Sailboat Owner's Guide to Corrosion - Galvanic and Stray Current Corrosion (Collier 6 & 7)

[Will Gilmore]

I was without my book for a few days, having left it in my daughter's car while we were out looking for a new car for her. I got it back last night and read a little on wiring to avoid stray current issues.

Avoiding multiple connections to ground really made sense as I read more in chapter 7 last night, and not something I would have thought twice about.

I'm putting this in overly simplistic terms because I don't have the book in front of me, so I can't be specific, so help me be sure I understand what Collier is saying about wiring.

There is a shore ground and there is a negative return wire, either green or bare and white and there is the static grounding of large metal equipment such as cases, engine blocks and boat structures that might become charged if a short should happen.

Tying the different paths to ground together can lead to sending a charge through parts of the boat that you don't want a charge going through. Collier said this was a common mistake made by electricians who don't have marine experience.

The shore side ground should not be tied into the grounded, or is 'bonded' the right term here, equipment chassis, etc. By doing this, any inadvertent stray current could pass through galvanically coupled systems as well as possibly lead to electrocution. The water itself can become a pathway that creates an electrical potential that becomes dangerous.

Is this condition alleviated by providing a full, low impedance connection to ground with as little resistance as possible or is it better to isolate these elements such that they have no pathway to ground at all?

-Will (Dragonfly)

 

[Davidasailor26]

Is this condition alleviated by providing a full, low impedance connection to ground with as little resistance as possible or is it better to isolate these elements such that they have no pathway to ground at all?

The problem is that it’s not really possible to isolate everything from ground reliably. Shore power is already grounded to earth. Let’s say we isolate all the AC appliances on the boat so that they don’t have any ground other than the white wire. If one of those components shorts internally its case will be electrified. Then if someone touches the case while touching anything with a connection to the water (sink, shower sump, bilge water, engine block, etc), they become a ground wire, which isn’t really something people are good at being. The green ground wire is the safety to prevent that condition.

 

[dLj]

There is a shore ground and there is a negative return wire, either green or bare and white and there is the static grounding of large metal equipment such as cases, engine blocks and boat structures that might become charged if a short should happen.

So first let me state that I am NOT a marine electrician - or any other kind of electrician - so anyone with more knowledge please correct me if anything I say here is incorrect or misleading.

Above you are talking about the AC electrical system. The AC electrical system and the DC electrical system are two totally separate electrical systems and should remain so in a boat - with one notable caveat.

In typical (modern) household AC single phase electrical systems, there are three wires:
1) the black (or I think sometimes red) wire, considered the "hot" wire.
2) the white often called the neutral or ground wire (why this gets confusing).
3) the green, or bare wire called the ground (Collier calls it the grounding conductor, I've never heard it called that before, but I don't know - so I'll use that term from here on). In boats, I've never seen the grounding conductor in bare wire, and would never permit it on my boat. I would only use a coated green grounding conductor to be used on my boat. But I"m sure open to anyone telling me how this may or may not be correct...

Do not confuse the white wire with the green or bare wire. They are not the same. In older homes the electrical system was a two wire system, essentially the black and white wires. As we learned more about AC electrical systems, we brought in the three wire system, adding in the green grounding conductor. In older homes a lot of household electricians would simply marry (connect) the new green wire to the white wire and call it a day. I'm not addressing that practice, just stating it as a fact. That's in household AC systems. Not at all the same environment as a boat.

In the AC system, the entire load that is fed to an appliance is brought in on the black wire, and returns to the energy source through the white wire. That means the white wire carries the full electrical load of the appliance.

The green wire, or grounding conductor sits at neutral, or ground. It is not meant to carry any current, ever, except in the case if there is a fault in the AC system where current "leaks" and needs to find a path to ground - the green wire provides that. This green wire is the only place where the two systems, the DC and the AC systems, connect.

I'd love to listen to someone with more knowledge here to talk about the galvanic isolator needed on the AC system.

Tying the different paths to ground together can lead to sending a charge through parts of the boat that you don't want a charge going through. Collier said this was a common mistake made by electricians who don't have marine experience.

See above talking about the common practice of "marrying the green and white wires in household systems.

The shore side ground should not be tied into the grounded, or is 'bonded' the right term here, equipment chassis, etc. By doing this, any inadvertent stray current could pass through galvanically coupled systems as well as possibly lead to electrocution. The water itself can become a pathway that creates an electrical potential that becomes dangerous.

The shore side white wire, not the grounding conductor (green wire), should not be tied to the equipment chassis.

Back when i did work in pipelines, we used the term bonded, or bonding, to mean electrical pathway connections made between sections of pipe in order to produce good electrical connections across the flanges separating out the different pipe sections. This would then allow the pipeline to be cathodically protected more efficiently.

Looking through the NACE dictionary, I see that term does not exist currently. What is found is:

continuity bond—a connection, usually metallic, that provides electrical continuity between structures that can conduct electricity.

This definition fits what I used to refer to as bonded, or bonding. Note to self: update internal vocabulary.

I am often hearing in the boat world, the term bonded or bonding. and it seems that this term is being applied to mean cathodic protection by establishing continuity bonds between submerged metallic fixtures and connecting them to a sacrificial anode.

Is this condition alleviated by providing a full, low impedance connection to ground with as little resistance as possible or is it better to isolate these elements such that they have no pathway to ground at all?

-Will (Dragonfly)

In practice you are doing both. You are isolating the AC power from the DC system as much as possible while providing a low impedance ground through the green wire.

I'm most certainly looking for comments on any/all of the above. I hope this at least helps a bit...

dj

 

[DArcy]

@dLj I am no more an electrician than you are but what you have presented seems in line with what I have learned. I will just add that the common vernacular among electrical engineers for "bonding" (at least the ones I have worked with) means to connect electrically to ground. It does not seem to be commonly used for other electrical connections.
For instance, Airbus uses MBN for the Metalic Bonding Network in composite aircraft. This is used for a DC return path as well as common ground. When away from "earth" ground you need to make your own common reference.

 

[dLj]

@DArcy - indeed, the term bonding does not appear in the NACE corrosion dictionary except as shown above. Bond, bonded, bonding all appear to have different meanings within different fields. Bonding is also used in metallurgy, adhesives and the like and has yet another fundamental definition. Makes me think it shouldn't be used as a single word. When applied to electrical connections when electrical continuity is desired, then continuity bond should be used. In your example, Metallic Bonding Network or MBN appears to be the proper term.

I do think (as I have stated here numerous times), using correct terms is really essential to not get confused. Just look at the simple example above from Will with the confusion between the white wire and the green wire. The same term is often used, confusing what the significant difference between them really is...

dj

 

[DArcy]

Terminology is important. With my background in mechanical engineering, I was used to "bonding" as a synonym for gluing so I use "electrically bonded" when talking about connecting to electrical ground. Continuity bond sounds good for this context.

 

[dLj]

@Will Gilmore - Should we put together Chapters 8 through 12? They are just all chapters on base materials used in the marine environment. Not sure why we'd want to separate them all out into individual threads.

dj

 

[Will Gilmore]

Funny you should say that. I just started it as Section 4

-Will (Dragonfly)