Role of engine in DC "boat ground potential" - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
 Not a Member? 
  #1  
Old 03-19-2009
Senior Member
 
Join Date: Jul 2008
Posts: 140
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 7
floatsome is on a distinguished road
Role of engine in DC "boat ground potential"

Back to basics. Here is a simple, theoretical schematic. I've never had formal training in electricity beyond high school physics, and I don't want to presume anything.

1. The engine block is in the negative side of the DC system. What creates "boat ground potential" in that system -- the connection to the negative posts of the batteries? What is the relevance of the fact that the propshaft is in the ocean and electrically continuous with the engine block -- is that important in the nature of DC ground, or does "boat ground potential" in that circuit just mean return to the negative battery pole?

2. What happens if the boater unhooks black cable E from the starter mounting bolt in order to disable the engine circuit but leaves house circuit intact and switched on? Is the house circuit still "grounded" because cable F completes the house battery circuit to the house battery negative post?

3. ABYC E-11 states "11.5.4.7.4 If the negative side of the DC system is to be connected to ground, the connection shall be made only from the engine negative terminal, or its bus, to the DC grounding bus. This connection shall be used only as a means of maintaining the negative side of the circuit at ground potential and is not to carry current under normal operating conditions." What does "not to carry current" mean? Does not the black wire in a standard two-wire DC system carry current, and so the engine block also carries current for devices that ground through their metallic attachment to the block?
Attached Thumbnails
Role of engine in DC "boat ground potential"-basic-dc-schematic-jt-d-.jpg  
Reply With Quote Share with Facebook
  #2  
Old 03-20-2009
Bender of Nails
 
Join Date: Jan 2009
Location: West Coast BC
Posts: 66
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 6
Deadeye is on a distinguished road
Marine applications are one of the few instances where "negative" and "ground" are not synonymous.

"Negative" deals with the current carried from a battery, genny, etc whereas "ground" refers to the current generated by dissimilar metals within an electrolyte (seawater).
Right away, you can see that the grounding system's purpose is to prevent the boat from becoming a battery by making sure the "battery's" electrodes (underwater metal components) are shorted out and allowed to dissipate through the sacrificial anodes (zincs)

The ABYC bit you quoted specifically forbids using any of this metal as part of the grounding(bonding) system. Essentially, you are electrically connecting all the underwater metal components. In your example, if the battery negative were disconnected, then obviously the engine wouldn't start. Ignoring that for the moment, the zincs would begin to corrode much faster. After being eaten away, more expensive metal begins to disappear.

In building aluminum power boats, we would bond all ac components and all significant hunks of metal together with 4 ga wire (green). Otherwise, the practical results are gremlins like hot water tank and heat exchanger zincs that don't last the season.

A marina shorepower hookup can make this even worse because now you can have two adjacent boats acting as individual electrodes.

This is all completely independent of the DC power system.
__________________
"Verbosity leads to unclear inarticulate things"
~Dan Quayle

Last edited by Deadeye; 03-20-2009 at 09:35 AM. Reason: clarity
Reply With Quote Share with Facebook
  #3  
Old 03-20-2009
kaluvic's Avatar
Senior Member
 
Join Date: Jan 2009
Location: Sana'a Yemen
Posts: 263
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 6
kaluvic is on a distinguished road
I'm considering not bonding my tru-hull fitting on my FG boat...I hope to hear more pros and cons on this thread.
__________________
James S
S/V Arctic Lady
Reply With Quote Share with Facebook
  #4  
Old 03-20-2009
Senior Member
 
Join Date: Jul 2008
Posts: 140
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 7
floatsome is on a distinguished road
I'd prefer for this thread not to go down the bonding rabbit hole, if that's OK. My question is simpler than that.

To better focus my question, is there anything wrong with sailing along using the house panel with the engine taken out of the circuit by disconnecting cable E at the battery in that theoretical schematic? (Other than the fact that none of the engine circuit devices will be usable, of course, and for safety cable A ought to be disabled too, using the battery switch.)

[My understanding: There would be nothing wrong with that practice, if the boat's wiring is installed and maintained to code.]
Reply With Quote Share with Facebook
  #5  
Old 03-20-2009
Senior Member
 
Join Date: Jul 2008
Posts: 140
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 7
floatsome is on a distinguished road
I am specifically chasing clearer understanding of "ground", "grounded" and "negative" in the DC circuits, not "grounding" or "bonding", which are not part of the current-carrying wires normally supplying devices in a DC system with current. I left AC green grounding, lightning and DC bonding wires out of that schematic to avoid all the distraction they create. Hopefully, we can leave bonding out of this thread as much as possible, and just focus on DC negative and the role of the engine block. My amateur understanding is shown in square brackets and open to correction.

E-11 gives these definitions:

Ground: The potential of the earth's surface ... established by a conduction connection (intentional or accidental) with the earth, including any conductive part of the wetted surface of a hull.

[My understanding: in a self-contained DC system in a fibreglass boat, "ground potential" is determined by the negative posts of the batteries. But, I wonder what role the propellor shaft's connection to the ocean plays in defining a boat's ground potential, and whether it is important.]

DC Grounded Conductor: A current carrying conductor connected to the side of the power source that is intentionally maintained at boat ground potential.

[My understanding: any DC device's black cable or wire that is ultimately connected to the negative terminals of the batteries is the negative wire, and can be considered "ground" = "boat's ground potential". These normally carry current, unlike bonding wires that some boaters might connect between a metallic throughull fitting and the boat's DC wiring.]

DC Grounding Conductor: A normally non-current carrying conductor used to connect metallic noncurrent carrying parts of direct current devices to the engine negative terminal ...

[My understanding: if a wire is connected from a DC device's casing to the boat's DC negative wiring, the purpose of that wire is to carry current only if there is a short in the device that would otherwise make the device's casing electrically hot. This seems equivalent to the third green or bare wire in an AC system, but I don't recall ever seeing a third, normally non-current conducting wire from DC device.]

[Calder (page 231) emphasizes that the term "grounded" is reserved for the current-carrying wires that normally supply current to DC devices, while "grounding" is reserved for wires that only carry current in unusual situations, such as stray current (bonding), or lightning.]

Polarized system DC: A system in which the grounded (negative) and ungrounded (positive) conductors are connected in the same relation to terminals or leads on devices in the circuit.

[My understanding: "Grounded" in this sense means that the black wire intended to complete the current-carrying DC circuit for a device is continuous with the battery negative terminals. So, it makes sense to me that "boat ground potential" (phrase used in E-11) might not be the same as earth (ocean) ground if the electrical connection is only to the negative posts of the batteries to create a local "ground" within the boat, and not to the ocean.]

[My understanding: The engine block in that schematic is a giant black wire, carrying current for devices that attach to the engine block and use the metal attachment as their current-completing path to the boat's ground (battery negative terminals). 1. That's why it is so important to keep the main DC ground cable connection pristine from engine block to battery negative post. 2. This analogy explains why I think it is OK to disconnect the engine circuit, but safely carry on using the house circuit.]

The propellor shaft connects the engine block to the ocean. That's not like in a car, where the engine block normally has no connection to earth. I am still fuzzy about a couple of things:

1. Whether the DC current from a device like an oil pressure sender or alternator that grounds through its connection to the engine block could return through the prop shaft to the ocean instead of back to the battery, and whether this is important. Calder (p 121) points out that current actually flows from negative to positive.

2. Whether the propellor shaft connection therefore puts the boat's ground potential to earth, over-riding the potential at the battery negative terminals.

2a. Whether point "2" is important.
Reply With Quote Share with Facebook
  #6  
Old 03-20-2009
Senior Member
 
Join Date: Jul 2008
Posts: 140
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 7
floatsome is on a distinguished road
Quote:
Originally Posted by Deadeye View Post
The ABYC bit you quoted specifically forbids using any of this metal as part of the grounding (bonding) system.
Ah yes, I see now what they were getting at. I was distracted by the "if" in "If the negative side of the DC system is to be connected to ground...".
Reply With Quote Share with Facebook
  #7  
Old 03-25-2009
Senior Member
 
Join Date: Jun 2006
Posts: 4,998
Thanks: 5
Thanked 18 Times in 18 Posts
Rep Power: 11
wind_magic has a spectacular aura about wind_magic has a spectacular aura about wind_magic has a spectacular aura about
Quote:
Originally Posted by floatsome View Post
Back to basics. Here is a simple, theoretical schematic. I've never had formal training in electricity beyond high school physics, and I don't want to presume anything.

1. The engine block is in the negative side of the DC system. What creates "boat ground potential" in that system -- the connection to the negative posts of the batteries? What is the relevance of the fact that the propshaft is in the ocean and electrically continuous with the engine block -- is that important in the nature of DC ground, or does "boat ground potential" in that circuit just mean return to the negative battery pole?

2. What happens if the boater unhooks black cable E from the starter mounting bolt in order to disable the engine circuit but leaves house circuit intact and switched on? Is the house circuit still "grounded" because cable F completes the house battery circuit to the house battery negative post?

3. ABYC E-11 states "11.5.4.7.4 If the negative side of the DC system is to be connected to ground, the connection shall be made only from the engine negative terminal, or its bus, to the DC grounding bus. This connection shall be used only as a means of maintaining the negative side of the circuit at ground potential and is not to carry current under normal operating conditions." What does "not to carry current" mean? Does not the black wire in a standard two-wire DC system carry current, and so the engine block also carries current for devices that ground through their metallic attachment to the block?
Floatsome, I just found this thread, and I'm trying to understand what it is exactly that you are asking. It sounds like what you are asking is if the negative potential in a circuit needs to be connected to earth ground, is that what you want to know ?

The answer to that question is, not really, you can float a ground. Airplanes do this all the time, they are not grounded to earth. Solar panel systems sometimes do this as well though I don't think that is to code. You can hook a solar panel up to a battery with two wires and the panel will charge the battery, but it is normal to connect the negative side of the circuit to an earth ground. What they mean by it not carrying any current is that the earth ground connection just insures everything is at the same potential, but that connection really doesn't carry current to do that, it's more like touching metal and getting a static spark, once it is discharged, there is no measurable flow of current after that.

If you disconnect your electrical system from earth ground it will still work, it is called a "floating ground".
__________________
What are you pretending not to know ?

Please support my
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
  #8  
Old 03-26-2009
Senior Member
 
Join Date: Jun 2006
Posts: 4,998
Thanks: 5
Thanked 18 Times in 18 Posts
Rep Power: 11
wind_magic has a spectacular aura about wind_magic has a spectacular aura about wind_magic has a spectacular aura about
Quote:
Originally Posted by floatsome View Post
I am specifically chasing clearer understanding of "ground", "grounded" and "negative" in the DC circuits, not "grounding" or "bonding", which are not part of the current-carrying wires normally supplying devices in a DC system with current. I left AC green grounding, lightning and DC bonding wires out of that schematic to avoid all the distraction they create. Hopefully, we can leave bonding out of this thread as much as possible, and just focus on DC negative and the role of the engine block. My amateur understanding is shown in square brackets and open to correction.
I will take a shot at this.

Quote:
E-11 gives these definitions:

Ground: The potential of the earth's surface ... established by a conduction connection (intentional or accidental) with the earth, including any conductive part of the wetted surface of a hull.

[My understanding: in a self-contained DC system in a fibreglass boat, "ground potential" is determined by the negative posts of the batteries. But, I wonder what role the propellor shaft's connection to the ocean plays in defining a boat's ground potential, and whether it is important.]
What you call self-contained is a floating ground system, a system that has a ground that is at an unknown potential. Connecting that ground to an actual earth ground will put earth and the negative side of your circuit at the same potential. Your prop, since it touches water, is at earth potential. If there is an electrical connection between the ocean and the negative side of your circuit, your circuit is grounded to earth and is no longer floating.

Quote:
DC Grounded Conductor: A current carrying conductor connected to the side of the power source that is intentionally maintained at boat ground potential.

[My understanding: any DC device's black cable or wire that is ultimately connected to the negative terminals of the batteries is the negative wire, and can be considered "ground" = "boat's ground potential". These normally carry current, unlike bonding wires that some boaters might connect between a metallic throughull fitting and the boat's DC wiring.]
Ground wires in a circuit carry current, in fact the negative side of your battery is the source of electrons and they flow backwards from the negative wires through the devices into the positive side of the battery, the "holes" left by the moving electrons flow from positive to negative (reverse direction).

Quote:
DC Grounding Conductor: A normally non-current carrying conductor used to connect metallic noncurrent carrying parts of direct current devices to the engine negative terminal ...

[My understanding: if a wire is connected from a DC device's casing to the boat's DC negative wiring, the purpose of that wire is to carry current only if there is a short in the device that would otherwise make the device's casing electrically hot. This seems equivalent to the third green or bare wire in an AC system, but I don't recall ever seeing a third, normally non-current conducting wire from DC device.]

[Calder (page 231) emphasizes that the term "grounded" is reserved for the current-carrying wires that normally supply current to DC devices, while "grounding" is reserved for wires that only carry current in unusual situations, such as stray current (bonding), or lightning.]
The only purpose of this conductor is to bring the negative side of your circuit to earth ground, or to bring the negative side of two floating grounds to the same ground potential. If you had two batteries, for example, that were not connected together in any way, and independent circuits connected to each battery's terminals, you would have two separate floating grounds and they could be at different potentials. Connecting the negative terminal of the two batteries together with a DC grounding connector would bring both floating grounds to the same potential, the result being one floating ground. Connecting them together and then to earth ground brings them both to earth ground.

Quote:
Polarized system DC: A system in which the grounded (negative) and ungrounded (positive) conductors are connected in the same relation to terminals or leads on devices in the circuit.

[My understanding: "Grounded" in this sense means that the black wire intended to complete the current-carrying DC circuit for a device is continuous with the battery negative terminals. So, it makes sense to me that "boat ground potential" (phrase used in E-11) might not be the same as earth (ocean) ground if the electrical connection is only to the negative posts of the batteries to create a local "ground" within the boat, and not to the ocean.]
Right, a floating ground.

Quote:
[My understanding: The engine block in that schematic is a giant black wire, carrying current for devices that attach to the engine block and use the metal attachment as their current-completing path to the boat's ground (battery negative terminals). 1. That's why it is so important to keep the main DC ground cable connection pristine from engine block to battery negative post. 2. This analogy explains why I think it is OK to disconnect the engine circuit, but safely carry on using the house circuit.]
Yes, but will your alternator still charge, and will your starter work ? If your starter is grounded to the engine block then the engine block has to be connected to the negative terminal of the starting battery to have a circuit, for example. Same for your alternator, it has to be a complete circuit.

Quote:
The propellor shaft connects the engine block to the ocean. That's not like in a car, where the engine block normally has no connection to earth. I am still fuzzy about a couple of things:
Some vehicles such as RV's occasionally drag along little wires that brush against the road to bring them to earth ground.

Quote:
1. Whether the DC current from a device like an oil pressure sender or alternator that grounds through its connection to the engine block could return through the prop shaft to the ocean instead of back to the battery, and whether this is important. Calder (p 121) points out that current actually flows from negative to positive.
I'm not sure what you are asking here, it sounds like you are wondering if you can use the positive potential from one circuit and the negative potential from another and the answer to that question is no because you wouldn't actually have a circuit.

Said another way, you can actually do this - if you have a solar panel and a battery and you hook the panel up to the battery with two wires, the panel will charge the battery and so long as you do not connect the negative terminal with earth ground you will have a floating ground. Now you can take ANOTHER battery, connect ITS negative terminal to the POSITIVE terminal of the first battery, and connect another circuit to the SECOND battery and it will all still work. You will have two floating grounds, essentially, the first battery's ground will float at some unknown potential, and the second battery's ground will float at the first battery's positive terminal potential, which would then make the second battery's positive terminal 24 volts (12 x 2) away from the first battery's negative terminal potential (another way to say that is that they are connected in series). If you can understand that, you got it. You can use a ground wire to bring your floating ground to any potential you want just by connecting it to something, same as reaching out to touch something to discharge static when you wear wool clothing.

Quote:
2. Whether the propellor shaft connection therefore puts the boat's ground potential to earth, over-riding the potential at the battery negative terminals.
Assuming it's electrically connected. Don't think of it as "over-riding", think of it as instantaneously bringing them to the same potential, like a static discharge, and then keeping them connected so that neither one of them wanders off and builds up a static charge again relative to the other. You can test this connection easily enough with an ohm meter - run a wire over the side of the boat into the water, connect that to one lead of your ohm meter, then touch the other lead of your ohm meter to whatever you are interested in, if it's something near zero ohms resistance then there is an electrical connection with earth ground.

Quote:
2a. Whether point "2" is important.
It's important if you don't want a floating ground. If your ground floats then that means that the negative side of your electrical circuit could at some point have a vastly different potential than earth ground, and in some situations that could lead to things like sparks jumping from your floating ground to earth ground for example. That's why they recommend you ground your car and yourself to earth ground before you pump gasoline at the filling station, because your car's electrical system floats and can become charged relative to the gas station's pump, possibly leading to a static spark that could ignite the fuel.
__________________
What are you pretending not to know ?

Please support my
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Last edited by wind_magic; 03-26-2009 at 01:08 AM. Reason: sp
Reply With Quote Share with Facebook
  #9  
Old 03-26-2009
Senior Member
 
Join Date: Jun 2006
Posts: 4,998
Thanks: 5
Thanked 18 Times in 18 Posts
Rep Power: 11
wind_magic has a spectacular aura about wind_magic has a spectacular aura about wind_magic has a spectacular aura about
Quote:
Originally Posted by wind_magic View Post
I'm not sure what you are asking here, it sounds like you are wondering if you can use the positive potential from one circuit and the negative potential from another and the answer to that question is no because you wouldn't actually have a circuit.
I thought of another way to explain this. If you have a solar panel hooked up to a battery with two wires and your ground is floating then you have no idea what the difference in potential is between your ground and earth ground. If you take a 12vdc light and hook it up to your battery, it will light up, because electrons will flow from the negative terminal of your battery through the light and back into the positive terminal of your battery, it is almost as if your battery were a pump moving electrons from one side of the battery to the other through your circuit. Now, if you were to hook the negative lead of your light to EARTH GROUND (instead of your batteries ground) and then you hooked up the positive lead to your batteries positive terminal, electrons are not going to flow from your battery into earth ground - instead what is going to happen is that your batteries potential is going to move relative to earth ground instead of pumping electrons and nothing is going to happen. Of course, if you were to hook the negative terminal of your battery up to earth ground, you've got a circuit, and the light would shine. Said another way, you could take a piece of wire and directly connect the positive terminal of a floating battery to earth ground and all it would do is change the potential of the negative terminal to -12vdc instead of 0vdc.

In addition to all of the above, there is yet another kind of ground on your boat, and that's the RF ground used in your radios, and that is something else entirely and has little to do with anything you've already talked about.

With your RF ground your radio is essentially acting as a pump, rapidly moving energy to and from earth ground through the radio and into and out of space through the antenna. If you don't have a good RF ground then the pump doesn't have a source for electrons and nowhere to send them and the radio doesn't work well, and if you don't have a good antenna on the other side then the same thing happens, the radio doesn't work efficiently. So you are trying to give your radio/pump a good antenna that is well connected with the sky and passes energy easily, and you are also trying to give your radio/pump a good connection to the earth so that energy also passes easily to/from the earth. That means you need a ground plane of some kind to bind your radio to the earth, this often means creating a capacitor in a fiberglass boat by basically spreading out lots of foil or metal mesh, etc, along the hull so that it is as close as possible to the water and and has a large area. One little wire or your prop shaft won't do the job, it's too puny, and RF energy won't pass through it well, it's like using a really little pipe on one side of your pump, the radio just won't be able to move energy well enough to do its job effectively. This ground is in addition to your other grounds, that's why your HF radios have two grounds instead of one. It doesn't really matter much for VHF.
__________________
What are you pretending not to know ?

Please support my
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Last edited by wind_magic; 03-26-2009 at 05:38 PM. Reason: correction
Reply With Quote Share with Facebook
  #10  
Old 03-26-2009
Senior Member
 
Join Date: Jul 2008
Posts: 140
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 7
floatsome is on a distinguished road
wind_magic - thanks. You filled in all the gaps nicely.
Reply With Quote Share with Facebook
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Nissan / Tohatsu 6h 4 stroke audeojude Outboard 255 01-31-2014 06:53 PM
Rebuild vs replace diesel engine Ilenart Gear & Maintenance 40 02-21-2010 10:23 PM
Overheating HELP !!! Culinary411 Gear & Maintenance 19 08-07-2007 05:33 PM
Atomic 4 replacement mr.fixit Gear & Maintenance 4 06-16-2005 04:48 PM
Cat30 Exhaust riser blew up - Engine won''t start E_N_Z_O Gear & Maintenance 2 07-31-2001 06:55 PM


All times are GMT -4. The time now is 10:48 AM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.