isolation transformer how to use?

[ericmark]

When taking power from an isolation transformer how does one protect? It is unlikely any 3 KVA transformer will give the 2.875 ohm impedance required to trip a MCB on the magnetic trip and whole transformer only feed from a 16A MCB in first place with volt drop being a likely problem not sure if active rather than passive RCD should be used. But I can see no point in using duel pole RCD except I don’t think one can buy single pole. All lighting comes from the inverter so it will only feed sockets so I think just a single duel pole RCD is all that is required?
Second question is in-rush an active RCD would ensure it is connected with no load but a 3 or 4KVA transformer may still trip a B16 MCB although one would hope marinas would used D16 but with a B16 can some simple soft start be used like a resistor which a few seconds after connection is shorted out like used with resistor start motors has anyone seen this done? Also losses is there a standard marking so one can tell how much off load current the transformer is going to draw before buying it? I know with 110 volt transformers the losses with a wall mounted transformer in a tin box are a lot less than those with the type fitted into a yellow plastic box with handle and I expect the same with isolation transformers. Price also seems high as a 3KVA 110 volt transformer is priced at around £54 but the isolation transformers seem to start a £200 for type in plastic case to £275 for type in tin box. Does anyone know where I can get cheaper options?
Eric

 

[Spark123]

The 110v transormers you speak of provide a 55v-0v-55v supply for a tool, the 3kw rating isn't continuous. Continuous may be as low as half this.
What sort of supply are you wanting to create? Earth free? - have a read up on electrical separation as opposed to ADS. RCDs don't work on electrically separate supplies the way you think they will. The only real way an RCD would be effective would be if you tied the neutral to earth (via an electrode >21ohms) which is back to it being a TN system.

 

[ericmark]

I realise that 3kVA portable transformers may have two rating one continuous and one max but seems a huge jump in price between the 55-0-55 and 0-230 volt outputs from transformers. The hull of the boat would be connected to one leg of the transformer very little point of an earth rod on the boat and certainly not anything copper.
It is totally against the rules to provide us with any TN system the marina supply should be TT but if they use copper coated earth electrodes this can cause electrolysis between electrode and boat which will soon eat away at the zinc anodes welded to boat hull.
There are two methods to stop this:-
1) Connect the earth wire to shore through diodes (at least 4) which will mean the small potential difference of less than 1.2 volt will not allow current to flow but any earth fault my go through the diodes and if they were to go open circuit protection would be lost and closed circuit would allow electrolysis to start and I would not be very happy with this method.
2) Isolation transformer and not connecting to any shore based earth system. This seems far more fail safe although it will cost more. But there are two unknowns.
A) How much power will the transformer use when there is no current draw from it.
B) What will be the inrush and will it need some method of soft start to allow it to be connected without tripping the supply MCB.
On the latter we must remember it may be plugged into to many marina power supplies throughout it's life and just because the marina which is used at the moment does not trip out it does not mean all marina's will be OK.
If before reading this you had been asked to wire a marina would you have fitted type D MBC's or would you have fitted duel pole RCBO at 16 amp at what ever the supplier happened to have in stock? I will admit until my son had this boat I would not have even thought about inrush on isolation transformers.
Eric

 

[NotHimAgain]

You have asked a number of question about this but you appear to be very confused about the solutions.

1) An isolating transformer is meant to provide isolation from earth YOU MUST NOT earth any part of the secondary winding to the marina supply. You do however bond the metal parts of the boat to it. (See GN 7)

Note that you will need to provide fault protection on the vessel (usually an RCD) and that overcurrent protection will also be required.

2) Marinas must not supply a TN-C-S connection to a vessel, they can and do supply TN-S connections (709.411.4).

The marina socket outlet for supplies to vessels will be RCD protected if it is installed to BS 7671.

3) Electrolysis initiated by the power supply can occur but it is not that common - the incorrect use of TN-C-S supplies will cause it. Most electrolysis is cause by the dissimilar metals on the vessel. However, if you are concerned about it, a properly installed isolating transformer or, ALTERNATIVELY a galvanic isolator in the cpc from the power supply can be used.

4) Transformers are heavy and expensive and could cause inrush problems - that is why they would not be my first choice. Inrush can be reduced if you leave a secondary resistive load (such as a cabin heater) connected before switch on.

 

[ericmark]

Weight is not a problem just remove some of the concrete ballast. And yes of course the marina can use a TN-S supply but unlikely.
I do see the argument for using a “galvanic isolator” which I think is a diode and the threshold voltage is enough to stop the electrolysis.
And yes I know I should not connect any part of secondary to marina supply.
Using a “galvanic isolator” is there any light or buzzer etc. to tell you if it has failed? I was always taught not to use semi-conductor devices on safety stuff. I remember going to great lengths to fit emergency stop isolator contactors before changing the functional contactors to semi-conductor type because I could not run an emergency stop through semi-conductor contactors.
I suppose I have more faith in an isolation transformer than some diodes.
But all things considered I think I will need to modify my ideas as the “galvanic isolator” is cheap and easy to fit. I would assume one would need to fuse the supply rather than use a MCB and use Semi-conductor fuses in the same way as when I used semi-conductor contactors? Or maybe both?
Eric

 

[NotHimAgain]

If I were you I would ask around a marina and see what solutions others have adopted. You could also try British Waterways they used to publish technical guides as part of their licencing setup.

Leisure vessels should be constructed to the RCD (Recreational Craft Directive).

I do not know of any monitor for a galvanic isolator, but I sure one could be made. However, whether that would be noticed before a problem occurred is another matter. I don't like galvanic isolators, but my boat is fibreglass so I don't have the same problem as you.

Isolating Transformers
According to GN 7 the hull and various other parts should be bonded and connected to the secondary of the transformer. They show a shore mounted transformer which IMO would be very not be common.

I have a copy of IEC 364-7-709 1994. This has now been superseded but much of the technical detail would be the same. This shows the isolating transformer - but the hull etc is NOT bonded to it. It is bonded to an earth bar but not to the secondary. It also shows an isolating transformer with no bonding at all.

I would be inclined to try and do without any of these solutions but you should ask around as I said above.

 

[ericmark]

Thank you this is of course why I am asking. My son tells me he has been advised that because of the leakage current caused by filters on white goods and items with switch mode power supplies that the diode method is not recommended and an isolation transformer should be used.
I still think living in a house would cure all the problems but he does not seem to agree!!!
Eric

 

[Spark123]

There may be some info here which is of use: http://www.theiet.org/publishing/wiring-regulations/mag/2007/23-marinas.cfm?type=pdf

 

[ericmark]

Thank you spark123 it is interesting to see the link between neutral and earth after the isolation switch. But I still have the problem in selecting a transformer as no one seems to say how much the losses are with their transformers or what the inrush will be.
My son have been advised not to use a “galvanic isolator” as where white goods and computers are used the leakage from the filters will remove the effectiveness of such units so it seems will have to be a transformer.
It seems most canal boat users know they need an isolation transformer but very few want to be able to run a washing machine so do not go to limit with the transformer a 1Kva being big enough it is fact he wants to go to limit of supply and use a 3 or 4kVA transformer which is causing the problem.
Eric

 

[Spark123]

All the n-e link is doing after the isolation transformer is tying the neutral to earth to create a TN supply. Doing it this way through its own isolation transformer ensures the earth is separate from that of the DNO and local to the boat only.
Any leakage back through the CPC of the circuit will only flow back to the isolation transformer via the N-E link as the secondary is the source of the supply.
Haven't a clue how much copper / iron loss you will suffer from an isolation transformer - you'd need to ask the suppliers.