Isolation transformer or galvanic isolator for shore power protection?

[Danny Jo]

When I dried out last week, I was horrified to see how much of my shaft and hull anodes had rotted away - much more than in the equivalent time last year. The main difference between this year and last was in my shore power connections in the weeks that I was in a marina. Last year I spent some time at Berthons at Lymington and had to hire an isolation transformers, but this autumn I am hooked up to the mains directly. I conclude that I need to isolate the boat from the mains electrically. What puzzles me is the big difference in cost of the options on offer.

Mastervolt market a suitable isolation transformer for £725 plus vat. To that I would have to add an RCD breaker at £138 plus vat. On the other hand "galvanic isolaters" are available over the web for around £70. I understand that these operate on the earth wire alone.

What benefits are conferred by an isolation transformer that are not delivered by the isolator (at a fraction of the cost)?

 

[VicS]

An isolation transformer isolates you from dangers that might arise from defects in the shore supply (eg loss of earth continuity, because you create your own on board, and reversed "polarity", because you create your own neutral and live) It is as well to remember that 240V and water remain as deadly a combination as ever and you should still have an RCD on the ship-side of the supply.

 

[rogerthebodger]

Re: Isolation transformer or galvanic isolator for shore power protect

Having looked at this very deeply I agree with Vic but the cost and Isolation transformers are also very heavy and quite large in size

On my new steel yacht I have gone for the galvanic isolator option plus neon lights to show earth faults and reverse line / neutral bases on lower cost, weight and the amount od room required.

 

[halcyon]

A few years ago I got involved with fitting out a steel yacht, with a owner paranoid with galvanic corrosion. We fitted a isolation transformer, but it was far far cheaper than £725, just look in yellow pages for a transformer manufacture, they will manufacture one, I think we paid not much over £100. There are special wiring regs for transformers, which I am floging through at the moment, together with galvanic isolater design.
On steel boats it is important not to have to much isolation voltage, as this can be as bad as to little.


Brian

 

[Gumpy]

I use one of these
http://www.airlinktransformers.com/transformer/wp3230-waterproof-transformer.asp

Can be placed on or off the boat.
Only downside is there is no softstart circuit.
Beware Iso transformers in metal cases they cannot be used safely onboard a boat despite what the likes of Victron say.

julian

 

[anniebray]

Your transformer looks good value if rather heavy at 35kg. Is soft start really necessary or what problems do you encounter without one ? Would a Honda 1Kw genny suffer any problems if plugged into your shorepower (tho of course the genny is already an isolated source)
Richard

 

[ean_p]

Hi
Can you expand on 'On steel boats it is important not to have to much isolation voltage, as this can be as bad as to little.' Don't quite understand what this refers to!

regards

Ian P

 

[Mudplugger]

Me Too! would like to know!

 

[derekmercer]

Re: Isolation transformer or galvanic isolator for shore power protect

A minor point.

You can now purchase isolation transformers that accept either 230V or 115V, by simply inserting a link (which can be swapped for a "changeover switch" rated at 16A). This makes you independant of the supply voltage as well, useful if thinking of going across the pond......

 

[William_H]

Re: Isolation transformer or galvanic isolator for shore power protect

I suspect from the comments that there is a lot of misinformation about 240V systems and earthing.
Firstly if you only use 240V shore power when you are on board then galvanic corrosion should not be a problem provided you disconnect the shore power lead. It is the 24/7 that permits corrosion.

Next you need to identify and consider all of your devices that use 240V. So an emersion heater electric fan heater/jug and many appliances are double isulated not earthed. ie with no metal body to connect earth wire to. These then are like an isolation transformer in themselves.
A battery charger is probably well insulated ie the negative output insulated from, the case and the mains earth. This needs to be checked. If it has a connection -ve o/p to mains earth then you (usually) have a connection directly mains earth to water via engine etc. An isolation transformer could be a good idea here. But just for the charger. ie smaller.
If you have a fridge with metal body that is earthed then things get sticky. i In this and many other cases it is desirable to ensure that the mains earth is connected to everything on the boat that is earthed to water to minimise electric shock. If there is no connection mains earth to ships earth (-ve) then a galvanic isolator can provide this connection for safety sake without providing the low voltage connection go galvanic corrosion.

So take every case on its own merits re what services are used. You don't really have a choice as to what method you use just different methiods for different circumstances. olewill

 

[Graham_Wright]

230/115 building site isolation transformers are relatively cheap. Put two back to back and you have your isolation.

 

[PhilipH]

You do not need the expense and weight of an islolation transformer - a galvanic isolator will provide you with adequate protection when connected to shore power. My boat is aluminium and I went this route. The other action I take to reduce the degradation of fixed anodes is to have hanging anodes, one of which is connected by a large crocodile clip to the propeller shaft - just remember to disconnect before putting the engine in gear! I made these up using large tear drop anodes, some old dinghy shrouds and old hosepipe for insulation.

 

[VicS]

[ QUOTE ]
a galvanic isolator will provide you with adequate protection when connected to shore power

[/ QUOTE ] The original question was: "What benefits are conferred by an isolation transformer that are not delivered by the isolator." I answered that, and was supported by Roger Shaw.
From the galvanic corrosion point of view there are no benefits, at least none of which I am aware. The benefits are those of enhanced electrical safety.

 

[Danny Jo]

Many thanks for all the contributions. I have opted to go down the transformer route because I want to leave the boat in the water as long as possible, and to run a duhumidifier continuously when in dock/marina (plus a small heater set to come on when the temperature drops below 7 degrees Celsius). A neighbour in the dock uses another strategm (mentioned above) for keeping the shore power earth separate from his boat earth - he runs the shore power into the boat as a conventional extension lead off which he takes his 240 lighting and heating (and very brief spells with a battery charger).

During the summer I tend to run the fridge only when the engine is running or when connected to shore power. Crews tend to be so picky nowadays - they seem to be unaccountably attached to unfermented milk and cold wine/beer, not to mention tasteless meat - and they connect me up to shore power as soon as we berth anywhere, long before I've had a chance to check how much I'm going to be stung for it.

As others have mentioned, 240 volts and water together are a potentially fatal combination. In reviewing my electrics yesterday, I discovered that the RCD device on board is an industrial type set to trip at 300 mA and capable of carrying 80 amps. I understand that the more appropriate 16 amp RCD tripping at 30 mA costs a lot more.

VicS's last comment is a useful reminder - safety first. So poor Freestyle will have to make do with her old number one genoa for another year.

 

[VicS]

[ QUOTE ]
VicS's last comment is a useful reminder

[/ QUOTE ] Heavens, it was not my intention to persuade you to fit a transformer, just a cold unqualified answer to your original question. However it's your call and it would be wrong, I suppose, to advise you against it. make sure you get sound advice on its installation if you are doing it yourself and make sure the shore-side earth is not connected to the on-board earth or you will be back at square one regarding the galvanic corrosion.

It's very basic but there is a little on mains installation in the "Electrical notes" on http://www.tb-training.co.uk/

 

[Danny Jo]

The decision was made before your post. Like many other boaty folks, I fancy myself as a Jack of all trades, but as I grow older I am becoming more aware of the meaning of the phrase "master of none". Learning by mistakes is all very well, but I don't fancy a lesson based on a corpse. Also I know I haven't got time to do everything that needs doing, which makes it easier to decide to contract out jobs.

Fitting by a professional is "free" within the purchase price, and one can winkle out buckets of advice on the strength of it. The nice man (a) explained how my boat electrics are set up - more valuable than a dozen books on the subject - and (b) demonstrated that the sophisticated voltage regulator piggy-backed onto the alternator was not doing its job because someone had connected the voltage sensor cable to the wrong point (subjecting my poor engine battery - a gel type - to dangerously high charging voltages).