Re: [Amel Yacht Owners] Re: Explanation of Bonding System
Bill Kinney <greatketch@...>
Kent,toggle quoted messageShow quoted text
You have the Amel system right, but the “other boat’s” do not use the bonding system as a ground return—at least not on purpose! That would be well and truly ugly in salt water. I think all the metal bits on any boat wired like that would dissolve in a week!
One of the problems is that in English, we use the word “ground” to mean many different things in electrical systems, so it is easy to mis-communicate.
All boats should use an “insulated negative return”. All power should flow from positive to negative terminals of the battery confined to a wire dedicated to that circuit. (Not strictly true, but if you think of it that way, you are on the right track…) This is different than most cars, for example, where the metal frame is used as the negative return.
A bonding circuit is to maintain all the underwater metals at the same potential to avoid “hot spots” and to allow them to “share” the protection of a single zinc.
An AC Safety Ground (the “green wire”) is also connected to the bonding circuit in several ways, making them almost the same “system” even though they are there to do different things. Since no significant current flows through either of these circuits in normal operation, multiple interconnections are not a problem.
Here is where “other boats” are different from Amels: In a standard marine electrical system there is ONE (and ONLY ONE) connection between the DC negative and the bonding/AC safety ground. Without this connection the DC negative return in an Amel “floats” at a different potential relative to the bonding/AC safety ground system, hence the term “floating ground”.
If there are multiple connections, intentional or otherwise, between the DC negative return and the bonding system then ground loop currents are virtually assured, and the resulting electrolytic corrosion can be devastating. This is because different parts of the DC negative return will be at slightly different voltages because of varying voltage drops in the system. Note that this is true in both a standard system and an Amel “floating ground” system. Multiple connections between DC return and bonding circuit are ALWAYS bad on any boat.
Why do almost all other boats make this connection between the DC negative and the bonding circuit? Two reasons. First, in the event of a short between an AC hot wire and any DC wire the entire DC system would be charged to AC voltages, and potentially become very dangerous. If the DC negative is connected to the AC safety ground, the AC voltage has someplace to go, and it trips a breaker. The second reason, is in a lightening strike, keeping all the wiring systems at the same potential as much as possible is supposed to reduce damage. I understand and accept the first, and while the second reason makes sense, I have no idea how important it really is.
Why does Amel NOT make this connection? To reduce the change of corrosion while plugged in at a marina. It does not eliminate all forms of electrolytic and galvanic corrosion, just one source. As you well know, there can be others! Although some people can come to blows arguing about which system is better/safer/smarter, both systems can be successful.