How to wire and test an isolation transformer

[Statistical]

Originally Posted by Colin Stone

Your diagram and words are fine. I was only after an explanation of why the Wing book has different case ground options.
This is the top of my transformer. The shield is connected to shore. The laminations are isolated, the frame is connected to case the which is now connected to ship ground. The case is electrically isolated from the ship steel work - infinity insulation resistance at 500v on megger.
Saving a couple of kg with a Victron in a 70 ton barge is not much of an issue!

There is more than one diagram because there is more than one way to do it when talking about the concept of an isolation transformer in an abstract sense which goes beyond boat installations.

The one thing you can't do is have a connection between shorepower and boat. So if the shield is grounded to shorepower it isn't grounded to the boat. If it is grounded to the boat it isn't grounded to shorepower.

That is it. It is that simple. Now isolation transformers on a boat produce a unique challenge in that at times you are not connected to shorepower so when it comes to the case while in theory you could have the case grounded to EITHER shorepower or boat (but not both) the former has the disadvantage of meaning the case is not grounded when disconnected from shorepower which is why ABYC requires the later.

Case grounded to boat.
Shield (if present) grounded to shorepower.
None of the ground or current carrying conductors on shorepower and boat sides have any direct connection internally or externally.

[Jammer]

Originally Posted by s/v Jedi

My diagrams show 9.33, i.e. the input terminal ground connects to shore ground and the output terminal ground connects to ships ground.

ABYC talks about an internal shield around the primary winding only, which connects to shore ground. These are a rarity but not surprisingly manufactured by some ABYC members who contribute to writing these recommendations

I do not believe there have been any on the market in the typical small-sailboat sizes (3kva +/-) since Charles Industries left the market a year or two ago.

Such transformers have been replaced by toroidal transformers that are far better and use insulation rather than metal shields.

I realize you're simplifying but... ABYC-spec transformers do of course use insulated wire. The difference is that the windings are not interleaved as they are on a toroidal transformer so it is feasible to design a safety shield between the windings so that if the insulation should fail the shore power breaker or GFCI will trip. Toroidal transformers also have more capacitive coupling between windings (since they are interleaved to reduce the magnetic flux through the toroid) so there is more common-mode AC leakage.

Absent such a shield there isn't any reason to bring the shore ground aboard at all IMO.

I think that it is hard to make the case that the neutral shouldn't be bonded to boat ground on the boat side of the transformer. If it is not then the ground will end up floating at around half of line voltage -- halfway between ground and neutral -- due to accumulative leakage currents throughout the boat. Wiring practice in nearly all applications has over the course of decades evolved towards bonding one of the conductors or an artificial midpoint (like the zigzag transformers used in 480 volt industrial applications) based on actual safety experience. The exceptions are exceptional and involve some kind of continuous or periodic (e.g. monthly) testing to confirm that leakage currents are within specification.

I think the ABYC guidelines as written are discouraging wider use of isolation transformers by driving up costs and complexity to an unreasonable degree.

[Statistical]

Originally Posted by Jammer

I think the ABYC guidelines as written are discouraging wider use of isolation transformers by driving up costs and complexity to an unreasonable degree.

Perfect being the good of enemy. Simpler cheaper isolation transformers would mean more boat use them with both the safety and stray current corrosion protection that brings.

[Colin Stone]

>former has the disadvantage of meaning the case is not grounded when disconnected from shorepower which is why ABYC requires the later.

I'm surprised that the Wing book doesn't simply explain that.

My Jemelec wasn't overly expensive, but I accept more than a GI.

Bonding N to E on a boat at each power source is no different to my house.

[s/v Jedi]

Originally Posted by Colin Stone

Your diagram and words are fine. I was only after an explanation of why the Wing book has different case ground options.
This is the top of my transformer. The shield is connected to shore. The laminations are isolated, the frame is connected to case the which is now connected to ship ground. The case is electrically isolated from the ship steel work - infinity insulation resistance at 500v on megger.
Saving a couple of kg with a Victron in a 70 ton barge is not much of an issue!

Very good

The problem with things people write or say about this is that you don’t know what level of expertise they have. Many here on CF are very vocal on this subject (almost as vocal as me ) but have no engineering degree at all, let alone in electrics. They can have 40 years experience as an installer, done all the ABYC courses (which is like a brainwashing followed by brain programming operation) and write books about it while still not knowing why things are done in certain ways and why alternatives exist and can be better options.

I often get the “but ABYC says…” reaction which is to be expected but as long as most boats don’t have isolation transformers, they don’t write papers about floating power distribution. I know they are working on it now because they asked me to help but unfortunately I don’t have the time to spare.

[JoeRobertJr]

Jedi,
From your very first post in this thread...

Originally Posted by s/v Jedi

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.

That breaker box looks perfect for my planned isolation transformer installation. Do you have a pointer to a source?

[s/v Jedi]

Originally Posted by JoeRobertJr

Jedi,
From your very first post in this thread...





That breaker box looks perfect for my planned isolation transformer installation. Do you have a pointer to a source?

Here it is: https://www.amazon.com/gp/product/B07S94Z7CF

You need to buy DIN rail breakers and terminals to fit this, all available on Amazon.

[JoeRobertJr]

Originally Posted by s/v Jedi

Here it is: https://www.amazon.com/gp/product/B07S94Z7CF

You need to buy DIN rail breakers and terminals to fit this, all available on Amazon.

Great, thanks! And thanks for this excellent thread! The discussions have been very helpful.

[Colin Stone]

I've just fitted one of these on the incoming supply.

63A Earth Leakage Over Under Voltage Protector Relay Circuit Breaker Surge Protection Energy Power kWh Meter Multimeter 230VAC

https://www.aliexpress.com/item/1005006045526583.html

[s/v Jedi]

Originally Posted by Colin Stone

I've just fitted one of these on the incoming supply.

63A Earth Leakage Over Under Voltage Protector Relay Circuit Breaker Surge Protection Energy Power kWh Meter Multimeter 230VAC

https://www.aliexpress.com/item/1005006045526583.html

That is good, now you can insert an isolation transformer between that relay and the rest of the boat so that you protect underwater metals as well as the crew and swimmer around the boat so they can’t be electrocuted by your boat

Remember, nothing can replace an isolation transformer when you have shore power. The only thing that is better is no shore power at ll.

[Colin Stone]

Yes, I've had an IT since 2004. This breaker replaces the original simpler breaker.

[s/v Jedi]

Originally Posted by Colin Stone

Yes, I've had an IT since 2004. This breaker replaces the original simpler breaker.

That’s great

ABYC recommends to use a ground fault protection breaker for the shore power inlet (after decades of not doing so while the rest of the world already did) but makes an exception for installations with isolation transformer for which a regular double pole breaker is all you need.

The breaker you installed adds extra features like surge suppression, which is great.

After we got our slip in Florida, with good shore power but lots of lightning storms, I added a 50A 12kW shore power inlet to Jedi and this connects to this contactor/monitor/protection device: https://www.amazon.com/gp/product/B003AN1UA8

I had used one for years in a motorhome where it performed very good. As my isolation transformer is only 3.6kW, I added a galvanic isolator to this inlet.

Of course this all happened before we went big with solar and we almost never visit marinas anyway. Last time was in Nassau en we didn’t use shore power at all as our solar provided all we need even for A/C.

The only reason for me to use shore power now, even in our own slip, is when I am using lots of power tools during our current renovation aboard.