Personally, I'd be strongly disinclined to bond neutral to anything on a boat. Unlike ground lines, neutral lines are supposed to be carrying current as a matter of normal operations. Since the lines aren't superconductive, there is resistance, so there will be voltage between points on the line. So if something neutral comes into contact with the water, there will be an AC voltage between it and whatever neutral's been bonded to. Supposing that the bond is to the engine (or in the case of a metal boat, the hull), half the time, the prop shaft will have a positive voltage relative to the accidentally submerged "neutral". Noting also that losing up to 10% of voltage in transmission is generally considered acceptable, this could be as much as 12Vrms on a 120VAC system. So this will corrode the prop shaft that half of the time, and pretty soon the stuffing box is going to be irrecoverably leaky and the boat may sink (I don't think I even need to talk about a hull bond here).
This isn't as much of a problem with a DC system, because the engine/hull/etc. get bonded to the negative side of the circuit, and negative potentials accumulate metal rather than corroding. You're still going to be screwed if the positive side of the DC system gets shorted to the sea via, say, a through-hull, but it you have a line or a device that just gets submerged (e.g. a leak/short gets formed in the depth sounder, or the knotlog impeller) you just lose the wire/device (which it's pretty safe to assume would happen anyway, since the damned thing fell in the drink already). Now some of you are probably wondering about the half the time that the AC system would give the engine/hull bond a negative voltage; wouldn't that deposit back the metal that got corroded off when it was positive. Well sure, sort of. Of course the metal that gets accumulated back to replace the corroded metal won't necessarily be the same metal that came off (after all, in salt water, there's all that sodium kicking around, so what are the odds that all the ions sticking to the shaft and turning back into metal as they get electrons from the bond, are going to be from the bronze/stainless/etc. that you just sloughed off). On top of that, your metal will completely lose any temper that it had, and has a decent chance of ending up all spongy and brittle. So even if your prop shaft doesn't dwindle down to the thickness of a toothpick, there's a good chance that you'll be able to just plain snap it in half after a while (ditto for metal hulls, and wouldn't that ever suck). So unless someone can give me a really compelling reason to bond my AC neutral, I'll be keeping those lines ungrounded, and solidly mechanically seperated from any machinery, through-hulls, etc. that might carry current to the water if shorted to either of the lines. (And if you didn't pick it up from what was mentioned above, it's also a really good idea to keep your positive DC wires the hell away from anything like that too, lest they get shorted to some (through-)hull.) Cheers, Kris On Wed, Dec 08, 2010 at 11:05:56PM -0500, banders...@earthlink.net wrote: > Ahoy Al, > > I understand the wisdom of connecting all the touchable metal parts of > electrical equipment to ship's earth so the bilge water and all touchable > metal parts will be at the same voltage as the earth and I do this on my > vessel. > > It is also a good idea to have a way to measure the ac voltage/current > between the grounding (green or bare) wires coming from your system, and the > grounding point to which you attach them, so you can be aware of any leakage. > > But would you please address the issue of connecting (or not connecting) the > ac neutral to earth. > > For example, my 240 vac loads (and those ashore) have no neutral. Why should > 120 vac loads be any different? Wouldn't it make things safer to isolate > *all* current-carrying conductors rather than connect neutral ( a current > carrying conductor) to the ship's hull or other similar earth-like structure? > > I seem to have read that the European electrical system has ungrounded > neutrals. Is this true or not? > > I believe in the US power that comes into a house is from a center tapped > secondary winding in the pole transformer which delivers 240 vac at the ends > of the coil and the neutral comes from the center tap. The grounding of the > neutral is only added in the house's breaker box by the neutral/grounding > jumper wire. Correct me if I am mistaken. > > I have had an ungrounded neutral 120 vac system for 30 years on an > electrically powerful and complex vessel and have never felt the slightest > tingle. > > I did this mainly because that was the way they did it on the many merchant > marine ships I served on and because I believed the safety aspects of this > scheme would work just as well on my own vessel. > > Norm > S/V Bandersnatch > Lying Julington Creek FL > N30 07.68 W081 38.47 > > > ----- Original Message ----- > From: Al Thomason > To: liveaboard@liveaboardonline.com > Sent: 12/8/2010 9:03:06 PM > Subject: Re: [Liveaboard] GALVANIC ISOLATOR > > > Hi: > > OK, in short I think there are two questions here: > > 1) Connect the onboard AC safety ground to the onboard DC Ground point: > Yes ? always. There has been several articles over the years on people dying > when swimming around a faulty boat w/o this connection. ABYC requires it. > Some will complain that this can cause issues with stray current and galvanic > issues, but bottom line: This bonding is a major safety issue, the other > concerns can be addressed below. And more so, if something does go very very > bad, and you do not have this on your boat can cause lots of problems for you. > Here is an article of a local boy who died as a direct result of the boat not > having this connection and a fault occurring: > http://www.abycinc.org/lucas_ritz.pdf > > > 2) Connecting of ships AC Safety Ground to Shore Safety Ground.: There > are basically two ways to do this: 1) make the connection via your shore > power cable - use of a galvanic Isolator can help reduce some of the stray > current concerns here. And 2) use a true isolation transformer. In this > case, the shore safety ground is only connected to the shield in the > transformer, and then a new bonding point is established by connecting the > ship safety ground and ship neutral onboard (and after the isolation > transformer). This approach will remove almost all issues with stray > currents. > > This is not really that complex of a topic, though some folks like to poke at > #1 and cause confusion. Note also that the above might be modified some for > non US boats (e.g. EU boats). I am just not sure the best practice for them > and the AC system can be slightly different then the US system. (Beyond > 120v/60hz..) > > Good luck! > > -al- > > > From: liveaboard-boun...@liveaboardonline.com > [mailto:liveaboard-boun...@liveaboardonline.com] On Behalf Of Emory Jones > Sent: Wednesday, December 08, 2010 4:30 PM > To: liveaboard@liveaboardonline.com > Subject: Re: [Liveaboard] GALVANIC ISOLATOR > > Thanks to all who responded! There does not seem to a consensus in regard to > the AC ground. Any other (additional) discussion would certainly help to > educate me. I am concerned about an AC to DC connection however my fears may > be ungrounded (sorry I couldn't resist). > Thanks again for all the input. > > Emory Jones > S/V Jacob Andrew > Solomons, MD > > _______________________________________________ > Liveaboard mailing list > Liveaboard@liveaboardonline.com > To adjust your membership settings over the web > http://liveaboardonline.com/mailman/listinfo/liveaboard > To subscribe send an email to liveaboard-j...@liveaboardonline.com > > To unsubscribe send an email to liveaboard-le...@liveaboardonline.com > The archives are at http://www.liveaboardonline.com/pipermail/liveaboard/ > > To search the archives > http://www.mail-archive.com/liveabo...@liveaboardnow.org > > The Mailman Users Guide can be found here > http://www.gnu.org/software/mailman/mailman-member/index.html _______________________________________________ Liveaboard mailing list Liveaboard@liveaboardonline.com To adjust your membership settings over the web http://liveaboardonline.com/mailman/listinfo/liveaboard To subscribe send an email to liveaboard-j...@liveaboardonline.com To unsubscribe send an email to liveaboard-le...@liveaboardonline.com The archives are at http://www.liveaboardonline.com/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/liveabo...@liveaboardnow.org The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
