Galvanic transformers

[BruceinSanity]

There are a few things to consider...

 

1) The power rating of the transformer should not be less than the power rating for the incoming connector. So; as in the majority of cases the inlet is 16A, 16 x 230 = 3680W or as near as you can get. Any smaller and it could get unintentionally overloaded and in addition will tend to be less efficient.

 

2) You need to consider how much power you will use for extended periods when attached to a shore line. For those who use incoming power for heating the water, ironing, kettle boiling and other such loads then an air cooled unit would probably be best.

So why? Well, the transformers in sealed cases (IP 67) whilst being good at surviving being located in a damp and cold environment such as on a swim, because of the encapsulation tend to lack cooling ability and overheat. Remember this sort of casing was intended for site work tools and these don't stress the transformer for long, are located where they can be easily seen (that's why they are yellow) and the loads they serve tend to be intermittent.

Conversely, the air cooled units (IP23 metal case) whilst handling higher extended loads better because of their cooling ability are prone to damage from damp. These really need to be located inside the cabin in a cupboard or some such warm and dry location.

 

3) It is important to have some sort of thermal overload cut off switch located inside the windings of the transformer and it is very sensible to be able to replace this if it fails (as they sometimes do). If the transformer starts to overheat, the heat is generated deep inside the windings (or at least the temperature rise is greatest) and if there is no thermal protection then the overheating fast becomes very dangerous.

 

4) Transformers usually generate some mains hum at 50Hz and this can become annoying if it has been located under a bed or near where you are likely to be enjoying peace and quiet. Also, since they operate on the basis of a cyclical magnetic field, any magnetic conductive item in the vicinity may well vibrate as well. So; it's best to locate them somewhere where they are not likely to be surrounded by clutter.

 

5) For the purposes of fault finding and diagnosis it is a good idea for them to be installed with male/female BS4343 ( blue three pin) connectors on the inlet and outlet arranged to that they can be temporarily bypassed.

 

6) As previously mentioned, they can and usually do generate a bit of heat if under load so it is essential that they are located in a space that has some ventilation and not tucked up in spare bedding, wet weather gear, towels, upholstery etc. etc.

 

So it's not as simple as it seems but then electrickery never was...

 

Hope this helps

 

Arnot

These are very good points, thanks. Our Airlink stands outside the cabin, either in the well deck or on the stern, depending which way round the boat is (we have bow and stern inlets) so that it's always as close as possible to the supply. It has a thermal cut out, and coped with washing machine and immersion heater loads without distress, as far as I could tell, albeit in chilly winter conditions.

[by'eck]

Don't see any need to have power handling capability of IT equalling that of shore power, as long as you have current limiting trip downstream of IT such that it cannot allow overload. That's why I suggested a suitably rated RCBO, providing double pole current as well as 30mA RCD trip.

 

I think having an IT capable of passing the 32 amps available at some marina's, may come in a tad expensive as well as bulky.

[Arnot]

Don't see any need to have power handling capability of IT equalling that of shore power, as long as you have current limiting trip downstream of IT such that it cannot allow overload. That's why I suggested a suitably rated RCBO, providing double pole current as well as 30mA RCD trip.

 

I think having an IT capable of passing the 32 amps available at some marina's, may come in a tad expensive as well as bulky.

Sorry, I possibly didn't make myself clear here.

 

In principle, current trips protect the wiring and/or equipment "downstream" of them not "upstream". It's not the shore power that is available that should be used as the marker for the transformer specification but the size of the input connector. It would obviously be impossible to use the power available for this because it would possibly change with the mooring and could not be anticipated. I am aware that there are some cheat leads out there with a 32A male leading to a 16A female. These should have some sort of current limit such as an RCBO in line but sadly rarely do.

 

Presumably if you have a boat with a 32A inlet then someone at some stage installed this for a good reason. It may be that there was no good reason or that the reason has disappeared. If that's the case, after suitable examination and test, either a large transformer should be used or the inlet should be changed.

 

Technically of course you are correct but another consideration is safety. Not just that of the person who installed it and (possibly) understood the risk but of all those who use the installation and work on it subsequently who are unaware of the risk taken on their behalf. If a smaller transformer is used, hidden in a cupboard or under a swim, someone at a later stage may want more current than the limiting protection device will pass, realising that the cable is good for it and not realising why the apparently undersized breaker was used, change it. In this case, the transformer will tend to generate a lot of heat and constantly trip the thermal protection but these are not designed for repetitive operation and will soon fail.

 

I suppose that in the final analysis, if the job's worth doing, it's worth doing properly...

 

Regards

 

Arnot

Edited May 6, 2015 by Arnot

[George94]

Don't see any need to have power handling capability of IT equalling that of shore power, as long as you have current limiting trip downstream of IT such that it cannot allow overload. That's why I suggested a suitably rated RCBO, providing double pole current as well as 30mA RCD trip.

 

I think having an IT capable of passing the 32 amps available at some marina's, may come in a tad expensive as well as bulky.

 

My IT is 7kW, so almost capable of handling 32A. It's dimensions are 36x26x22 cms, so not too bulky, but it does need air to cool it. As for cost, it protects the hull more reliably than a GI, so I think the cost (<£1,000) is acceptable.

[ditchcrawler]

Our Airlink IT is a standalone waterproof unit, just like a 230 - 110 site transformer. It has a couple of leads with blue ceeform connectors on and goes between the shore supply and the boat inlet. We put it away under the bed when cruising.

They dont work under the bed, it says so in the instructions.

[NMEA]

I do think the subject has been adequately covered, certainly as far as the OPs question goes and really can't add more. Apart from one VERY important factor concerning Isolation transformers, it will by its nature only crop up occasionally and so is often forgotten. Some (not all) GIs have a requirement for the PE to be internally bridged when the boat is lifted otherwise you will have no protection, read the manual carefully, the alternative is an earth strap connected to the boat and a conductive stake driven into the ground.

 

Edited to change Galvanic isolator to Isolation transformer.

Edited May 6, 2015 by NMEA

[ditchcrawler]

I do think the subject has been adequately covered, certainly as far as the OPs question goes and really can't add more. Apart from one VERY important factor concerning galvanic isolators, it will by its nature only crop up occasionally and so is often forgotten. Some (not all) ITs have a requirement for the PE to be internally bridged when the boat is lifted otherwise you will have no protection, read the manual carefully, the alternative is an earth strap connected to the boat and a conductive stake driven into the ground.

Are you saying that you rely on hull water conductivity for safety?

[by'eck]

Are you saying that you rely on hull water conductivity for safety?

 

Sorry, but you are completely missing the point if thinking along those lines. Think more of electrical conductivity of your body to surroundings. When on land a local connection to ground/earth is highly relevant. When on a boat remote from such, its the boats hull.

 

Once out of reach of land the boats conductivity to such is irrelevant from a safety point of view.

 

NMEA's comments regarding earth connectivity are important when a boat is lifted onto land. I would suggest that a local ground stake option is preferable though. Its certainly a requirement in homes with external power feeds to remote sheds and garages. Typically the earth conducting armour on the power cable should be broken at the remote end and local ground conductor used instead to provide the nominal safety earth.

Edited May 6, 2015 by by'eck

[ditchcrawler]

 

Sorry, but you are completely missing the point if thinking along those lines. Think more of electrical conductivity of your body to surroundings. When on land a local connection to ground/earth is highly relevant. When on a boat remote from such, its the boats hull.

 

Once out of reach of land the boats conductivity to such is irrelevant from a safety point of view.

 

NMEA's comments regarding earth connectivity are important when a boat is lifted onto land. I would suggest that a local ground stake option is preferable though. Its certainly a requirement in homes with external power feeds to remote sheds and garages. Typically the earth conducting armour on the power cable should be broken at the remote end and local ground conductor used instead to provide the nominal safety earth.

Totally confused by the posting and reading carefully starts by saying isolating TX and then GI and reading it again I see he has edited it. Compare it to what I replied to.

[jonathanA]

I do think the subject has been adequately covered, certainly as far as the OPs question goes and really can't add more. Apart from one VERY important factor concerning Isolation transformers, it will by its nature only crop up occasionally and so is often forgotten. Some (not all) GIs have a requirement for the PE to be internally bridged when the boat is lifted otherwise you will have no protection, read the manual carefully, the alternative is an earth strap connected to the boat and a conductive stake driven into the ground.

 

Edited to change Galvanic isolator to Isolation transformer.

I'm struggling with this makes no sense to me.....

[Arnot]

Apart from one VERY important factor concerning Isolation transformers, it will by its nature only crop up occasionally and so is often forgotten. Some (not all) GIs have a requirement for the PE to be internally bridged when the boat is lifted otherwise you will have no protection, read the manual carefully, the alternative is an earth strap connected to the boat and a conductive stake driven into the ground.

 

Edited to change Galvanic isolator to Isolation transformer.

I'm not sure I understand this... surely the whole idea of an isolation transformer is that it isolates. In effect the boat becomes an enclosed system, a sort of faraday cage. Any current flow has to be through this enclosed isolated system and would not interact with any external power source. There should be no need to provide any external earth given, as it should be, any outlet in the boat is protected by an RCD. Even if the boat system is used to supply power tools used on the hull or nearby, even if the boat is hooked up to a shore line at the same time, it still shouldn't need any conductive stakes or similar contrivances.

 

All of this however presumes that it is a transformer that is used and that the PE has NOT been bridged or tampered with in any other way. I would be interested to know which if any manufacturer of isolation transformers requires PE bridging. I certainly would not recommend it and in fact would think it highly unwise.

 

There is an argument for a galvanic isolator being bridged when a boat is lifted out and parked up but even this is both technically dubious and does not stack up in safety terms really. The galvanic isolator will increase the fault touch voltage but not significantly and given that the boat (if wired correctly) approximates to a domestic PME system in that the neutral and PE are bonded at source the PE impedance will be so low that the fault touch voltage will never get anywhere near a dangerous level under any foreseeable circumstances. My assessment is that having people mess with the wiring at all for such a fallacious justification only introduces a needless risk.

 

Sorry if this is a bit obtuse but there isn't really a simple way of phrasing it and it needs to be said. Unfortunately there is quite a lot of misinformation surrounding the subject of protective earthing especially when it comes to boats and mobile applications albeit well meaning.

 

A lot of this cloudy thinking and misinformation actually has sound foundations and dates from when RCD's were unusual on boats and the earthing systems in a lot of boatyards and boat builders were themselves unprotected by RCD's or RCBO's and provided by an untested spike in the ground. Of course now we have RCD's, PME earthing systems and regular inspection, test and certification of commercial installations so many of the historic precautions no longer have any relevance.

 

So; here is my firm advice. If you have an isolation transformer or a galvanic isolator and have your boat lifted, don't mess with either of them. Attaching a spike or some such can be done but it won't improve the electrical safety apart from marginally and at the same time the introduced trip hazard will be greater so I wouldn't bother with that either.

 

The whole idea behind the way installations are carried out now is to make them safe in normal usage regardless of any intervention.

 

I hope (but somehow doubt) that this helps...

 

Arnot

 

Edited for smelling pistakes

Edited May 6, 2015 by Arnot

[ditchcrawler]

Well I have been down to the pub, they had 5 beers on and I can't get my head around it.

[by'eck]

I'm struggling with this makes no sense to me.....

 

I think he is suggesting that because an IT provides a break in shore earth, when boat is lifted ashore, someone standing on terra firma may not be protected.

 

In the Victron isolation transformer manual download here, it very clearly states on page 7 that a jumper between boat and incoming PE must be inserted for safety reasons when its ashore. In this case though the neutral/boat earth strap should be removed first.

[smileypete]

Just install a reasonably decent GI like the Safeshore ones with LED monitor, job done!!!

 

The LEDs will warn if current is passing through the GI and so give some peace of mind.

 

cheers, Pete.

~smpt~

Edited May 6, 2015 by smileypete

[Arnot]

 

I think he is suggesting that because an IT provides a break in shore earth, when boat is lifted ashore, someone standing on terra firma may not be protected.

 

Ahh but protected from what? A boat without an earth that is isolated from an incoming supply is little different from a vehicle or any other largish conductive structure without any connected mains wiring attached to it. The incoming supply should be protected by an RCD and thus if there is any dangerous leakage current it will trip. If there isn't any RCD then connecting the hull to the incoming earth will actually increase the risk of electrocution. Any supply generated from within the electrical system of the boat should also be RCD protected and its PE would be bonded to the hull to ensure this worked.

 

 

In the Victron isolation transformer manual download here, it very clearly states on page 7 that a jumper between boat and incoming PE must be inserted for safety reasons when its ashore. In this case though the neutral/boat earth strap should be removed first.

 

An interesting case. Actually I have an issue with the Victron isolation transformer in that it is not wired in accordance with the accepted norm as shown in post 15 and here.

 

Instead the transformer case is connected to the outgoing PE and the incoming PE is unconnected internally. This is why they give that advice. I just think they don't wire their isolation transformers correctly. Possibly they do it because they use a toroidal transformer where it is difficult to connect to the core. A protective screen can be wound in, it just costs a bit more. Here is the internal wiring for the Victron unit..

IMHO the problem with this is that if, as is probably the most common and dangerous form of failure, the transformer windings overheat and short together, there is no path for the leakage to trip an RCD so only the overcurrent protection can prevent the incoming wiring burning out. This won't however prevent the transformer going into Chernobyl mode and even the thermal protection would be unlikely to act in time to save the day.

 

So; that's why I don't install them...

 

Regards

 

Arnot

[GUMPY]

I have to agree with Arnot when a boat is on an iso transformer in a dock there is no return path for any fault current back to the transformer. So even if the hull is "luve" there is no way the current can pass through you back to the transformer.

Victron transformers are a special cas as they are wired in a potentially dangerous manner.......

[Keeping Up]

I have to agree with Arnot .......

So do I. Arnot has been the voice of good sense on this thread!

[BruceinSanity]

So do I. Arnot has been the voice of good sense on this thread!

Hear, hear. It's like having a calmer version of Gibbo back. ISTR that gentleman agreeing that the Victron wiring was unsatisfactory for the same reason.

[ralphclaydon]

Hi

i have been looking on the AirLink site, The BT3231 ( 3600va, 230v x 230v ) would seem to be my preferred choice BUT the size of the case is given as H 355 L320 and

W 355 seems very big to me! Thats 12" x 14" x14" high in old money!

Has anyone got this model?

[boots]

Think I have the model before that one, but the size seems about right,

[ralphclaydon]

Blimmy where do you find that sort of room on a narrowboat?

[BruceinSanity]

Hi

i have been looking on the AirLink site, The BT3231 ( 3600va, 230v x 230v ) would seem to be my preferred choice BUT the size of the case is given as H 355 L320 and

W 355 seems very big to me! Thats 12" x 14" x14" high in old money!

Has anyone got this model?

That sounds right, same size as a 230 > 110 site transformer. Yup, it's big, there's a lot of copper in there with some air space above it. Oh, and be warned, it's H-E-A-V-Y!!!

[boots]

When on our mooring (in use) it lives on either the front or back decks (as close to the shore post as securely possible). When out cruising it lives in the engine 'ole (cruiser stern)

[jonesthenuke]

We have an Airlink BT3231. Currently installed in a cupboard at the rear of the boat immediately next to the incoming shore supply socket, thus limiting the cable length on the boat before the transformer.

 

In future I will examine a location below the back cabin floor (they may be space - just). The case looks resilient enough to place it there but I will give it some careful thought first regarding IP ratings etc.

 

It seems well enough made, my only recollection was that the seal for the top cover is relativly flimsy plastic foam. In service it has a slight hum. I would probably find this irritating in a bedroom but its fine where it is on our boat.

[jonathanA]

 

I think he is suggesting that because an IT provides a break in shore earth, when boat is lifted ashore, someone standing on terra firma may not be protected.

 

In the Victron isolation transformer manual download here, it very clearly states on page 7 that a jumper between boat and incoming PE must be inserted for safety reasons when its ashore. In this case though the neutral/boat earth strap should be removed first.

 

yes I was replying to the pre-edit post - referring to GI which made no sense.

 

but to answer the boat out of water using Iso transformer correctly. - arguably thats very safe as there is no way anyone standing on terra firma can get a shock off the boat even if the live is directly connected to the hull. when i was a spotty youth science labs in schools were all fitted with isolating transformers so kids couldn't get a shock - thats the whole point of them, we happen to use them on boats to prevent galvanic currents rotting our hull/fittings but thats not what they were designed to do.

 

but if your out of the water you shouldn't be using the transformer (no point) and then I'd agree I don't think I'd want to rely on the earth/CPC of whatever dodgy extension leads and mains distribution lash up the boat yard has...

 

ETA- realised Arnot has already made the same point more elegantly earlier on.

Edited May 7, 2015 by jonathanA