Isolation Transformer

[Chris Lingwood]

I remember reading here about a isolation transformer from RS (436-8805) which at 1650VA should give me 7amps continuous (3000va peak for 15 minutes). I have a 5 amp fuse in my shoreline anyway and have only blown it once when I hit both switches on the fan heater by accident so that's plenty.

 

Some one did point out another potential problem with it (other than the potentially low capacity). The problem is I think its in the galvanic isolator thread and I don't really want to venture back in there. I think it was to do with the way that it is earthed. I don't want to buy something that costs loads (the Victron one) if I don't have to, but equally if it doesn't do the job then I might as well not fit it.

 

I've found the manufacturers website so I should be able to find the answer to any questions from there http://www.carroll-meynell.com/

 

I seem to remember a forum member was trying to get a cheaper isolation transformer sourced but I can't remember who it was...did they have any joy in the end?

 

Some boats in my marina are coming out with some odd deposits on them so there might be a problem with the wiring. I thought I'd investigate this again as I'm redoing some of my wiring anyway and galvanic isolators are debatable at best (PLEASE DON'T DEBATE THEM!!!! PLEEEEEEEAAAAAASSSSSSEEEEE!!!!).

[chris w]

1. Take note that the Isolation Transformer (IT) you're looking at is rated at 1650VA NOT 1650 Watts.

 

2. The frame of the IT should be earthed to the shore NOT the boat

 

3. The neutral on the boat side of the IT must be earthed (ie: bonded to the hull at or very near to where the battery negative is bonded) in order to make your RCD's function correctly

 

4. RS stock the one you highlight at £85 + VAT (part no. 436-8805)

 

5. Galvanic Isolators are not "debatable at best", they are excellent at best.

 

Chris

Edited July 18, 2007 by chris w

[Gary Peacock]

The Victron range of units is the cheapest on the market. (Some like them some moan about them so I aren't getting involved in that debate.)

 

Mastervolt are a bit more expensive.

 

This is a Marine Isolation Transformers for them from another company but I have no details of prices.

 

We have a customer who built his own (But his business was in that market.) afterwards I think the price was very near the Victron without the work put into designing and building it. I could ask him next time I see him for a price to build two or three if there was the interest.

[carlt]

4. Galvanic Isolators are not "debatable at best", they are excellent at best.

 

Chris

For some people. Useless for others.

[chris w]

The Victron range of units is the cheapest on the market.

 

The Victron equivalent to the RS components (Carroll-Meynell) isolation transformer is £350 + VAT rather than £85 + VAT so they're hardly the "cheapest on the market"

 

Chris

 

 

For some people. Useless for others.

 

 

I can make you a nice solid oak version for your wooden hull

Edited July 18, 2007 by chris w

[carlt]

The Victron equivalent to the RS components (Carroll-Meynell) isolation transformer is £350 + VAT rather than £85 + VAT so they're hardly the "cheapest on the market"

 

Chris

I can make you a nice solid oak version for your wooden hull

Phew you got the joke I thought it was all going to kick off again.

 

That'd be great, the balsa wood anodes are working a treat!

[Chris Lingwood]

1. Take note that the Isolation Transformer (IT) you're looking at is rated at 1650VA NOT 1650 Watts.

I noticed that. I was under the impression that this is only of significance when there is a low power factor. My 5 amps have proved to be plenty which is 1150W so with 1650VA that allows for a power factor of 0.7? That sounds like plenty of headroom bearing in mind I probably very rarely use that 5amps.

 

Having said that the wiki of pedia suggests that for switch mode power supplies the factor could be as low as 0.55. And most of my loads I think are switch mode apart from my hifi amp.

For example, SMPS with passive PFC can achieve power factor of about 0.7–0.75, SMPS with active PFC, up to 0.99, while SMPS without any power factor correction has a power factor of only about 0.55–0.65.

 

They are however very recent so may well have either passive of active PFC. My computer for instance has:

-Active PFC and high efficiency design for superior environmentally friendly operation

so its power factor might be about 1ish...which is good! Dunno about the sterling charger or tv though.

 

2. The frame of the IT should be earthed to the shore NOT the boat

Yup otherwise I've done what's commonly known as defeating the object

 

3. The neutral on the boat side of the IT must be earthed (ie: bonded to the hull at or very near to where the battery negative is bonded) in order to make your RCD's function correctly

So I would be connecting my neutral and earth wires together just as happens somewhere off in the national grid normally. Makes sense. I take it this connection MUST be made on the IT side of the RCD otherwise it wouldn't detect a current imbalance if you held onto the hull and the live wire. Is this connection not made inside the transformer? The earth bonding would serve this purpose then?

 

4. Galvanic Isolators are not "debatable at best", they are excellent at best.

That sounds like debate of galvanic isolators Mr. W I think there's been more than enough of that.

 

I seam to remember something about earthing a safety screen. What ever it was its internal to the transformer and could be wrong for our purposes.

[chris w]

Phew you got the joke I thought it was all going to kick off again.

 

That'd be great, the balsa wood anodes are working a treat!

 

 

You still owe me for them - pay up or I'll send the drillers round

[Chris Lingwood]

For some people. Useless for others.

ARGH!!!

 

The Victron equivalent to the RS components (Carroll-Meynell) isolation transformer is £350 + VAT rather than £85 + VAT so they're hardly the "cheapest on the market"

Exactly! Now have Victron made something subtly different or have they stuck a marine sticker on something and quadrupled the price. As far as I know there's nothing special to a 1:1 wound transformer which should justify £400 other than economies of scale.

 

Phew you got the joke I thought it was all going to kick off again.

 

That'd be great, the balsa wood anodes are working a treat!

Phew!

[chris w]

So I would be connecting my neutral and earth wires together just as happens somewhere off in the national grid normally. Makes sense. I take it this connection MUST be made on the IT side YES of the RCD otherwise it wouldn't detect a current imbalance if you held onto the hull and the live wire. Is this connection not made inside the transformer? NO The earth bonding would serve this purpose then? YES

 

 

 

 

I seem to remember something about earthing a safety screen. What ever it was its internal to the transformer and could be wrong for our purposes.

 

That's the internal frame earth which, as discussed previously, needs to be earthed to the shore

 

 

 

Chris

[Gibbo]

Transformers are rated in VA because that's what matters. A 1000 VA transformer is safe up to 4.3 Amps at 230 volts.

 

The wattage is *not* the correct thing to consider. A load of 500 *Watts* with a power factor of 0.1 would pull 22 amps and therefore need a 5000 VA transformer. As you have a 5 amp limited supply, it matters not one jot what the power factor of the load is. It will either be under 5 Amps (irrespective of power factor) in which case the transformer is sufficient. Or it will be be above 5 Amps, the transformer will not be sufficient, but the circuit breaker will pop anyway.

 

Before the pedantry starts the one proviso here is that highly distorted loads can cause core heating in the transformer. This is something that has to be considered for huge transformers (like on the national grid) but not for small ones such as this. It makes almost no difference on this scale and can be safely ignored.

 

Your wiring should be as chris w described but remember that the transformer chassis must not be accessible to touch.

 

Gibbo

 

Edited to correct broken fingers.

Edited July 18, 2007 by Gibbo

[Chris Lingwood]

So I would be connecting my neutral and earth wires together just as happens somewhere off in the national grid normally. Makes sense. I take it this connection MUST be made on the IT side YES of the RCD otherwise it wouldn't detect a current imbalance if you held onto the hull and the live wire. Is this connection not made inside the transformer? NO

 

Oh ok. Thats good to know.

 

I seem to remember something about earthing a safety screen. What ever it was its internal to the transformer and could be wrong for our purposes.

 

That's the internal frame earth which, as discussed previously, needs to be earthed to the shore

 

Chris

 

Splendid. Right gotcha, I thought you were just talking about the outer casing of the transformer rather than any internal component when you said frame.

 

I can't see any reason then why this wouldn't be as appropriate as Victron one.

 

Surely there's a flaw with this setup though. I can't think of a reliable way that mains would trip if you managed to short your shorepower live wire onto the hull (before the IT). The mcbs on the shore wouldn't trip because no great current would flow. The RCD might trip but that depends on how much current the water conducts back to ground. I think I'm right it saying that nothing on the boat would trip either.

 

Relying on the RCD on the shoreline to trip uses the same logic that says you shouldn't ground your hull...logic I'm not fond of.

[Gary Peacock]

So if I was to get one of these to have a play with like for like with the Victron what are we talking specification wise?

 

If it seems worthwhile I will get someone to try one out in lieu of a galvanic isolator.

Edited July 18, 2007 by Gary Peacock

[chris w]

So if I was to get one of these to have a play with like for like with the Victron what are we talking specification wise?If it seems worthwhile I will get someone to try one out in lieu of a galvanic isolator.

 

 

They are the same spec

 

Chris

 

 

Surely there's a flaw with this setup though. I can't think of a reliable way that mains would trip if you managed to short your shorepower live wire onto the hull (before the IT). The mcbs on the shore wouldn't trip because no great current would flow. The RCD might trip but that depends on how much current the water conducts back to ground. I think I'm right it saying that nothing on the boat would trip either.Relying on the RCD on the shoreline to trip uses the same logic that says you shouldn't ground your hull...logic I'm not fond of.

 

The shore RCD WOULD trip because the impedance of the water between a steel narrowboat with a 300 to 400 ft² base plate and the marina piling is about 3 ohms (Allan Jones on "Keeping Up" and I measured it extensively some months ago). So there would be a current imbalance and, even if that didn't trip because of a fault, the current would be around 70 amps instantaneously so the shore MCB would also trip.

 

Chris

Edited July 18, 2007 by chris w

[Gary Peacock]

They are the same spec

 

Chris

The shore RCD WOULD trip because the impedance of the water between a steel narrowboat with a 300 to 400 ft base plate and the marina piling is about 3 ohms (Allan Jones on "Keeping Up" and I measured it extensively some months ago). So there would be a current imbalance and, even if that didn't trip because of a fault, the current would be around 70 amps instantaneously so the shore MCB would also trip.

 

Chris

 

Thanks for that Chris,

 

We will give one a try and see how it performs. The only downside I can see is that the thermal trip if it is like the units I used to work with you have to remove the lid to reset it. (Saying that most of the Mastervolt kit is just the same!)

 

Obviously it will take a few months to know if it performs but at that price or whatever it costs sourced directly from the manufacturers it's worth a trial.

Edited July 18, 2007 by Gary Peacock

[Chris Lingwood]

The wattage is *not* the correct thing to consider. A load of 500 *Watts* with a power factor of 0.1 would pull 22 amps and therefore need a 5000 VA transformer. As you have a 5 amp limited supply, it matters not one jot what the power factor of the load is. It will either be under 5 Amps (irrespective of power factor) in which case the transformer is sufficient. Or it will be be above 5 Amps, the transformer will not be sufficient, but the circuit breaker will pop anyway.

So when I said 1150W i meant 1150VA. So the transformer is OK up to just over 7 amps in reality. I've reread the wikipedia article and I think I've got it now.

 

They are the same spec

 

Chris

The shore RCD WOULD trip because the impedance of the water between a steel narrowboat with a 300 to 400 ft² base plate and the marina piling is about 3 ohms (Allan Jones on "Keeping Up" and I measured it extensively some months ago). So there would be a current imbalance and, even if that didn't trip because of a fault, the current would be around 70 amps instantaneously so the shore MCB would also trip.

 

Chris

That's a much lower resistance than I would have expected but then again I've never measured it!

 

I may also buy one, if they do what the specification says they should then it seams almost silly not to! Shame I can't find a stockist other than RS for this model. I might try them directly.

[chris w]

That's a much lower resistance than I would have expected but then again I've never measured it!

 

I agree. My intuitive guess would have been a few hundred ohms or a couple of Kohms. But it turned out not to be. We spent a day taking measurements as part of our treatise on (wait for it) galvanic isolators.

 

Chris