Help with shore power earthing! (i think..)

[Firefly625]

Hi, I seem to be having a senior moment... please help. I installed to my Merry Fisher 625 shore power last summer. All installed I believe correctly & safely, waterproof adapter through cockpit, running to RCD unit, running to 2 outlets in the cabin, & green/yellow earth running from RCD to DC negative in battery switch box. This is the part I am now struggling with (I think). This is an outboard powered boat, DC negative goes straight from battery cut off unit to engine. I know AC earth needs to go to engine, therefore links to 12v DC negative by default. To keep 12v battery topped up I have been using a battery conditioner which has worked well, but if battery low does take some time to charge up as it is only charging under 2amps, but this is permanently connected to the battery and has been fine. But I have purchased a 6amp sterling battery charger and fitted it yesterday, switched on and it immediately blew RCD unit. Thought I had done something stupid so checked all connections and all OK, so (as we all do) tried again hoping that the same thing would not happen & guess what, it didn't, this time it blew the internal fuse in the charger... Now I have been in contact with charger supplier who have said they will send me a new unit, but I am now just considering the circuit I have. Is it correct? Is the charger faulty or is my circuit.... In essence connecting the 240v shorepower earth to the boats earth (the engine) I have linked straight to the 12v DC negative therefore linking both 240v & 12v AC/DC together... But I am sure this is correct, although my confidence is now reducing..so any advice would be welcome. Otherwise I am off to a marine electrician! Many thanks in advance!!

 

[Cliveshep]

I cannot see anything wrong with cross-bonding your A/C earth to your engine metalwork however you achieve that aim. BTW don't forget to bond your s/s sink and taps as well.

I suspect that your charger might in fact have been faulty and that it is not a function of bonding your engine. Note here, are you sure that your engine is negative earth? I could see problems with bonding metalwork and battery negative if not.

 

[Firefly625]

- Thanks for your response... I wish my boat was large enough to have s/s sink and taps but it really is only tiny! a moulded GRP sink and ally pump tap probably don't require earthing!! /forums/images/graemlins/grin.gif

 

[sailorman]

240v & 12v should be totally seperate.
the 240v is earthed when pluged into the shore connection & then when disconnected the whole 240v is totally isolated from the 12v

 

[Firefly625]

OK.... but what I worry about is that by cutting the 240v shore power earth in the boat the AC current loses its return path for leakage, therefore anyone who went into the water when the boat was at its mooring could get a v nasty shock....

 

[sailorman]

short out the 240v live with any 12v wire all your 12v is now 240v /forums/images/graemlins/shocked.gif.
the 3 wires from the consumer unit should go ashore via shore connector, no other earth is req

 

[Firefly625]

I think anyone with shorepower would avoid shorting out between 12v & 240v, but a worth while point made.

I just seem to remember being taught you should never treat a boat like a floating house and an entire boat should be grounded, but maybe this is where I am going wrong....

 

[sailorman]

on a 12v system there is no earth wire, only the pos & return ( neg )

 

[Firefly625]

quite correct..sorry I was refering to 240v, not 12v.

I guess you have shore power on your boat which goes through a RCD unit?

 

[rickp]

See the section 'Breaking the Galvanic circuit' on page 61 of Professional Boatbuilder for Nigel Calder's view on things.

 

[Firefly625]

Thanks for that... that's a bit of bedtime reading for me..

 

[Hurricane]

I wondered how long it would be before someone posted a more professional explanation here.

AFAIK sailorman is wrong here.
The reason to connect the shorepower ground to the negative side if the battery is to ensure that all the metalwork on the boat is connected to ground thus making it safer from electric shock. The negative side of the battery is normally a "ground" for the 12v system anyway. For example the main body of the engine will also be connected to the negative as in any "negative ground" system.

So, if a fault occurs in the 240v system, all the boats exposed metalwork will be protected.

Now, for bigger boats with more permanent installations another problem exists. In a marina lots of boats are connected to the common shore supply's ground and some may well be leaking small currents to the common ground. The ground itself is connected to the shore so a current may then exist in this gounnd supply. Where there is a current flowing there will be a voltage - there has to be. This is a voltage between the water and the boat's ground system. More importantly it can cause havoc with the boat's galvanic protection, on the underwater metalwork, the propeller and anodes etc . So, if you are just simply connecting a shore supply to the boat and dont want to upset your boat's galvanic protection - dont connect the earth but this makes the whole thing dangerous IMO. The simple solution is to install a galvanic isolator in the connection between the shore supply's earth and the connection to the boat's 12v earth.
A galvanic isolator is just a couple of big diodes connected back to back thus creating a "blockage" to any current flowing as a result of the small voltages but will let current generated from large voltages pass through.

AFAIK RCDs measure the current in the supply against the current in the neutral and if they dont match then they trip. I dont think that they have any connection with the earth so I dont think your problem is in the method that you've connected the earth. However, the new battery charger may not be isolated from its earth. I think the question may be more along the lines of what the new battery charger is rather than whether its faulty.

 

[Laundryman]

Okay, so what happens to the earth when running a 240v generator on board?

 

[sailorman]

sintered bronze earth plate
unless Im totally wrong again /forums/images/graemlins/grin.gif

 

[Firefly625]

Thank you Hurricane, I am a new user here, but do have a little bit of boat owning experience so I do know about galvanic issues etc, Having read some pages of rickp's posting I am coming to the same conclusion as you, i think the Sterling pro-budget Charger is a bit too "budget!". I am now looking to throw money at the problem and buy a more expensive charger... somehow that always seems to be the answer when it comes to boats. /forums/images/graemlins/wink.gif

 

[Laundryman]

What's a sintered bronze earth plate and where do i put it? Thank you

 

[sailorman]

http://www.yachtbits.com/dynaplate/sintered_bronze_grounding_plates.php

 

[Laundryman]

Thanks for that.

 

[rickp]

Don't most marine gensets (and inverters) tie earth to neutral on the device? I've yet to see a powerboat with a sintered plate for an earth - thought they were only fitted by those using HF radios.

 

[Firefly625]

This is interesting ... imo..... just read page 62 of Professional Boat Builder and although it does spend 6 pages recommending the AC gounding to DC Negative it then goes onto state that standards for European boat building permits disconnecting the AC grounding to DC Negative if the incoming shore-power cord is protected by a Ground Fault Circuit Break aka Residual Current Device (RCD).

Oh what would I do if I didn't have a boat to worry about! /forums/images/graemlins/confused.gif