Electronics query on chargers

roly_voya

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Anyone know if marine battery chargers (mine is sterling 40a) provide galvanic isolation between 230v earth and 12v neg. Need to know weather I can connect the charger before the isolation transformer. The reason is that with a low power dock supply the surge current to the transformer trips the dock RCD /forums/images/graemlins/frown.gif but if I can cut out the transformer and just leave the charger connected I can use power from the inverter and have the dock supply recharging the batteries /forums/images/graemlins/ooo.gif - but not if it's at the expence of frying the zinc/underwater fittings /forums/images/graemlins/blush.gif
 

William_H

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An excellent question. I don't have an answer but will be keen to find out. The connection of the 240V earth to the output negative would (assuming -ve connects to the water via engine etc) by pass any galvanic protector. Lets hope it is isolated for you and others. olewill
 

Gumpy

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it doesnt matter what the charger does.
The earth for the 240v system on board should be bonded to the 12v negative to comply with various regulations and to make the system safe.
Therefore connecting the charger directly to the mains WILL give you galvanic problems.
Why not look at adding a softstart to the transformer?

Julian
 

andy_wilson

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I\'ve alway admired that regulation

If the marina (or any of the many wires and connections between it and the boat) has an earth fault and you have an appliance that shorts to earth, all of your 12v -ve including the engine, seacocks possibly and maybe even the sink will be at 230V.

I think I'll keep 12v and 230v separate thanks.
 

roly_voya

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Sorry Parglena but for european boats this is not quite true (although seems a common misconseption). The ISO standard to comply with RCD states that if a 'whole ship 30ma RCD is fitted at the shore power inlet then the 230v and 12v grounds need not be connected'. The advantage of not doing so is that you reduce complications in galvanic protection but only providing there is no earth route through either the inverter or charger (or any other mixed 12/230v system. Under fault conditions within the 230v system current is drained to earth via the onboard ground or the earth wire back to the RCD. Inverter & charger cases plus sink etc are connected to the onboard grd. If there is a short between 230v and 12v -ve this would ground via the anode which is also an immersed ground point. Any of these instances woul trip the RCD. Common building practice is to strap the 12vgrd and 230vgnd together which does meet regulations and is cheaper since it avoids fitting an RCD into the shorepower inlet. The disadvantage is that you can add to galvanic problems, it is also possible to have a situation where the earth tags become disconnected from the ground point but still connected to 12vgnd which would lead to the whole boat being live.
 

roly_voya

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Re: Electronics query on chargers - Poblem solved

The answer is No, can't be done like that because although the 230v earth and -ve are isolated the 23v earth is grounded to the case which must also (for safety) be grounded to the onboard earth which would make a galvanic conection to the onboard earth. This would not put shps fittings etc at risk as there would be no connection to the fittings connected to the zinc but would mean that the onboard earth itself could be destroyed by residual current corrosion which would be an electricution risk although not a risk of sinking as it would be if you lost underwater fittings.
 

William_H

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Re: Electronics query on chargers - Poblem solved

The whole question is one of the earthing of the mains system. An earth system is designed to provide a reliable earth to the metal parts of any appliance so that any stray connection of the active to the appliance body will result in a fuse blow. Or at least reduction of the potential of the appliance body to minimise injury.
The fact is that a human body can usually find a contact with a good earth (plumbing in a house) while handling the appliance. So we must ensure there is no chance of the appliance being live.

Now likewise on a boat there is always a good chance your body is connected to earth via the ocean, plumbinng etc. This makes it vital that metal bodied appliances be connected via the mains earth to gound also.
It follows that a connection directly from the appliance body to the ocean and all the grounded items in the boat is the a safe backup way. This will bypass or supplement the mains earth. However you can usually rely on just the mains earth connection. Unfortunately however due to faults on yours or other boats you can get a potential on the mains earth. hence another earth via your boat is good.
This is all very fine but a connection form the mains earth to the boat negative egine hull fittings propeller etc may carry current. Even a little current can cause accelerated elctrolytic corrosion.
So a galvanic isolator is used. This is a connection from mains earth to the boats negative which is effective in conducting any dangerous current. It however is not a connection for small voltages. It uses the nature of silicon diodes which require at least .7 volt to get any conduction. (non linear conduction) 2 of these in series then requires 1.4 volts of difference to get a connection. You need another 2 diodes connected the opposite way around to conduct AC. Thsi isolation of anything less than 1.4 volts is usually enought to isolate the small voltages related to galvanic (electrolytic) corrosion.
Hence any connection battery charger mains earth (case) to the output negative can create electrolytic corrosion problems. And bypass any galvanic isolator. Hope this is correct and explains

Appliances with plastic body and no electrical external connection are described as "double innsulated" and don't need any earth.
There is a lot to be said for total power disconnection when you leave the boat.
olewill
 
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Re: Electronics query on chargers - Poblem solved

Nice description of a galvanic isolator olewill, but he's using an isolation transformer.

Why not put a meter between them and see what you get?

I have a sterling wired in without any form of galvanic protection and my anodes are fine.
 

john_morris_uk

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[ QUOTE ]
The earth for the 240v system on board should be bonded to the 12v negative to comply with various regulations and to make the system safe.


[/ QUOTE ] With respect, I wish you wouldn't make sweeping generalisations (especially referring to 'regulations') that are NOT TRUE.

There might be reasons why the 240v earth should be bonded to the ships 12 or 24 volt negative rail, but it is not necessarily a legal requirement. There are circumstances where it is not necessary or desireable.
 

lw395

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An RCCB and an isolated, double insulated supply is a pretty good answer.
Trying to earth every piece of metal on a plastic boat, such that it will take a 30A mains fault wouldn't be my choice.
Main thing is to physically protect the mains cable.
Take care and make sure you understand whatever system you have!
Better still, unplug the bugger and go sailing!
 
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