The same size would seem logical.1. what size should this cable be - I am assuming it ought to be at least equal to the incoming mains earth cable?
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.2. is it better to go to a common ground (eg. engine) or to the battery negative?
Results 11 to 16 of 16
08-02-10, 22:59 #11Sea Wych Owners Association: www.Seawych.org
08-02-10, 23:39 #12Registered User
- Join Date
- Oct 2006
Last edited by goboatingnow; 08-02-10 at 23:43.
08-02-10, 23:57 #13
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.
Last edited by gasdave; 08-02-10 at 23:59.
09-02-10, 06:30 #14Registered User
Location : Oakland, California outside San Francisco
- Join Date
- Dec 2009
09-02-10, 16:24 #15Registered User
- Join Date
- Jul 2007
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.
09-02-10, 23:24 #16Registered User
Location : West Australia
- Join Date
- Jul 2003
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