Battery charger earthing

[prv]

Suppliers permitting, I should be installing Ariam's new battery system at the weekend. I've just been reading the manual for the battery charger I've ordered.

As well as the usual earth conductor in the AC input cable, and the common negative terminal on the DC side, it also has an earthing stud on the case. The instructions are most insistent that this be "grounded", but they aren't clear about how or to what.

I realise this relates to the minefield of galvanic corrosion, bonding of seacocks, IEEE standards, etc, but I'd be grateful for the forum's informed thoughts (and promise not to be too scathing about the inevitable uninformed ones ).

For information, the boat is GRP and is fitted with a saildrive that is electrically isolated from the engine. The only anodes are on the saildrive itself and the prop (which is isolated from the driveshaft) - there is no hull anode and no bonding of seacocks etc. The shore power system is also generally independent of everything else - currently it consists of a small consumer unit with MCBs and RCD, a couple of radials to 4 double sockets, and a connection to the calorifier element. The old battery charger was plugged into a socket.

So, connect the charger case terminal to the mains earth? To the DC negative? Both? Neither?

Cheers,

Pete

 

[boatmike]

The reason this and other AC kit has an independent earth is to provide a direct ground that does not rely on the shore supply. On shore the current building regs require the earth supply to be seperately grounded to plumbing or other "earth" line to avoid reliance on what may in fact be a less than reliable system earth otherwise. On a boat sitting in the water this is even more important but the "earth" in this case should be direct to sea. So it should ideally be earthed to a seperate ground plate under the boat. Most small boats don't have one of these so you can connect directly to an anode bolt or if your skin fittings are bonded, to that. The problem is then that any earth leakage through this system can add to the problem of galvanic corrosion so it's wise to connect through a "galvanic isolator" which blocks small stray currents flowing but in the case of a dead short will allow a short directly to earth.
Its also wise to connect through an onboard RCD which will trip in milliseconds if there is a short. This advice is the same for any onboard AC. Connecing to DC- won't achieve much unless your DC- is also grounded to sea (which IMHO it should be) Even then the most direct connection to seawater (through a GI) is better than connecting to DC neg. In your case, if there is no anode I would consider fitting a small one to provide a ground plate or connect to a metallic skin fitting through a GI. Glad to see you already have RCD protection. Hope this helps!

 

[nigelmercier]

So, connect the charger case terminal to the mains earth? To the DC negative? Both? Neither?

Mains earth.

If you want your anodes to deplete, also connect to DC negative. There may be a new regulation saying you should do this if your boat is brand new, otherwise I suggest you don't. If you do, you should also fit a Galvanic Isolator, and make sure it is the only thing connected to the incoming earth.