem to be built by people who have poor knowledge of electrics. My own catalogue also includes undersized poorly terminated cables and inadequate chargers. Starting from a shell it would be comparatively easy to do a 'Proper Job' instead of trying to patch it all up after the event. Now if only my carpentry skills matched my electronic skills ............

 

Rather a lot of canal boats seem to be built by jumped-up welders with no knowledge of boats at all.

[ditchcrawler]

Surly to do it properly the IT shouldn't even be on the boat?

[George94]

Surly to do it properly the IT shouldn't even be on the boat?

 

That is the perfect scenario, and is what I would do if I had a houseboat and a safe place to mount the IT.

[arcsyst]

Surly to do it properly the IT shouldn't even be on the boat?

Why?

 

The purpose of the IT/GI is simply to break the earth loop. Doesn't matter where it is.

[Paul C]

Why?

 

The purpose of the IT/GI is simply to break the earth loop. Doesn't matter where it is.

 

How do you connect it to shoreline, if its installed on the boat?

[arcsyst]

How do you connect it to shoreline, if its installed on the boat?

How do you connect your GI?

Edited March 10, 2016 by arcsyst

[George94]

 

How do you connect it to shoreline, if its installed on the boat?

 

There's this stuff called wire, AKA electrical cable.

Why?

 

The purpose of the IT/GI is simply to break the earth loop. Doesn't matter where it is.

 

It doesn't matter if everything is working as it should. But if it catches fire...

[Paul C]

How do you connect your GI?

 

 

 

There's this stuff called wire, AKA electrical cable.

 

I was hoping for an intelligent debate but it seems unlikely here. Never mind.