Isolating transformer

[Brian Davis]

Hi I am looking at fitting an isolating transformer for mooring at the marina. Does the shoreline earth terminate (finish) at the transformer or carry on through. 

[Tony Brooks]

It finishes at the transformer, otherwise it would not isolate the hull from shore side earth voltages. The boat's earth is the derived from the transformer outlet neutral, so it is totally isolated from the shoreside mains wiring. It is important to ensure the earthing screen in the transformer is connected to the correct side. I understand that Victron got that wrong on some units.

[Brian Davis]

Great thanks 

[Tony Brooks]

It is also important to know which side any metal casing is earthed to unless it is double insulate in an outer metal case, because if it is the shoreside earth then mounting the transformer to, or standing it on, metal could allow the hull to be at incoming earth potential, which is not what you want.

[IanD]

48 minutes ago, Tony Brooks said:

It finishes at the transformer, otherwise it would not isolate the hull from shore side earth voltages. The boat's earth is the derived from the transformer outlet neutral, so it is totally isolated from the shoreside mains wiring. It is important to ensure the earthing screen in the transformer is connected to the correct side. I understand that Victron got that wrong on some units.

Victron got it right for the usual case (like mine) where the isolation transformer is mounted on the boat (screen/case connected to boat GND).

 

IIRC they actually say in their data sheet that for the rare case where it's shoreside, there's an option to get it connected the other way round.

Edited July 16, 2024 by IanD

[GUMPY]

If the screen is connected to the boat side then in the unlikely event of a transformer insulation meltdown the boat could go live.

If it's connected to the shore it can't.

Best option is screen connected to shore side and transformer in a plastic case. It was this that led to the Airlink Transformer being in a plastic case.

 

[IanD]

12 minutes ago, GUMPY said:

If the screen is connected to the boat side then in the unlikely event of a transformer insulation meltdown the boat could go live.

If it's connected to the shore it can't.

Best option is screen connected to shore side and transformer in a plastic case. It was this that led to the Airlink Transformer being in a plastic case.

 

Why don't you offer to act as technical advisor to them, I'm sure they'd be grateful -- because they obviously don't have any electrical engineering expertise of their own, in spite of selling huge quantities of gear to the marine market... 😉 

 

If you have anything in a metal case on a boat -- like a Victron isolation transformer -- the case should be grounded to the boat GND for the usual safety reasons.

Edited July 16, 2024 by IanD

[GUMPY]

Did I mention a metal case? 

No

I mentioned the interwinding screen.

 

A transformer mounted on a boat should be in an insulated case not a metal one.

I pointed this out to Airlink nearly 20 years ago and they then produced the plastic cased transformer.

 

 

 

 

Edited July 16, 2024 by GUMPY

[Brian Davis]

Thanks everyone now a lot clearer about the earthing 

[IanD]

14 hours ago, GUMPY said:

Did I mention a metal case? 

No

I mentioned the interwinding screen.

 

A transformer mounted on a boat should be in an insulated case not a metal one.

I pointed this out to Airlink nearly 20 years ago and they then produced the plastic cased transformer.

 

I hope you and Airlink are very happy together then... 🙂

 

Meanwhile, this is what Victron say for their ITs -- which come in metal boxes, like all their other big heavy (31kg for mine) kit:

 

"By connecting all metal parts to the neutral output on the secondary side of the transformer, a GFCI will trip or a fuse will blow in case of a short circuit."

 

That includes the interwinding screen.

 

I expect their technical experts are well aware of all the possible failure modes of such a device, and chose this configuration for very good safety reasons. If you disagree, don't buy one 🙂 

[GUMPY]

2 hours ago, IanD said:

That includes the interwinding screen.

I missed where it said that  can you point it out please.

Can you also explain to me how an RCD on the secondary of the transformer will trip when the fault (failure of the transformer making the screen go live) is before the RCD? 

 

I might add the chance of a catastrophic failure of the transformer resulting in a live screen is very rare. I have seen one in a lifetime of using transformers 

Edited July 17, 2024 by GUMPY

[IanD]

3 minutes ago, GUMPY said:

I missed where it said that  can you point it out please.

Can you also explain to me how an RCD on the secondary of the transformer will trip when the fault (failure of the transformer making the screen go live) is before the RCD? 

 

Thanks

It doesn't say that, the drawing shows it (green/yellow dashed line).

 

I presume the RCD (GFCI) they refer to is the one on the bollard, which AFAIK they're all equipped with for safety reasons, being outside and near water. If the screen goes live and therefore the boat hull does, the ground fault current will be plenty high enough to make this trip. I also assume Victron have thought about this, since they have to meet all the international safety standards... 😉 

Edited July 17, 2024 by IanD

[GUMPY]

Please show me the fault path that will trip the bollard RCD as it's not obvious due to the resistance of fresh water 😲

[IanD]

32 minutes ago, GUMPY said:

Please show me the fault path that will trip the bollard RCD as it's not obvious due to the resistance of fresh water 😲

You do know about capacitive currents, I assume? And also the fact that if you're standing on a live boat *without touching anything on shore* you'll feel no effects whatsover? At least unless you do touch the shore and the boat at the same time, in which case the RCD will trip just like any other earth leakage fault.

 

An IT wired like this is universally adopted as the safest solution; if you disagree, you should be arguing with all the safety authorities worldwide who set the rules for device certification, not me... 🙂 

 

Edited July 17, 2024 by IanD

[GUMPY]

This appears to be a yearly discussion 

Last done August last year so I'm out

BTW there is no interwinding screen in that diagram🤭

[IanD]

2 minutes ago, GUMPY said:

This appears to be a yearly discussion 

Last done August last year so I'm out

BTW there is no interwinding screen in that diagram🤭

But there is in the other one I posted. You do realise these are descriptive drawings, not accurate wiring system schematics?😉 

 

Would you like a list of marine/electrical approval authorities so you can write to them and tell them that Victron ITs are dangerous? 🙂 

 

(yes you've ridden this particular hobby-horse before, and fell at the same fence...)

[Peanut]

Diagrams show the earth connected to the hull, which is all well if you have a steel boat. With a yoghurt pot, do you need a grounding plate on the outside?

 

Or do you have a centre tapping on the transformer, or some other connection?

[IanD]

9 minutes ago, Peanut said:

Diagrams show the earth connected to the hull, which is all well if you have a steel boat. With a yoghurt pot, do you need a grounding plate on the outside?

 

Or do you have a centre tapping on the transformer, or some other connection?

With a plastic (or wood) hull all the equipment neutrals connect together and back to the IT neutral, all the equipment grounds (including metal cases) connect together and back to the IT ground, and the ground and neutral are linked at the IT -- pretty much like a house installation on land in a non-metal building with neutral-ground bonding at the consumer unit...

Edited July 17, 2024 by IanD

[jonathanA]

Hmm I think its a fair question by Gumpy. I am struggling to see how a shore RCD would operate with the screen connected to boat "earth".  I note that Gibbo on the smartgauge site advocates screen connected to shore earth.   

I'm not picking a fight just trying to learn and check my understanding of how these things work.  Putting cards on table I don't need either a GI or IT as I'm off grid. Although do have a GI for those times we have access to shore power. 

 

Accepting the failure mode of primary to screen is pretty unlikely but if it did.....

 

 

[IanD]

23 minutes ago, jonathanA said:

Hmm I think its a fair question by Gumpy. I am struggling to see how a shore RCD would operate with the screen connected to boat "earth".  I note that Gibbo on the smartgauge site advocates screen connected to shore earth.   

I'm not picking a fight just trying to learn and check my understanding of how these things work.  Putting cards on table I don't need either a GI or IT as I'm off grid. Although do have a GI for those times we have access to shore power. 

 

Accepting the failure mode of primary to screen is pretty unlikely but if it did.....

 

 

Either primary to screen or secondary to screen is indeed an unlikely failure mode, most toroidal transformers have a double layer of insulation between windings (and windings and screen if they have one). A turn-to-turn short within a winding is a more likely failure mode.

 

The circuit is symmetrical, so whichever side the screen is grounded to, a short to a winding (which one?) will either blow a fuse or mean the boat GND has a voltage on it. Which is fine so long as you're on the boat; if you form a bridge to shore then the RCD will trip, just like in a house.

 

There are other reasons to connect the screen to the boat ground, especially in a metal-cased IT -- which Gumpy doesn't like, but is what Victron use, possibly because a plastic case isn't robust enough for something that weighs 30kg or more.

 

The advantage of an IT over a GI is much better isolation, especially with long cable runs (e.g. in marinas) where there may be significant voltage on the incoming neutral or GND, and the fact that if a GI fails it usually fails short-circuit which is not detected -- except it no longer works as a GI. However a GI is much cheaper than an IT, which is probably why most canal boats use them...

Edited July 17, 2024 by IanD

[Guest]

2 hours ago, Peanut said:

Diagrams show the earth connected to the hull, which is all well if you have a steel boat. With a yoghurt pot, do you need a grounding plate on the outside?

 

Or do you have a centre tapping on the transformer, or some other connection?

My Broads cruiser had one, bolt on the inside to a plate on the outside, all earths on the boat terminated on it. That included the one to the inverter. Now it was under the floor and fitted out in the 80s so don't think it was ever messed with

[blackrose]

28 minutes ago, IanD said:

The advantage of an IT over a GI is much better isolation, especially with long cable runs (e.g. in marinas) where there may be significant voltage on the incoming neutral or GND, and the fact that if a GI fails it usually fails short-circuit which is not detected -- except it no longer works as a GI. However a GI is much cheaper than an IT, which is probably why most canal boats use them...

 

If a GI fails it should fail short circuit to protect those onboard and anyone else who happens to touch the boat. Some non AYBC compliant GIs may fail open circuit so they're protecting the steel of the boat rather than the people. In this event the boat owner might be dead but at least their boat won't suffer from galvanic corrosion. 

 

Anyway, if an AYBC compliant GI has a status monitor and it's looked at occasionally then both of the disadvantages you mention above are ameliorated. That's why I fitted one. Also I'm largely off-grid these days.

[GUMPY]

Being really picky an IT should be at the supply and not on the boat.

Think of the scenario when the IT is on the boat and the cable going to the boat gets damaged and shorts to the boat, if it's the earth then you lose your galvanic  protection and you might never know.

Edited July 17, 2024 by GUMPY

[Brian Davis]

Thanks for the information 

[Guest]

2 hours ago, GUMPY said:

Being really picky an IT should be at the supply and not on the boat.

Think of the scenario when the IT is on the boat and the cable going to the boat gets damaged and shorts to the boat, if it's the earth then you lose your galvanic  protection and you might never know.

They were selling inline ITs in the cables on an advert I saw a couple of years ago?