AC ground to DC negative connection

[VicS]

1. what size should this cable be - I am assuming it ought to be at least equal to the incoming mains earth cable?

The same size would seem logical.

2. is it better to go to a common ground (eg. engine) or to the battery negative?

ISO 13297 to which I have provided a link says the connection to the DC should be as close as possible to the battery negative.

[goboatingnow]

Maybe I misread you. Surely the GI does not disconnect the RCD isn't the whole point that it isolates low voltages (from galvanic sources) but allows current from higher voltage sources (the mains power supply) to flow so that the RCD and the MCB or fuse still give the normal protection

yes sorry, I meant that the GI effectively disconnects the AC protective earth anyway. It doesnt do anything with voltages but what it should do is prevent low stray earth currents from entering the boat, coupling across into the dc negative and then attacking you underwater connected metals. What it does in practice is another thing alltogether.

[gasdave]

I for one am not in favour of connecting the protective earth ( misnamed AC ground) to the DC negative, provided a whole boat RCD is fitted. Its worth pointing out that a Galvanic isolator in effects basically disconnects that anyway.

In my experience its often found that there are significant fault currents in the earth circuit, this can cause a GI to close, effectivly nullifying their existence.

I think the point about the RCD is that there is a reported failure rate of (I think) 10 - 20% on testing. The device is safe when it works.

The GI is designed not to conduct under normal circumstances (unless it has a capacitor allowing small AC current flow - your "fault current") thus providing galvanic protection. When there is a voltage leak it is designed to close/conduct thus providing person protection. The voltage at which it closes is determined by the number of diodes.
My understanding is that the GI does not "disconnect" the RCD - it simply provides a safe return to earth of leaked AC. The RCD is still able to sense the difference in current between the live and neutral, so can trip.

Calder's and presumably the American argument for using the AC earth to DC negative connection includes the scenario where you may have an AC leak into the DC circuit (eg. fault in a battery charger). This may not generate enough current flow through the DC ground (water) to trip the shore or boat RCD and without the connection has no other means of returning to earth. You then have a very "hot" DC system, and potentially dangerous surrounding water.

There is also something about adequate lightening protection requiring the two grounding circuits being held to the same voltage potential.

The downside is that there is a risk of having your DC equipment "fried" in the event of a failure on the AC earth to shore return line at the same time as an on-board leak or short - when "all the holes in the Swiss cheese line up"! The upside is that you may survive it.

[Stu Jackson]

Some observations here:

http://c34.org/bbs/index.php/topic,4685.0.html

[Jegs]

FWIW,

A possible failure of an RCD was mentioned. In my limited experience, a total of three, they simply repeatedly trip: which is, of course, the way you'd want it.

John G

[William_H]

Some observations here:

http://c34.org/bbs/index.php/topic,4685.0.html

Thanks Stu that was an interesting link (West Marine) Re that link what is a Ground Fault Circuit Interupter? Is that another name for RCD An RCD as I understand it compares current in the active to current in the neutral. Any difference is current via earth or through a person so shuts off.Or is that a core balance relay? A different device or different name?

I am not sure about the earthing details given in that article. I dont think many baots have the negative battery isolated from the engine. Mostly connection via stater alternator and pressure sensors mean engine is very much connected to batt negative. Only a few have isolated staters Alternators etc
I would not connect an earth wire to the chain plates as suggested for lightning protection unless you have galvanised steel wire. Stainless steel has a significant electrical resistance just enough to get really hot and lose its temper (heat treatment) with any large current. The loss of strength could see a failure some time after a strike. I think the earthing of the Al mast should be the only ground path for the lightning current. Of course you could say the low resistance of the Al mast should bypass lightning current from the stays. i think stays best left not earthed.

I am not at all sure about his assertions that salt water in a marina carries currents which can take an alternative path through interconnected skin fittings. I am no expert but doubt it.
Even well written articles need to be scrutinised.
For comment olewill