Proper Grounding for AC and DC

[davisr]

I've spent a lot of time reading Don Casey, Nigel Calder, and the many helpful postings by MaineSail on this and other forums regarding rewiring projects. I have almost finished my wiring plan, but one area remains a stumbling block for me – the grounding of my AC and DC systems.

Like many owners of boats in the 27 foot range and under, my Ericson 25 has not an engine, but an outboard, specifically a Yamaha 9.9 high thrust with an electric starter. Neither Casey nor Calder address the issue of outboards, although Calder does state the following, "although it is common practice to select a substantial bolt on the engine block as the DC main negative bus, this is not the best choice. It is preferable to set up a separate bus bar," Boatowner's Mechanical and Electrical Manual, p.232.

Calder goes on immediately after this to say that this separate current-carrying bus bar, which he calls the DC Negative Main Bus, should, in turn, be connected to another noncurrent-carrying bus, which he calls the Grounding Bus. He notes that this Grounding Bus is also known by the following names: Common Ground Point, Central Bonding Strap, Equalization Bus, and DC Grounding Bus. Calder then says that this Grounding Bus should itself be connected to an immersed Ground Plate or Ground Strip. Calder has a good diagram which helps to simplify things, but to simply things with words I'll summarize as follows:


DC Negative Main Bus --> Grounding Bus --> Ground Plate


Calder's suggestions will likely cause many to bristle, since the issue of ground plates/ground strips and lightning protection is a hotly debated one, but I bring all this up only for the sake of the issue at hand -- finding a ground for my AC and DC systems. Back in May of this year, there was an active thread on another forum with the title "A/C Grounding Question." On that thread, Maine Sail posted the ABYC regulations on this subject. I have placed in bold the section that I believe pertains to my boat.

Maine Sail said:

If you want to wire to current suggested and accepted marine standards:

ABYC E-11

11.5.5.3: The main AC system grounding bus shall be connected to

11.5.5.3.1 the engine negative terminal or the DC main negative bus on grounded DC systems, or

11.5.5.3.2 the boat’s DC grounding bus in installations using ungrounded DC electrical systems.

11.5.5.4: In AC circuits, all current carrying conductors and the grounding conductor shall be run together in the same
cable, bundle or raceway.

11.5.5.5: There shall be no switch or overcurrent protection device in the AC grounding (green) conductor.

11.5.5.5.1 Where isolation of galvanic currents is desirable, only devices meeting the requirements of ABYC A-28, Galvanic Isolators, shall be used in the grounding path


I believe that since my Ericson 25 has an outboard motor instead of an engine my DC electrical system would be considered ungrounded (but I'm unsure). At any rate, the ABYC, in this section, uses the term DC grounding bus. This is one of the terms that Nigel Calder says is used to refer to the bus located near the Ground Plate or Ground Strip.

The Ericson 25, apparently unlike larger Ericsons, was built with a Ground Plate or Strip of some sort. On my E25 (and I know on at least one other), there is a bolt in the bilge near the mast compression post. To this bolt are connected wires from the chainplates (starboard, port, and forward). Based on everything that I've said above, it seems that I should use this bolt, or at least a bus that I install near it, as my Grounding Bus or Central Grounding Point for my AC and DC systems. I've attached a diagram that will hopefully make things a little more clear.

My question, then, to the Cruiser’s Forum is this: am I interpreting the ABYC standards correctly?

Thanks,
Roscoe

Ericson 25, #226

[goboatingnow]

ABYC requires that the DC negative should be conncted to the AC protective earth. As to calder recommendations I'd ignore them quite frankly.

dave

[Maine Sail]

Originally Posted by davisr

I believe that since my Ericson 25 has an outboard motor instead of an engine my DC electrical system would be considered ungrounded (but I'm unsure). At any rate, the ABYC, in this section, uses the term DC grounding bus. This is one of the terms that Nigel Calder says is used to refer to the bus located near the Ground Plate or Ground Strip.

In your case your DC system would be an "ungrounded" one unless you connected it to the mast base lightning grounding buss.

If you connect your DC system to the lightning ground/keel and your AC green to the DC the AC grounding conductor (green) will have two paths back to earth for safety. The shore power cord and through the ships grounding path. If the shore power grounding conductor path becomes compromised, as they certainly can and do, you will still have a path for AC to find earth.

Perhaps some ABYC definitions will help minimize confusion:

11.4.1 AC grounded conductor - A current carrying conductor that is intentionally maintained at ground potential. (eg: white wire)

11.4.2 AC grounding conductor (green or green with a yellow stripe) - A conductor, not normally carrying current, used to connect the metallic non-current carrying parts of AC electrical equipment to the AC grounding bus, engine negative terminal or its bus, and to the source ground.

11.4.5 DC grounded conductor - A current carrying conductor connected to the side of the power source that is intentionally maintained at boat ground potential. (these are your black or yellow DC neg wires)

11.4.6 DC grounding conductor - A normally non-current carrying conductor used to connect metallic non-current carrying parts of direct current devices to the engine negative terminal, or its bus.

Originally Posted by davisr

The Ericson 25, apparently unlike larger Ericsons, was built with a Ground Plate or Strip of some sort. On my E25 (and I know on at least one other), there is a bolt in the bilge near the mast compression post. To this bolt are connected wires from the chainplates (starboard, port, and forward). Based on everything that I've said above, it seems that I should use this bolt, or at least a bus that I install near it, as my Grounding Bus or Central Grounding Point for my AC and DC systems. I've attached a diagram that will hopefully make things a little more clear.

This sounds like a lightning ground point and is likely connected to the keel if you can't see a copper plate or strip outside the hull.

Originally Posted by davisr

My question, then, to the Cruiser’s Forum is this: am I interpreting the ABYC standards correctly?

Thanks,
Roscoe

Ericson 25, #226

It seems as if you are on the right track if you intend to wire to the current standards.

The key is that the DC wires connected to that grounding point are not current carrying and just grounding wires. Only with an AC fault would AC current potentially pass through this grounding point and you'd hopefully spot the reverse polarity alarm/light quickly and fix the issue.

[delmarrey]

Originally Posted by goboatingnow

ABYC requires that the DC negative should be conncted to the AC protective earth.

NO WAY! Not on my boat! Just what I need is a short backing up into the DC system.

[amarinesurveyor]

Listen to what Calder and ABYC have to say, they know what they are talking about...it will make your boat safer! The purpose is to protect the people on board against shock.
Brian

[goboatingnow]

Originally Posted by delmarrey

NO WAY! Not on my boat! Just what I need is a short backing up into the DC system.

Actually I fully agree, and under European rules, it is not required to connect the two together once a RCD is installed. Its much better electrically to not have a connection between Dc negative and AC ground, as it prevents transients and noise on the AC earth being coupled into the DC system ( not to mention what happens when the earth gets raised up at times).

ABYC however is the code in the US as it predates the mandatory use of RCD or ELCIs

Dave

[goboatingnow]

Originally Posted by amarinesurveyor

Listen to what Calder and ABYC have to say, they know what they are talking about...it will make your boat safer! The purpose is to protect the people on board against shock.
Brian

calder will actually outline both the European and ABYC rules, which are different. Its a trade off of safety factors.

Dave

[djmarchand]

Well, irrespective of where is the proper ground point, you should be aware that an aluminum outboard motor represents some special problems. First can it be raised to the point that it is not in the water at all when it isn't in use. That is preferrable since aluminum outboards can corrode badly if left in the water long term.

Secondly the aluminum needs special protection if left in the water. I would first make sure that the paint is in perfect shape- touch up all scratches rigorously. Make sure that the zincs are in perfect shape and replace them if even partially wasted.

I would prefer that if the outboard is left in the water that it wasn't connected to the boat's general bonding/grounding system. I would prefer that it be electrically isolated so that anything wrong with the boat's main DC system won't affect the outboard and that it will depend on its own independent zinc anode protection system. This may or may not be possible. It depends on if the outboard's DC system has a floating ground, ie the negative power for starting, etc is not connected to the engine block.

David

[delmarrey]

Originally Posted by amarinesurveyor

Listen to what Calder and ABYC have to say, they know what they are talking about...it will make your boat safer! The purpose is to protect the people on board against shock.
Brian

Never heard of anyone ever getting a shock off of a 12v DC system. Otherwise there would be a lot of dead mechanics out there.
As for the AC, it should be grounded to the shore. AC produced on board should have a ground plate under the hull.

[davisr]

The outboard in question is a Yamaha 9.9 High Thrust with power tilt and electric start. The prop will be out of the water when under sail and when docked or at anchor. I'm currently re-powering, so I do not have personal experience with this motor, as I've not yet picked it up from my local dealer. I did today, though, contact the customer sales department at the US headquarters of Yamaha Outboards. I was told that all electrical functions on the motor were controlled by one positive and one negative wire and that the motor itself was not grounded. Therefore, in the diagram that I posted at the beginning of this thread, I was mistaken to label the orange wire leading from the outboard as the "ground." I should have called it the "negative." Other than that, it appears, based on what several have said, that my diagram is sound, at least according to ABYC standards.

The DC Negative Main Bus and the AC Ground Bus are separate, at least in their origin. Grounding wires from both of these separate busses meet at the Grounding Bus. From here a single wire runs to the nearby Grounding Bolt/Plate. To this bolt/plate are also connected the lightning grounding wires from the chainplates.

Currently, the mast itself is not a part of this grounding system. I've indicated this on the original diagram which I here attach again. According to Nigel Calder, a grounding wire should run from the mast to the grounding bolt/plate.

Will such a mast grounding wire be prudent and necessary for my boat to be compliant with ABYC regulations?

Thanks,
Roscoe

[Maine Sail]

Originally Posted by davisr

Will such a mast grounding wire be prudent and necessary for my boat to be compliant with ABYC regulations?

Thanks,
Roscoe

Lightning grounding is optional under ABYC so you're fine but I'd add a jumper anyway.. Double check the underside of the mast step you may just find a wire going to the bilge and that grounding bolt. Many builders did ground the spars though it could have been removed too.

Keep in mind that your boat does not "need" to be compliant with ABYC unless you have an insurance company and insurance surveyor who are sticklers. Thing's like AC/DC grounding, other wiring mishaps, bilge pumps & propane systems are some items that will often get flagged with an insurance survey..

[davisr]

Many thanks for all the help Maine Sail. I can't tell you how much your articles and your many photos and diagrams have helped me in this project. If you ever put all that great info and knowledge into a handy book, I think you'd have a hit. I know I'd buy a first run copy.

Regards,
Roscoe

[goboatingnow]

Originally Posted by delmarrey

Never heard of anyone ever getting a shock off of a 12v DC system. Otherwise there would be a lot of dead mechanics out there.
As for the AC, it should be grounded to the shore. AC produced on board should have a ground plate under the hull.

Rubbish on board produced ac has no ground path nor is earth referenced, Connecting in a ground plate does nothing and I've never seen such " ground plates"

[senormechanico]

Originally Posted by delmarrey

Never heard of anyone ever getting a shock off of a 12v DC system. Otherwise there would be a lot of dead mechanics out there.
As for the AC, it should be grounded to the shore. AC produced on board should have a ground plate under the hull.

It wasn't lethal (obviously) but I DID get a somewhat painful shock from 12 volts when I was working on my first car.
I was wearing a thin T shirt, leaning over the front fender of a '53 Oldsmobile with very worn, checkered paint.
It was raining hard, I was soaked, and I leaned over the battery with my bare arm.
My stomach muscles really tightened!
I could still feel those muscles the next day.

[Maine Sail]

Originally Posted by goboatingnow

Rubbish on board produced ac has no ground path nor is earth referenced, Connecting in a ground plate does nothing and I've never seen such " ground plates"

Dave,

Please expand on this? PerhapsI am not understanding what you are trying to say..?

Generators and inverters are grounded most often to the DC grounding buss then back to the main engine or a grounding strip or plate to get to "earth". I have a number of boats I work on with grounding strips external to the hull but most ground the AC green to the DC system / engine which has a path to the earth...

What on-board AC sources like inverters or generators lack is an alternate path to earth that you get while plugged into AC shore power. This ofcouse is not really necessary as the wiring on the boat is often more reliable than the dock wiring.

When Buffy & Skippy take off in their brand spankin' new Sea Ray 380, with the shore power cord still connected, and rip it out by the roots, what often happens? The marina sends Darrell and Darrell down there to re-wire the dock post. Because they don't really know much about "lectricity", and are inter-related a little too closely, they drop the green grounding wire back to shore/earth..

Hence the reason why the ABYC standards insist that AC green/grounding is to be connected to the vessels DC grounding system for an alternate path to earth. It protects for the events where the shore power cord, dock or dock post offer an unreliable fail safe to earth. I see it quite often even with what few docks we have here in Maine. I can imagine dock wiring issues are far worse in areas with a higher number of slips.


Were you saying on-board generators and inverters are not grounded to the vessels grounding/earth system?? Confused on that one...