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Old 12-11-2022, 17:45   #1
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Step down transformer for isolation on boat.

I have a steel hull vessel with 240VAC 50Amp 60Hertz connection currently with no isolation transformer. I know of another similar boat that is using a step down transformer for isolation. The transformer is a single phase insulated dual coil that can be configured for 240VAC input and 240VAC output. This is possible since it is a dual coil transformer.
I also have one of these step down transformers that I have been testing in the shop and I am considering using in the boat for shore power isolation.
Attached is the name plate of the transformer I have.
I would like to ask the community what, if any, downsides are there to using a step down transformer for isolation in a marine environment?
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Old 12-11-2022, 19:04   #2
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Re: Step down transformer for isolation on boat.

The first thing to realize is that there are no ABYC-approved isolation transformers in production at this point since Charles Industries discontinued production last year.


The ABYC standard requires a grounded metal shield between the primary and secondary windings. This allows a GFCI/RCD in the primary to detect and trip in the case of any insulation failure in the primary that would undermine the isolation provided by the transformer.


Toroidal transformers made to European standards, including those sold by Vicrtron, don't have such a shield. Neither do the general-purpose industrial step up/step down transformers like the one you are considering.


Whether the level of isolation provided by such transformers is sufficient, is a judgement call upon which experts disagree.
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Old 12-11-2022, 19:08   #3
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Re: Step down transformer for isolation on boat.

Just because some guy on the dock does it, doesn't mean it is right. Even if he has a boat like yours (which of course means he is smarter than average).

The FIRST question is...

Is the shore power side safety ground ACTUALLY fully galvanically isolated from the output ground? That's what makes it an "isolation" transformer. Which safety ground is connected to the case?

It is not unusual for the neutral sides to be cross connected in a "regular" transformer which would defeat any attempt at isolation. We had a simple step-up transformer on our boat that was like this. It has since been replaced with a "proper" marine isolation transformer.

There are other technical details for the internals of a proper isolation transformer. Read the subject wikipedia page for a start.
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Old 12-11-2022, 21:18   #4
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Step down transformer for isolation on boat.

The ABYC shield requirement is rather nonsensical. As most electro static shields are not designed as an electrical fault protection .Hammond a major Uk transformer maker confirmed to me that they would regard the ABYC requirement as dangerous as a safety shield is completely different then that normally provided as an electro static shield.

( the standard electro static shield is not rated for full input current )

Note that Hammond for example can add a electro static shield if needed but its not a safety device.

The transformer you reference will be more then sufficient for the intended purpose, of course there should be no connection between input L N and earth and output lines

RCBOs on input and output will protect against shock faults or overcurrent including primary secondary break-over

Isolation transformers can be best mounted in a strong ABS case hence no earth wiring is involved , shore power earth in this case may never need to be brought any further turn the shore power AC input socket.
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Old 12-11-2022, 21:41   #5
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Re: Step down transformer for isolation on boat.

Are you going to turn the boat into a 100a 120v boat?

You don’t want a step down transformer. You want a center taped 1:1 transformer so you can get 120/ 240v out. You need 3 wires (plus ground) from the iso transformer.

If you ran the series way and center tapped you’d only have 60v phases.

Hubbell might be the only one making a 50a 120/ 240v iso transformer now.

https://hubbellcdn.com/literature/Wi...ansformers.pdf
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Old 12-11-2022, 22:00   #6
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Re: Step down transformer for isolation on boat.

The only problem with it is it is not a true isolation transformer. But have your favorite electrician check it out and make sure that the secondary is actually isolated from shore power, including ground. You want the secondary circuit to be the point of origin for your boat's AC electrical system. Otherwise you are better off with another solution for isolation.

I have a Charles isolation transformer, rated for 50Hz as well as for 60hz, with multiple hookup configurations available, handy if I should ever sail to a place that does not use the US power standard. If I ever sell the boat, I'm taking it with me! I didn't know Charles is no longer making them.
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Old 13-11-2022, 00:48   #7
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Step down transformer for isolation on boat.

Quote:
Originally Posted by GrowleyMonster View Post
The only problem with it is it is not a true isolation transformer. But have your favorite electrician check it out and make sure that the secondary is actually isolated from shore power, including ground. You want the secondary circuit to be the point of origin for your boat's AC electrical system. Otherwise you are better off with another solution for isolation.

I have a Charles isolation transformer, rated for 50Hz as well as for 60hz, with multiple hookup configurations available, handy if I should ever sail to a place that does not use the US power standard. If I ever sell the boat, I'm taking it with me! I didn't know Charles is no longer making them.


How can you say its not a true isolation transformer no galvanically isolated transformer has secondaries bonded to primaries . I humbly suggest you not on reliable ground here.

Looking at the specs says its absolutely a isolating transformer.
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Old 13-11-2022, 05:21   #8
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Re: Step down transformer for isolation on boat.

Quote:
Originally Posted by goboatingnow View Post
Isolation transformers can be best mounted in a strong ABS case hence no earth wiring is involved , shore power earth in this case may never need to be brought any further turn the shore power AC input socket.
I understand the logic of this when afloat, but my understanding is that the AC system on the boat must be connected to the shore side safety ground when on the hard to comply with most local electric codes. My isolation transformer includes jumpers for this.

My understanding of the logic is to prevent any part of the boat that could be touched from outside (say the prop shaft) being at anything other ground potential. Not that every boat complies with all local electric codes...
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Old 13-11-2022, 05:40   #9
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Re: Step down transformer for isolation on boat.

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Originally Posted by smac999 View Post
You dont want a step down transformer. You want a center taped 1:1 transformer so you can get 120/ 240v out. You need 3 wires (plus ground) from the iso transformer.

If you ran the series way and center tapped youd only have 60v phases.



The wiring diagrams on the nameplate are just examples. He can run the primary in parallel at 240 volts and the secondary center tapped in series. The boat will then require 240 volt shore power, but it's not unheard of for larger boats to be wired that way.
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Old 13-11-2022, 09:07   #10
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Step down transformer for isolation on boat.

Quote:
Originally Posted by ItDepends View Post
I understand the logic of this when afloat, but my understanding is that the AC system on the boat must be connected to the shore side safety ground when on the hard to comply with most local electric codes. My isolation transformer includes jumpers for this.



My understanding of the logic is to prevent any part of the boat that could be touched from outside (say the prop shaft) being at anything other ground potential. Not that every boat complies with all local electric codes...


Well In my case the presence of RCBOS removes that requirement.

Nor is your logic correct te on the hard as there is no connection to on board Metals
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Old 13-11-2022, 10:30   #11
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Re: Step down transformer for isolation on boat.

Thank you all for contributing to the discussion.
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Old 13-11-2022, 12:48   #12
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Re: Step down transformer for isolation on boat.

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Originally Posted by goboatingnow View Post
Well In my case the presence of RCBOS removes that requirement.

Nor is your logic correct te on the hard as there is no connection to on board Metals
Whether his logic is correct or not, it is technically a requirement of the NEC for the grounding conductor at each AC outlet to be connected to a system ground that has a ground electrode (such a rod driven into the earth or some other approved method). The requirement is present regardless of whether there are metals anywhere in the area. As you may be aware, the NEC is the electrical code in force in most of the states in the USA. When shore power is used, the ground conductor of the shore power connection provides this, because the shore power circuit is required to be grounded.

The grounding requirement also exists for other separately derived sources. If you have an inverter or generator, you're supposed to drive a ground rod, and it is actually done more often than not in large (non-boat related) temporary portable power installations such as construction sites and special events.

For boats on the hard, I doubt if this is an area of enforcement interest for the various code enforcement authorities, of which there are quite a patchwork. Whether a boat on the hard is included in their jurisdiction is an interesting question. Whether a boat afloat is included in their jurisdiction is something I would prefer they don't think much about.
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Old 13-11-2022, 19:23   #13
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Re: Step down transformer for isolation on boat.

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Whether his logic is correct or not, it is technically a requirement of the NEC for the grounding conductor at each AC outlet to be connected to a system ground that has a ground electrode (such a rod driven into the earth or some other approved method). The requirement is present regardless of whether there are metals anywhere in the area. As you may be aware, the NEC is the electrical code in force in most of the states in the USA. When shore power is used, the ground conductor of the shore power connection provides this, because the shore power circuit is required to be grounded.

The grounding requirement also exists for other separately derived sources. If you have an inverter or generator, you're supposed to drive a ground rod, and it is actually done more often than not in large (non-boat related) temporary portable power installations such as construction sites and special events.

For boats on the hard, I doubt if this is an area of enforcement interest for the various code enforcement authorities, of which there are quite a patchwork. Whether a boat on the hard is included in their jurisdiction is an interesting question. Whether a boat afloat is included in their jurisdiction is something I would prefer they don't think much about.
Jammer,

Thank you for understanding my question and re-articulating it in a more technically coherent form.

I certainly did not mean to suggest that there were inspectors wandering around the boatyards looking for boats not properly grounded. It's just that given a choice, I prefer to follow the codes because they have been refined over the years to consider an awful lot of unusual cases.

I can follow standards very well, and certainly have enough understanding of electricity on a practical and theoretical level to be able to understand all the everyday issues that come up. But I do rely on the standards organizations be it ABYC or NEC to think deeper about those oddball cases that are not normal and can cause unexpected problems.

In general, I find that people who blow off the standards organizations are not really impressing me with their clear thinking. "I know better than all those idiots on the expert committee..." Yeah--maybe...
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Old 13-11-2022, 20:45   #14
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Re: Step down transformer for isolation on boat.

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Nor is your logic correct on the hard as there is no connection to on board Metals

There is no connection to on board metals when everything is in its proper place and working as it should be.

Shoreside electrical wiring codes are intended to provide protection in cases where something is not in its proper place or working as it should be, for example, if insulation fails and/or a wire comes loose, and an energized conductor makes contact with metal ductwork, railings, piping, structural elements, appliance cases, etc.
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Old 13-11-2022, 21:28   #15
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Re: Step down transformer for isolation on boat.

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In general, I find that people who blow off the standards organizations are not really impressing me with their clear thinking. "I know better than all those idiots on the expert committee..." Yeah--maybe...

I agree wholeheartedly.


The NEC was written with the ashes of buildings that burned down and with the blood of building occupants, first responders, and electricians who died as a consequence of poor wiring practices. Its knowledge was gained at a terrible price and is not the mere excogitations of the learned and the fools. It works as a system both internally and in concert with typical North American building practices and traditions.


As an aside, there are other electrical codes that differ considerably, most notably that in England. As different as the English practices are from those in the USA they work well as a system and in concert with the very different overall building norms and practices in that country.


That said, the NEC has its limitations in certain novel and specialized areas where it is perhaps not originally intended to apply or where it is perhaps not thought through. What to do with the bonding jumper in an isolation transformer is one of these unusual cases.


A guiding principle of the NEC is that it should never be possible to create an unsafe situation using operating controls -- switches, breakers, plugs, and so on. So there can't be a three-way switch to lift the ground from the isolation transformer's neutral output (where it is connected while the boat is in the water) and bond it to the shore power ground (where it is supposed to be connected while the boat is on the hard), because setting the switch wrong would create an unsafe condition and there's no way to make an interlock. As a result there has to be a jumper behind a closed panel held shut with fasteners that require tools to remove, to do this in a compliant way.


I don't think it's worth it, at least not for boats that are on the hard seasonally for storage, that have shore power to charge the batteries and maybe run a dehumidifier.



But in any case, that jumper can be wired regardless of whether the transformer is one specifically manufactured to be an isolation transformer. The qualified person wiring it just has to know how to do it.
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