Isolating transformer

[Peanut]

6 hours ago, peterboat said:

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

Magic, they require rather hefty transformers.

[jonathanA]

8 hours ago, peterboat said:

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

I think they were actually GIs, I'm pretty sure safeshore (?) Still sell a plug in version.

[peterboat]

1 hour ago, Peanut said:

Magic, they require rather hefty transformers.

 

2 hours ago, Peanut said:

Magic, they require rather hefty transformers.

 

10 minutes ago, jonathanA said:

I think they were actually GIs, I'm pretty sure safeshore (?) Still sell a plug in version.

I did wonder about them as the ITs I had seen were large

[IanD]

12 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.

In an ideal world, perhaps -- though again you're cherry-picking one fault scenario to make your case and ignoring all the others, some of which favour it being onboard.

 

However in real life shoreline bollards don't have (big, heavy, expensive) ITs permanently fitted to them, so you'd have to keep one on the boat and lug it onto shore (heavy!) when you wanted to use it -- assuming it was high enough IP rated to be left outside in the rain (many are not), and you chained it down to stop it being nicked (valuable). Plus when somebody gets lazy or forgets they don't use it, so there is no isolation at all.

 

All of which makes this an impractical option in reality -- which is why they're almost always fitted onboard, as shown in the diagrams I posted... 😉 

[GUMPY]

Give me an example of a fault where having the IT on the boat is safer. 🤔

 

 

[TheBiscuits]

1 minute ago, GUMPY said:

Give me an example of a fault where having the IT on the boat is safer. 🤔

 

I once dropped a mobile phone in the canal if that counts.

 

Having it on the boat would have been much safer!

 

Ah, wrong TLA 🤣

[IanD]

1 hour ago, GUMPY said:

Give me an example of a fault where having the IT on the boat is safer. 🤔

 

 

Like I said, in an ideal world you're correct that having an IT on shore is preferable, with any screen connected to shore GND.

 

But for all the reasons I gave, that's not practical in most cases -- not least because having a portable one means it doesn't have to be used if it's too much bother. And there's your example, because you can't avoid using a built-in IT on the boat... 😉 

 

And if the IT is on the boat, the interwinding screen -- if there is one, which there often isn't -- should be connected to the boat GND, as should any metal case.

 

Incidentally, you suggested earlier that if a hull went live not much current would flow due to the high resistance of water so an RCD might not trip -- well, you're wrong. According to published data typical inland fresh water resistivity in a canal/marina (clay bottom) is about 100ohm-m or lower, plug in typical narrowboat sizes and water depth and distance to bank when moored and the likely resistance to ground is a few ohms (maybe 1ohm-10ohm range depending on various factors).

 

https://www.reddit.com/r/geophysics/comments/70dn6p/what_is_the_resistivity_of_fresh_water/

 

Edited July 18 by IanD

[GUMPY]

Reddit😂😂😂

 

[IanD]

31 minutes ago, GUMPY said:

Reddit😂😂😂

 

Or you could do a Google search and follow up various results including scientific papers, which are much more verbose and less easy to understand but which all come up with similar results... 😉 

 

Any significant fault-induced voltage (big enough to even give you a mild tingle) on the boat GND/steel hull -- which is big, and submerged in quite conductive water, not deionised -- will easily give enough leakage current to trip any RCD, on shore or boat.

 

Go on, man up and admit you were wrong... 😉 

Edited July 18 by IanD

[jonathanA]

That's interesting, I still can't help think that even with a properly installed on board It the screen should be connected to the shore earth. It seems to me that a primary to screen fault relies on the shore rcd triping when someone bridges boat to shore and that's fine I suppose. However adding unlikely occurrence to the already unlikely, if someone is in the water and touches the live hull they could receive a dangerous shock, potentially leading to drowning. 

 

I think its largely academic/theoretical but I can't see an obvious fault mode where screen to boat earth is preferable to screen to shore. 

 

I think the most likely fault is, as you said earlier, insulation breakdown possibly due to overloading/overheating and I can't see it matters much where the screen is connected so long as the appropriate breaker trips. 

[nicknorman]

It does depend on which failure mode you are considering, but most of the scenarios and thus the best practice points to connecting the screen to shore earth. Best to have a plastic case, so that the question of what to connect the case to doesn’t arise. Discussed in detail here…

 

http://www.smartgauge.co.uk/iso_wire.html

 

Oh dear, looks like Ian got it wrong, he will have to rewire his. Or alternatively I could be that he is right and the rest of the world is wrong, so thousands of boats will have to be retired. Yes, that is much more likely.

3 hours ago, jonathanA said:

… if someone is in the water and touches the live hull they could receive a dangerous shock, potentially leading to drowning. 

 

The thing is, they don’t need to touch the hull. Just being in a strong electric field (strong potential gradient) means that the current partially flows through them and it doesn’t take much to cause paralysis, with drowning following shortly afterwards.

[IanD]

4 minutes ago, nicknorman said:

It does depend on which failure mode you are considering, but most of the scenarios and thus the best practice points to connecting the screen to shore earth. Best to have a plastic case, so that the question of what to connect the case to doesn’t arise. Discussed in detail here…

 

http://www.smartgauge.co.uk/iso_wire.html

 

Oh dear, looks like Ian got it wrong, he will have to rewire his. Or alternatively I could be that he is right and the rest of the world is wrong, so thousands of boats will have to be retired. Yes, that is much more likely.

The thing is, they don’t need to touch the hull. Just being in a strong electric field (strong potential gradient) means that the current partially flows through them and it doesn’t take much to cause paralysis, with drowning following shortly afterwards.

Like I said, there are thousands of boats around the world with Victron ITs in them, all wired like mine -- oh yes, and not in plastic cases for good reasons. You'd better write to Victron and tell them they've been doing it wrong all these years, and you're absolutely sure of this because a website says so.

 

You can't get a strong electric field in the water without lots of volts on the hull, which will cause large leakage current to ground via the water and trip the RCD, either in the bollard or on the boat. At least, unless Ohm's Law was repealed when I wasn't looking... 😉

[GUMPY]

9 hours ago, nicknorman said:

Discussed in detail here…

 

http://www.smartgauge.co.uk/iso_wire.html

I'd forgotten that page existed. Says it all far better than I could.

 

[nicknorman]

9 hours ago, IanD said:

Like I said, there are thousands of boats around the world with Victron ITs in them, all wired like mine -- oh yes, and not in plastic cases for good reasons. You'd better write to Victron and tell them they've been doing it wrong all these years, and you're absolutely sure of this because a website says so.

 

You can't get a strong electric field in the water without lots of volts on the hull, which will cause large leakage current to ground via the water and trip the RCD, either in the bollard or on the boat. At least, unless Ohm's Law was repealed when I wasn't looking... 😉

It depends on whether you are happy to presume that the boat is plugged in via a functional shore RCD. That presumption seems to be at odds with the relevant ISO and the BSS, which require (recommend, in the case of BSS) an additional RCD  on the boat despite stuff downstream of the bollard RCD being already protected. Still, it is reassuring to know that they are wrong and you as always are right.

[David Mack]

1 hour ago, GUMPY said:

I'd forgotten that page existed. Says it all far better than I could.

 

But I notice even Gibbo says it is a nuanced decision:

"I therefore believe, on balance, mainly due to the extreme danger of the boat hull becoming live due to faulty wiring that it is safer to bond the transformer safety screen and chassis to the incoming shorepower earth."

I also note that the various fault situations Gibbo considers do not include the shore side cable chafing through so that the shore side earth wire connects to the hull - not dangerous, but with the safety screen connected to the shore side earth this would negate the point of having an IT in the first place.

[GUMPY]

37 minutes ago, David Mack said:

I also note that the various fault situations Gibbo considers do not include the shore side cable chafing through so that the shore side earth wire connects to the hull - not dangerous, but with the safety screen connected to the shore side earth this would negate the point of having an IT in the first place.

It needs no connections or transformer on the boat just a cable connected to shoreside could create that problem.

[IanD]

2 hours ago, David Mack said:

But I notice even Gibbo says it is a nuanced decision:

"I therefore believe, on balance, mainly due to the extreme danger of the boat hull becoming live due to faulty wiring that it is safer to bond the transformer safety screen and chassis to the incoming shorepower earth."

I also note that the various fault situations Gibbo considers do not include the shore side cable chafing through so that the shore side earth wire connects to the hull - not dangerous, but with the safety screen connected to the shore side earth this would negate the point of having an IT in the first place.

 

Also that this relies on a good shoreside ground connection, which is not always guaranteed. And that the transformer actually has an interwinding shield, which many don't (there are good reasons for this). The quality of the shield ground connection on the boat is in your control, the one on shore isn't.

 

And to quote from Gibbo's webpage:

 

"The hull of the boat now has 230 volts on it with respect to the actual ground outside. As this current has to travel through the water to return to earth, it is far from certain (particularly in fresh water) that sufficient current will flow to blow the incoming shorepower fuse. Obviously this situation is highly dangerous. If an RCD is fitted to the shorepower then this may well trip, but again it is far from certain. It is however highly likely. "

 

It's more than highly likely, it's damn well certain given actual values for freshwater conductivity -- which I gave but you've ignored, and presumably Gibbo didn't look up either, he just "assumed". I'm pretty sure that all UK shorepower points (in marinas and canalside) have to be fitted with an RCD for obvious safety reasons, and the current via a live hull will be way *way* bigger than needed to trip these -- and probably even big enough to trip a fuse, but this doesn't matter. Given that he got this simple fact wrong, excuse me if I don't take everything he writes as gospel... 😉 

 

2 hours ago, nicknorman said:

It depends on whether you are happy to presume that the boat is plugged in via a functional shore RCD. That presumption seems to be at odds with the relevant ISO and the BSS, which require (recommend, in the case of BSS) an additional RCD  on the boat despite stuff downstream of the bollard RCD being already protected. Still, it is reassuring to know that they are wrong and you as always are right.

 

Of course there is an RCD on the boat, as ISO and BSS require/recommend 🙂

 

Not me, Victron and other IT suppliers. I'm happy to be in the same boat (ho ho...) as thousands of other customers, using equipment designed and reviewed by experts at a reputable supplier and tested by all the certification authorities, and that they almost certainly know better than you and your guru Gibbo whose opinion -- with some basic errors in his assumptions, see above -- is that on balance it's better his/your way.

 

But if you think Victron are wrong, I suggest you write to them and point out the error of their ways -- I'm sure they'd be grateful to know that they've sold thousands of ITs with an inbuilt safely issue that you and Gibbo have spotted but they and the regulatory bodies didn't... 😉 

Edited July 19 by IanD

[GUMPY]

I wonder how many marinas have their RCD properly tested on a regular basis.

In marinas I have visited there was a case of reverse polarity in the bollard and two RCD that worked on the test button but were way out of spec plus a final one where the test button didn't work but it tripped within spec.

Yes I was sad enough to carry on RCD tester with me and I used it if I was going to be in the marina for a while.

 

[IanD]

52 minutes ago, GUMPY said:

I wonder how many marinas have their RCD properly tested on a regular basis.

In marinas I have visited there was a case of reverse polarity in the bollard and two RCD that worked on the test button but were way out of spec plus a final one where the test button didn't work but it tripped within spec.

Yes I was sad enough to carry on RCD tester with me and I used it if I was going to be in the marina for a while.

 

They should test them regularly (like appliances should be PAT tested -- a PITA for bands...) but I bet most don't.

 

An RCD tester is a good idea for general safety testing, they also report any wiring errors which as you say are not unknown 🙂 

 

However given that a live steel hull is going to leak far more than 30mA -- even allowing for paint! -- any shoreside RCD that isn't totally defunct or has contacts welded shut would still trip in the much-argued case above. Assuming the one on the boat doesn't trip, obviously... 😉 

Edited July 19 by IanD

[GUMPY]

Unless the law has changed since I retired there is no legal requirement to have any equipment PAT.

 

The law states that you have to be able to prove that the equipment is safe and PAT has become the accepted way of doing it but there are other ways.

 

PAT is simple and unless the law has changed recently you don't need to be qualified to do it, just competent. So why bands don't buy a tester and do it themselves has always been a mystery to me.

 

It always struck me as a sensible idea that boats and caravans should be tested in the same way.😲

[IanD]

52 minutes ago, GUMPY said:

Unless the law has changed since I retired there is no legal requirement to have any equipment PAT.

 

The law states that you have to be able to prove that the equipment is safe and PAT has become the accepted way of doing it but there are other ways.

 

PAT is simple and unless the law has changed recently you don't need to be qualified to do it, just competent. So why bands don't buy a tester and do it themselves has always been a mystery to me.

 

It always struck me as a sensible idea that boats and caravans should be tested in the same way.😲

 

There is no legal requirement, but many venues won't let you bring in your own equipment -- not just PA but also things like backline instrument amps -- unless you can provide proof of PAT testing.

 

(yes I know this is like ATM machines, but it's what everyone calls it)

 

We did exactly what you said (except the tester was borrowed) but it's still a PITA when you've got a lot of gear -- which we had with a 10-piece band with full PA and 6 foldback channels and lots of backline... 😞 

Edited July 19 by IanD

[blackrose]

1 hour ago, GUMPY said:

I wonder how many marinas have their RCD properly tested on a regular basis.

In marinas I have visited there was a case of reverse polarity in the bollard and two RCD that worked on the test button but were way out of spec plus a final one where the test button didn't work but it tripped within spec.

Yes I was sad enough to carry on RCD tester with me and I used it if I was going to be in the marina for a while.

 

 

About 8 years ago I was moored in a small marina on the Warks Avon. A friend on another boat with a Victron combi lost all his mains electrics when his combi stopped working. We didn't know what the issue was, it worked ok when not plugged in drawing from the batteries, but he was a liveaboard without solar so it was a bit of an inconvenience for him.

 

Eventually, we disconnected it, took it off the wall and he took it back to the place he'd bought it. I think it found it's way back to Victron who tested it and said it was fine. When he got it back we reinstalled the combi but it still didn't work with the mains plugged into the boat. He was a boating novice so he was at the point of giving up and moving back to land and I'm not an electrician so I didn't know what to do. A couple of days passed and I thought I'd plug my socket tester into one of my boat's mains sockets. It just hadn't occurred to me before that the mains supply might be faulty, but I was shocked to see about 250v - 255  on the tester.  None of my cheap Sterling equipment was affected but Victron is a bit more sophisticated and sensitive and wouldn't work at those voltages. I told the marina manager and someone came to fix it a couple of days later. 

Edited July 19 by blackrose

[IanD]

9 minutes ago, blackrose said:

 

About 8 years ago I was moored in a small marina on the Warks Avon. A friend on another boat with a Victron combi lost all his mains electrics when his combi stopped working. We didn't know what the issue was, it worked ok when not plugged in drawing from the batteries, but he was a liveaboard without solar so it was a bit of an inconvenience for him.

 

Eventually, we disconnected it, took it off the wall and he took it back to the place he'd bought it. I think it found it's way back to Victron who tested it and said it was fine. When he got it back we reinstalled the combi but it still didn't work with the mains plugged into the boat. He was a boating novice so he was at the point of giving up and moving back to land and I'm not an electrician so I didn't know what to do. A couple of days passed and I thought I'd plug my socket tester into one of my boat's mains sockets. It just hadn't occurred to me before that the mains supply might be faulty, but I was shocked to see about 250v - 255  on the tester.  None of my cheap Sterling equipment was affected but Victron is a bit more sophisticated and sensitive and wouldn't work at those voltages. I told the marina manager and someone came to fix it a couple of days later. 

 

That sounds strange, most Victron gear like inverters is fine up to 270Vac input. It needs to be because if you use an IT this has 5% stepup at no-load -- with 253V input (UK limit) this gives 266V at the inverter input -- and I saw this at Uplands marina sometimes.

 

It could be that the maximum input voltage limit was set lower than this since it's programmable...

Edited July 19 by IanD

[GUMPY]

19 minutes ago, IanD said:

We did exactly what you said (except the tester was borrowed) but it's still a PITA when you've got a lot of gear -- which we had with a 10-piece band with full PA and 6 foldback channels and lots of backline... 😞 

An afternoons work. 🤭

It wasn't my job directly  but try doing it for the largest music industry lighting company in the UK it's a full time job for at least  two people with added people at busy times.  Don't miss it one bit.

 

 

[IanD]

5 minutes ago, GUMPY said:

An afternoons work. 🤭

It wasn't my job directly  but try doing it for the largest music industry lighting company in the UK it's a full time job for at least  two people with added people at busy times.  Don't miss it one bit.

 

 

Plus the time needed to get all the band members to bring their kit in when some live miles away -- basically, most of a day that could be better spent doing other things. Twice as long as a gig but for no money... 😞 

 

And given that all the risk is to the band members not the public and we're happy not to bother, it seems pretty pointless. The only issues we've ever had have not been with our kit but with venue supply problems, one of which actually blew up our guitarist's rather nice valve amp... 😞 

Edited July 19 by IanD