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Old 06-12-2022, 11:32   #1
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How to wire and test an isolation transformer

I see so much confusion about this… and get questions on the issue as well so I decided to write this post.

Attached is the diagram that I have posted often. It shows the shore power inlet, isolation transformer and an inverter/charger that has an internal transfer switch. It also shows the distribution side with the breaker panel. Here you must determine if you have single pole or double pole breakers. This matters later on.

There are guidelines that you need to adhere to. First the location of shore power inlet, breaker box and isolation transformer. The wiring between the inlet and the breaker box must be as short as possible and not longer than 3 meter / 10 feet.

The second photo shows the breaker box that I recommend. These are very affordable, as are the breakers. Install both double pole breakers that connect to the transformer in it, as well as some DIN rail terminal blocks, then wire the inlet to the input of the first breaker, which must be 30-32A. The ground conductor is wired to a terminal block.

Wire the transformer input to the load side of the input breaker and the ground conductor to the same terminal block that the inlet ground was connected to.

Wire the transformer output to the input side of the output breaker , which is 30A for 120V boats and 16A for 230V boats and the ground conductor to a second terminal block, that is not connected to the first.

Last but not least is the cable towards the inverter/charger or your system, whatever you have aboard. Connect it to the load side of the output breaker and the ground conductor to the same terminal block that the output from the transformer connects to.

In case you have single pole breakers on your AC panel: connect a jumper between the transformer output Neutral and the transformer output Ground. Sometimes the transformer has a jumper for this, do it in the breaker box if there is no room at the transformer.

In case you have double pole breakers on your AC panel: you can choose to ground the neutral like for single pole breakers, which is recommended by the ABYC in the US. I prefer not to but only leave it out when you understand this floating neutral concept. It’ll probably get discussed in this thread again.

Above I wrote that the input breaker must be 30-32A, even for 230V boats. This is because the transformer can take 110-120V shore power and double that for use aboard, which is very convenient. As the voltage double, the current is back to the regular 16A for 230V boats so that is why the output breaker is 16A in that case.

Now comes the testing. There must be no connection between the input ground and output ground. None at all. Same for the other conductors, they should all be completely isolated between input and output.

Feel free to ask about any additional testing etc. in this thread
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Old 06-12-2022, 11:44   #2
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Re: How to wire and test an isolation transformer

My primary issue would be the I’m Lucio. Of a bond to “ ships ground “ I do not personally advocate that approach
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Old 06-12-2022, 11:50   #3
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Re: How to wire and test an isolation transformer

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Originally Posted by goboatingnow View Post
My primary issue would be the I’m Lucio. Of a bond to “ ships ground “ I do not personally advocate that approach
I have no idea what you are trying to say… please don’t kill this thread with crazy stuff
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Old 06-12-2022, 12:05   #4
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Re: How to wire and test an isolation transformer

Think he means connecting to the shore ground to ships ground is not a good idea as you are not isolated from any stray current from a bad installation and not even a galvanic idolater ! Good way to eat anodes.
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Old 06-12-2022, 12:21   #5
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Re: How to wire and test an isolation transformer

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Think he means connecting to the shore ground to ships ground is not a good idea as you are not isolated from any stray current from a bad installation and not even a galvanic idolater ! Good way to eat anodes.
I completely agree with that, which is why I stTe that it must be ISOLATED

So, my idea is great, which is why this is a reference diagram
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Old 06-12-2022, 12:54   #6
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How to wire and test an isolation transformer

Quote:
Originally Posted by s/v Jedi View Post
I have no idea what you are trying to say… please don’t kill this thread with crazy stuff


If you install an isolating transformer you do not need to involve any form of ship board ground circuit it’s completely unecessessarily to re- introduce a fault path that doesn’t need to be re introduced
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Old 06-12-2022, 13:02   #7
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Re: How to wire and test an isolation transformer

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Originally Posted by goboatingnow View Post
If you install an isolating transformer you do not need to involve any form of ship board ground circuit it’s completely unecessessarily to re- introduce a fault path that doesn’t need to be re introduced
That is not true. Ships ground wiring is very effective because outlets as well as appliances that are not double insulated have ground pins that connect to ground wiring. This prevents metal surfaces of two appliances becoming hot and eliminate the risk of electrocution.

Even when the neutral is floating, the ships ground still provides that safety.
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Old 06-12-2022, 13:04   #8
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How to wire and test an isolation transformer

Quote:
Originally Posted by s/v Jedi View Post
That is not true. Ships ground wiring is very effective because outlets as well as appliances that are not double insulated have ground pins that connect to ground wiring. This prevents metal surfaces of two appliances becoming hot and eliminate the risk of electrocution.



Even when the neutral is floating, the ships ground still provides that safety.


But the whole Point is there is no return path if an appliance powered by a transformer live becomes live. Hence establishing a boat wide ground point actually increases the risk of electrocution you are then force to rely on RCD devices to re establish safety
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Old 06-12-2022, 13:29   #9
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Re: How to wire and test an isolation transformer

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Originally Posted by goboatingnow View Post
But the whole Point is there is no return path if an appliance powered by a transformer live becomes live. Hence establishing a boat wide ground point actually increases the risk of electrocution you are then force to rely on RCD devices to re establish safety

Microwave internal insulation fails and ship's L1/Hot makes contact with the microwave case. AC wire gets loose, rubs against exhaust manifold, insulation melts, ship's L2/Neutral makes contact with the exhaust manifold. Now touching the microwave and pretty much any other large metal object on the boat will fry you.
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Old 06-12-2022, 13:34   #10
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How to wire and test an isolation transformer

Quote:
Originally Posted by ChrisOwens View Post
Microwave internal insulation fails and ship's L1/Hot makes contact with the microwave case. AC wire gets loose, rubs against exhaust manifold, insulation melts, ship's L2/Neutral makes contact with the exhaust manifold. Now touching the microwave and pretty much any other large metal object on the boat will fry you.
Leaving aside your fantastical fault scenario no system protects you where you insert yourself completely into the live and neutral circuit

Which I and everyone else will install a. Rcbo on thd output of the isolating transformer in this case the rcbo will trip

You install isolating transformers to create. Floating supply system to remove the fault mechanism associated associated with earth referenced mains.

Re introducing an earth reference makes nonsense of the whole idea.
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Old 06-12-2022, 13:50   #11
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Re: How to wire and test an isolation transformer

Jedi,
Thanks for posting this, it's not something that I have fully considered. But I have to agree with Goboatingnow. The isolated side is sort of like an AC battery where one can touch the L1 and earth and no current flows. Same with L1/neutral, no current will flow to earth. So why the need to create that path?
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Old 06-12-2022, 22:06   #12
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Re: How to wire and test an isolation transformer

Jedi,

Thank you for posting the diagrams for different systems; they are very helpful.

For a 120 volt system with an isolation transformer, 30 amp double-pole breaker at the AC panel (Blue Sea 8488), and no inverter, is this the correct diagram (see attached)?

I created this basic diagram for this scenario based on yours that has an inverter/charger.

Thanks again for your diagrams and explanations.
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Old 07-12-2022, 00:44   #13
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Re: How to wire and test an isolation transformer

I would agree with rcbos on both transformer input and output again I would query a Connon earth point especially silky if that is sea water or earth referenced.
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Old 07-12-2022, 10:41   #14
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Re: How to wire and test an isolation transformer

Since the isolation transformer is the power source in the boat. Any ground fault current must find its way back to the source. Having a common ground providing this path is a good idea IMHO
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Old 07-12-2022, 11:59   #15
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Re: How to wire and test an isolation transformer

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Originally Posted by scherzoja View Post
Jedi,

Thank you for posting the diagrams for different systems; they are very helpful.

For a 120 volt system with an isolation transformer, 30 amp double-pole breaker at the AC panel (Blue Sea 8488), and no inverter, is this the correct diagram (see attached)?

I created this basic diagram for this scenario based on yours that has an inverter/charger.

Thanks again for your diagrams and explanations.
Yes, you have moved the transformer output breaker to the breaker panel, which is okay but it is better to keep it as close to the transformer as possible, which is why I put it in the same box as the input breaker.

The idea is to keep fuses and breakers as close to the power source as possible
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