Do I need a galvanic isolator & AC safety question

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bluedragon

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I’ve been searching the forum and the WWW for opinions on galvanic isolators. My shore power installation is at present little more than an extension lead with an RCD, not bonded to the boat’s DC ground, anode, etc, and is used only intermittently for appliances such as kettle and fan heater. My reading of the situation is that a galvanic isolator would serve no purpose…anyone got a different view?

Now the second (and related issue) is SHOULD my AC installation have it’s earth connected to boat DC –ve for safety purposes. Codes of Practice say it should…there is even a suggestion on one supplier’s website that it is illegal not to do so! But in the unlikely event that the marina supply earth is poor or even disconnected, won’t the RCD trip-out anyway if I touched AC live and avoid electrocution? I’m sure this happened recently when I was testing the installation with a simple mains tester screwdriver (earthed through me via a resistor). I’d welcome some expert opinion on this…
 
Absolutely no need for a galvanic isolator if you aren't using the shorepower for battery charging....... its an entirely separate system with no ability to create a galvanic circuit if its not connected to anything on the boats 12V circuits....

If you use the 240V through a battery charger, then yes, best practice seems to suggest that bond the AC and DC earths is the right way forward, and that a galvanic isolator is a good idea..... I do just that.

But many on here don't and there are as many in the against camp as there are in the for......
 
Concur with MC - we don't have a permanent battery charger fitted so the AC system is not connected to the DC -ve - so no galvanic isolator there ...

all we have is: Pontoon supply -> Our Lead(s) -> RCD installed in aft locker -> 4 x 13amp sockets.

All we run is:
Kettle
Toaster (yup got one of them!)
DeskLamp (energy saver bulb - brighter than our 12v lights)
Laptop (only if we want to charge the laptop battery or watch a movie)
 
I've been following this debate for a long time and I'm still not convinced that connecting the AC earth to the DC earth and anodes is a good idea.

Surely, if theres's a fault on the AC side everything on the DC side will become live. This sounds like a terrible idea to me. I must be missing something?

When I wired up our boat years ago the best practice then was to keep everything isloated which meant using alternators and starter motors etc which were not earthed to the engine.
 
Read 10 books .... get 10 answers

It seems that no-one actually gives an answer that all can agree ... I have various books on electrical installation in boats ........ and they all have different answers - even variations on same ...

Pretty pictures of swimmers getting electrocuted, others of boats becoming "live" ... all sorts of things happening .... but they all have one thing in common - they don't actually answer the question !

I have trailing lead ex caravan shop to socket in cockpit. Breaker box inside cabin then going out via MCB's to sockets ...

I run small heater, PC charging and possibly a CD-Radio set. I did think about taking the Tesco £5 kettle ... but haven't yet ...

As to connecting DC -ve and AC -ve .... nope. Not till I read a definitive and agreed BY ALL chapter and verse.
 
I'm still not happy about the idea of connecting live AC to the DC circuit no matter how momentary it might be. So I've kept everything separate same as Fireball (no toaster though).
 
Its a widely debated subject area isn't it....!

I also wondered that if you use the 240V to charge the batteries then a failure in the charger could make the DC circuit live, without the benefit of the AC earth if they weren't bonded....

Although I believe the justification is more to do with floating earth potentials......
 
The reason for a so called earth connection on any mains AC instalation is to provide an alternate path for the current to flow if there is a fault between the metal case of an appliance and the live supply connection.

If a fault occures either the fuse or circuit breaker would blow due to excess current flow to earth.

The earth and neutral line connected at the sub station.

If the appliance is not metal cased as is common now no earth is supplied as the case cannot becone live so no path to earth.

The danger is then a path directly from live through a person to earth so the use there days of a RCD (residial current dectector),. This measures the different in current flow between live and neutral and if it exceeds 20 mA it would trip.

Thats the theory.

Now where to connect the earth lead of metal cased appliances on a boat.

On a boat this must be in contact with the sea through a ground plate, metal skin fitting or your anodes under water, which will become common ground, like the metal plumming fittings in a house. If the 12v -ve is also connected so be it.

Now with this setup and the use of shore power your anode will be connected to passible all the anodes on the boats on the same circuit so if the boat next to you has no anodes yours will protect his boat thus be used up your anodes quicker. As the anodic voltage is low (1.2vdc max) as compaired to mains voltage, a device is placed in the earth lead running to the shore power that will block this low voltage and associated current, problem solved. This device is the galvanic isolator being 4 diodes inline and back to back simular to a bridge rectifer with a max current greated the your mains fuse or curcuit breaker.

You should also fit a RCD in the incomming line to prevent dangerous levels os current leaking from the circuit and possible through you.

Just to consider what is the possibility of someone being in contact with the sea and a faulty mains electrical appliance at the same time on a GRP boat. I think passible but unlikley and because of the possibility a RCD MUST be fitted even if the regs do not require.

On my new yacht being a steel hull and deck, its a must that a RCD abd galvanic isolator be fitted as there could very well be an electrical path through the steel hull to the sea
 
As already stated metal cases of mains appliances are earthed to the earth wire of the power plug. If there is no metal case there is no earth connection. However a battery charger may or may not have a connection from the negative lead to the case. If it has this conection then your negative battery hence all the engine prop etc is connected to the mains earth. If left connected long term this can cause corrosion or deteriation of anodes. Hence you use an GI. It is still a problem if the battery charger -ve connects to the mains earth.
So to answer the question ...all plastic appliances no worries as you are. No don't connect the earth to the ships gound but if you do you need a GI for long term use. olewill
 
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which meant using alternators and starter motors etc which were not earthed to the engine.


[/ QUOTE ]

How does that work then? Every starter motor I've ever come across has always relied on it being bolted to the engine to get an earth!!!!

Peter.
 
There are many marine diesels (our own included, a Volvo) which have the cranking motor, alternator and sensors all isolated from the engine itself (so the negative is isolated from the engine). But not sure if that is what the poster was alluding to though.

John
 
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