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Old 10-04-2008, 16:30   #31
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If the boat has a DC grounding system and the AC ground is not connected to it, should the AC hot lead come in contact with the DC ground 120V AC current will flow from the DC ground sea contact to the AC ground on shore. This will create a shock hazard in the water surrounding the boat. If however, the AC and DC grounds are tied together the breaker will trip. The best way to bring AC aboard is through an isolation transformer.

For boats that do not have a bonding system, have an isolated DC ground system, or an isolated RF ground system have an additional risk of electrolysis, a "sneak" path for an AC shock hazard, and flashovers during lightning strikes.

Lightning will take ALL possible paths to get from ANY metal metal object above the waterline to the sea. To prevent "flashovers" during a lightning strike all metal objects aboard should be connected to a common seawater ground. This includes stays, shrouds, mast, poles, sail tracks, lifelines, stanchions, anchor(s), anchor chain(s), tanks, through-hulls, electrical components, electronic equipment, RF grounding plates, engines, etc. If you use low inductance connections to connect these items together you will by default have a very good RF ground system. Low inductance connections can be made using copper or stainless steal tape and braid. The stainless will last longer. Some common sense is required when making these connections. If an anchor is laying on a already grounded anchor roller then it too is grounded.


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Old 10-04-2008, 18:11   #32
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All of this talk about lightning strike reminds me of something that happen when I was 9 years old living in Colorado. One afternoon during a lightning storm, a pine tree in our front yard was struck by lightning. The bolt stripped all the bark off the tree, travelled down the entire length, jumped from the bottom branch, passed through the kitchen window aluminum frame blowing glass everywhere, passed across all the pots and pans hanging from a rack, and then grounded itself in our refrigerator melting the outlet and laying waste to all the electrical appliances, lamps, tv's, clocks etc. All this happened faster than I could blink my eyes while sitting at the kitchen table.

Our house had lightning rods and grounding cables every 20 ft across the roof. So when someone tells me that they can bond a boat and protect it from lightning, I'm going to smile and nod my head with disbelief.
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Old 10-04-2008, 18:26   #33
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Can someone explain the logic to me behind the ABYC requiring AC & DC shared ground?
In the modern world there isn't one. I suspect they have a lot of catching up to do.

There are plenty of boats, especially metal ones, built around the world that do not do so and the classification societies allow it. My own boat is also so built.

Comment has been made about the risk of galvanic corrosion to engines if not so - there is no risk if an isolated ground engine is used (and many come from the factory like that now) and there is no electrical connection to its block, etc from batteries, shaft or any other external metal part in the sea, that regardless of whether there is sea water in the engine's cooling circuits or not. Reason - because there is no complete electrical circuit. In fact there is less risk because the possibility of electrolytic corrosion driven from the battery connections is pretty much non existant, as is the risk from arcing accidents from broken or otherwise shorting of the battery conductors to the engine causes.
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Old 10-04-2008, 18:34   #34
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In the modern world there isn't one. I suspect they have a lot of catching up to do.
As I suspected
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Old 10-04-2008, 18:40   #35
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Trim50, did you read the article I referenced a few posts back? I think it explains it very well. It's a human safety factor.
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Old 10-04-2008, 18:48   #36
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Originally Posted by Viking Sailor View Post
If the boat has a DC grounding system and the AC ground is not connected to it, should the AC hot lead come in contact with the DC ground 120V AC current will flow from the DC ground sea contact to the AC ground on shore. This will create a shock hazard in the water surrounding the boat.
No, the boat's Ground Fault protection will isolate the AC and in civilised places the same will happen at the shore end connection as well. Also, isolated ground boats are fitted with 2 pole circuit breakers in the DC system (most sail boats are fitted with single pole protection).

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For boats that do not have a bonding system, have an isolated DC ground system, or an isolated RF ground system have an additional risk of electrolysis
No, metal boats, especially, are built with isolated DC systems for a specific reason to decrease the risk of electrolysis.

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To prevent "flashovers" during a lightning strike all metal objects aboard should be connected to a common seawater ground. This includes stays, shrouds, mast, poles, sail tracks, lifelines, stanchions, anchor(s), anchor chain(s), tanks, through-hulls, electrical components, electronic equipment, RF grounding plates, engines, etc.
In a small sail boat if you get hit by a lightning strike you are going to have problems regardless (think someone else has made that point too ). The types of protection along some of the lines you mention require conductor sizes, etc to take a strike beyond sensible fitting on other than very large sailing vessels (you may care to hunt out the specifications for those). A known risk of what you propose is the blowing out of through hulls and the subsequent loss of the boat through flooding if they are integrated into any lightning protection system on a plastic boat with conductor sizes capable of carrying lightning strike currents.

I don't know what the complete answer is for lightning and small sail boats - I suspect luck is the most important (certainly the most important if one is lucky enough to never be struck ) and also suspect a metal boat is likely to have fewer problems.
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Old 10-04-2008, 19:06   #37
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Trim50, did you read the article I referenced a few posts back? I think it explains it very well. It's a human safety factor.
I think you have to be very careful when reading articles by Nigel Calder. I read the article you linked to when it was first published and am not about to read it again, so my comments here are not aimed specifically at it as I don't recall the detail.

I have a lot of time for Calder's books and articles and see him as great value, but when it comes to boats with isolated DC systems, for some reason (perhaps because of ABYC) he avoids mention of them.

For example, in his Boatowners' Mechanical and Electrical Manual (at least up to the third edition, in case there is now a fourth) he does not even acknowledge the existance of isolated ground small marine diesels (certainly in this part of the world ALL small marine diesels from one major manufacturer are now so isolated and many have been for more than a decade) and much advice he gives is not appropriate to them. He makes a passing reference to isolated alternators but not in any connection to this subject. The local distributor of that range of engines tells me he is often called to sort out amateur installations or associated cabling of such engines.

I think what many are doing here is considering their own boat and just imagining that an isolated DC boat will only be different by having the DC negative isolated from the AC and bonding and all else on the boat will be the same. I have already mentioned Ground Fault protection (as has another, and which in any event should be on any AC supplied boat) and 2 pole breakers on DC circuits for protection. It is obviously more expensive to build such a boat - people who spend that money do so for the benefits, there are NO downsides.
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Old 10-04-2008, 19:25   #38
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If the boat has a DC grounding system and the AC ground is not connected to it, should the AC hot lead come in contact with the DC ground 120V AC current will flow from the DC ground sea contact to the AC ground on shore.
A question this time - I am a little puzzled by how the AC fault gets to the sea through an isolated DC system? The idea is there is no DC system contact with the sea and it is normally monitored to alarm if through some incorrect cabling or fault there is.
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Old 11-04-2008, 03:56   #39
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DC Negative & Grounding
Fig. 18,
from ABYC E-11
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Old 11-04-2008, 04:24   #40
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Don't know if is just me or not but the bottom part of the diagram that shows the common ground bar is chopped off for me GordMay.

John
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Old 11-04-2008, 04:28   #41
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Sorry - Goto
DC Negative & Grounding Diagram” at:
http://www.cruisersforum.com/gallery...p?i=4190&c=500
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Old 11-04-2008, 07:58   #42
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John,
I'm not quite sure what your rant about Calder and DC isolated diesel engines has to do with this discussion. If you can point out any other "expert" advice on the matter of whether AC and DC grounds should/should not be connected I would take a look. Calder certainly addresses the use of GFCI devices and states why he disagrees with the use of them as a "whole boat" protection. On this he is in agreement with ABYC recommendations and in disagreement with ISO standards and he gives, what appear to me, valid reasons for his views.

Once again, can you direct me to any valid research that says the ABYC guideline is wrong and the ISO standard is correct. I would be happy to read it, maybe even twice.
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Old 11-04-2008, 16:38   #43
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John,
I'm not quite sure what your rant about Calder and DC isolated diesel engines has to do with this discussion. If you can point out any other "expert" advice on the matter of whether AC and DC grounds should/should not be connected I would take a look. Calder certainly addresses the use of GFCI devices and states why he disagrees with the use of them as a "whole boat" protection. On this he is in agreement with ABYC recommendations and in disagreement with ISO standards and he gives, what appear to me, valid reasons for his views.

Once again, can you direct me to any valid research that says the ABYC guideline is wrong and the ISO standard is correct. I would be happy to read it, maybe even twice.
Hey, I didn't "rant" about Calder, in fact I stated that he was good value. I also did not claim that ABYC was wrong I have only described another way vessels' electrical systems can be configured - my only comment about ABYC was to the effect that ABYC does not seem to have caught up yet. So I suggest that you do not create your own fabrications of what I have said in order to try and gain ground in your arguments.

You clearly do not understand what I am talking about as you seem to think I am talking about isolated engines only, whereas I am talking about a whole electrical system, and you have started talking about GFCI's in a manner entirely unrelated to such systems.

What I have said must be clear to some others though because some have come back and told me that their DC systems are as that which I describe.

I suggest in response to your request for information validating what I say that you go off and find it for yourself. I have no desire to assist those who consider I "rant" and who ask for information in a tone already dismissive of it. But I will suggest that a good source of learning would be a discussion with the electrical engineers at any sizable yard building quality aluminium boats (they will be very familiar with what I am saying) - I only suggest that so's you can go away and argue with them, telling them they don't know what they are talking about, rather than me whose view you have already dismissed.
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Old 11-04-2008, 16:56   #44
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I think a person needs to be onboard during a lightning hit to fully appreciate the drive train's ability to channel this energy into the water. Having been there, I can't agree that drive shafts should be isolated. Sure things get blown up ,rather spectacularly I may add, but the hulls are undamaged.

As for Hot marinas.... isn't that what zinc anodes are for?
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Old 11-04-2008, 17:42   #45
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it's getting warm in here, time for a cold one...
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