Isolation transformers

danielefua

danielefua

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It probably already happened but I'd like to trigger a discussion on isolation transformers. I plan to install one because it seems a very sensible thing to do for a number of reasons and I'd like to hear people on the following questions: toroidal or not, switching (a quite recent possibility), brands and costs, internal screens, hums, rust, cases, etc etc and whatever comes into your mind.
I appreciate your cooperation
Daniel
 
One thing I learnt from an old friend (ok he wasn't old at the time...) always overrate your transformer ... and all associated cabling.... mind u - this was for an audio amplifier - but it never ran out of power!!
 
I've just bought one so can offer a bit of help here. If I've been informed correctly and done enough research, the following are the pertinent facts:

1/ The torroidal ones are about 20% lighter but are limited in capacity. If you want a large capacity (to use the 32A shorepower) you can't get a torroidal.
2/ There's [--word removed--] all difference between the marine ones (except the price of course) and having a custom one made by a transformer manufacturer.
3/ You need a soft start to prevent blowing showpower.
4/ You can get them to step up/down so if you there a chance that you might go to somewhere with 100/120 electric, get one with a second 'leg' on the input.
5/ Victron, amongst others, do some good marine versions. I found a company that makes them for Sunseekers and got them to make one to my spec (about 30-50% of the cost of an 'official' marine version)
6/ They are bloody heavy - I can only just lift the one I've got.
7/ Humming etc shouldn't be noticable if it's in a locker etc.
8/ If you've got a decent frame transformer, it shouldn't need venting with a fan - air vents in the casing will suffice
9/ I was told switch mode wasn't possible for the size I wanted.
9/ Get one that's continously rated for the load you are going to draw.
10/ It's mounted in a metal case that's been painted with hammerite
11/ Size: the case for my 50A output one is about 16*16*12 inches
10/ You don't need a galvanci isolator if you've got an isolation transformer - BUT, it's not going to stop electrolysis from other boats around you using shorepower (nor is an isolator)
11/ Good practice to fit an MCB on the input (overload cutout).

Hope that helps.

Nigel
 
Tank you very much Nigel. You do indeed answer all my questions and more in an extremely helpful way. I wonder if there someone out there who has other ideas on your almost exaustive list of points.
Regarding the other people who answered: 1) my question was written in plain english - I am afraid - even if it is not my mother language so that I don't know what "flippinek" means. 2) installing an isolation (or isolating, I believe both are correct) transformer is a pretty sensible choice to avoid a possible source of electrocution when a shore power circuit is badly wired as it often happens (last year it happened to me). Moreover it avoids the chance of stray currents from your boat to shore even if, as Nigel writes, it does not avoid the influence of possible galvanic currents from other boats.

Thanks to all, with friendship
Daniel
 
Nothing to add to Nigels post, butI was a bit concerned about your comments regarding electrocution.

Despite the isolation transformer you are still dealing with a potentially lethal voltage. You reduce but not eliminate the Live-Earth potential but 240v is still dangerous and must be treated with some respect.

But I'm sure you appreciate that.

BTB

Your English is far better than my Italian, and I suspect other poster's Italian.
 
240VAC are to be treated with a lot of respect, you're perfectly right! My experience was the following: for a reason that can be explained but is not at all forgivable, in the marina where last year I kept my boat, both power wires were live: no neutral/live but live/live. Their tension with respect to the marina "ground" wire was 120VAC, half their nominal tension. This is already a discouraged situation but, even worse, the "ground" wire was at 120VAC with respect to the sea. Notice that all the marina outlet have differential breakers but, correctly, they would not trip unless there are other problems.
I accidentally noticed it because I have several 240VAC outlets in the boat whose ground is connected to the shore ground and not to the boat ground and I got a shock touching at the same time the case of an AC appliance and the sink.
I am sure that the marina had this problem for long time but none ever realized it: a powerful and silent stray current from most boats that no breaker or gauge ever sensed.

Daniel
 
One comment from bitter experience:

Never size anything critical like this to your maximum current draw. Add in as big an expansion factor as you can. I'd suggest at least double if at all possible.

Perhaps 20 years ago, a certain Mr Gates designed a limit of 640K memory in his operating systems 'cos that was as much as anyone could possibly ever need...
 
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