Proper Grounding for AC and DC

[foggysail]

Yes, 12vdc can be a problem. Although 40vdc is considered safe in our country, one should be very careful when coming into contact with ANY low voltage, high energy source. For example, you would be most uncomfortable to have your wedding ring or a wrist watch melted on your body because either came between a DC source and its return. And the physical harm will happen in microseconds, not minutes!

Foggy

[delmarrey]

Originally Posted by senormechanico

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.

Gee Steve, you're not suppose to put your tongue across the terminals!

That amperage can get ya if full force but not enough to kill ya at 12V. I use to solder generator commutators with just a 12v battery, two leads and a carbon arc rod.
And have burned up a couple battery cables just for fun (tried to blow up the battery). But the only shock I've received from 12V in the past 60 years is from a capacitor, better known as a coil. That'll wake you up in the morning.


Marine Electrical Check List

Common Ground Point (ship's ground)

grounds from batteries, engine, switch-panel negative bus bar, bonding system, auxiliary power generator, underwater ground plate, ship's 120 Volt safety-ground, and LORAN signal ground all meet at one point

This point must be a heavy bus bar or bracket with bolted connections.

Note: When referring to 12 Volt wiring, 'ground', 'negative' and 'ground return' are all equivalent terms.
easy to access and located as far above bilge levels as practicable
labelled as Common Ground Point

The picture is for lighting grounding, for those in question. Immersed grounding
.

[goboatingnow]

Originally Posted by Maine Sail

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...

The reason ABYC recommends DC negative to AC ground Is too ensure that any metallic DC only devices which inadvertantly, come in contact with live AC are included in the AC protective earth and trip the fuses. In Europe this is achieved by relying on RCBs and no ground interconnection is needed.

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.

Your comment makes no electrical sense. Isolated on board AC production from invertors, generators and isolating transformers are not earth referenced, unlike mains, hence the fault path never includes earth. Even if the shore power lead is still connected fault currents generated via on board sources never run out the earth in the shore lead. ( which is why so called grounding plates are useless and not required

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.

As I said above you've got this backwards. The interconnection is not to provide an alternative fault path through the DC negative. ( that would be dangerous) it's to ensure that inadvertent mains faults that' come in contact with DC only equipment, like your engine, trigger the mains protection devices.

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

By convention they are. But that internal earth system must be then connected to the on board negative at the generating equipment. If it isnt then the earth wire does nothing and is electrically useless. On board generation doesn't really need a ground line a, similar to " double insulated " power tools, you cannot get a shock between earth ground and live. You can only get a shock if you insert yourself into the live and neutral circuit. What the ground wire achieves for on board supplies is it provides a method to trigger circuit breakers and RCBs, even though you can't get a shock from it.

Remember the big fault mode with conventional mains is that it is earth referenced generation, hence the planet provides an alternative return circuit, to ensure we have a low resistance path we use a neutral. But full faults currents can flow through a person merely by one side in contact with the earth and the other being in contact with the AC live wire. This fault mode doesn't exist in on board or any isolated mains production system.

Dave

[Maine Sail]

Originally Posted by goboatingnow

The reason ABYC recommends DC negative to AC ground Is too ensure that any metallic DC only devices which inadvertantly, come in contact with live AC are included in the AC protective earth and trip the fuses. In Europe this is achieved by relying on RCBs and no ground interconnection is needed.



Your comment makes no electrical sense. Isolated on board AC production from invertors, generators and isolating transformers are not earth referenced, unlike mains, hence the fault path never includes earth. Even if the shore power lead is still connected fault currents generated via on board sources never run out the earth in the shore lead. ( which is why so called grounding plates are useless and not required



As I said above you've got this backwards. The interconnection is not to provide an alternative fault path through the DC negative. ( that would be dangerous) it's to ensure that inadvertent mains faults that' come in contact with DC only equipment, like your engine, trigger the mains protection devices.

By convention they are. But that internal earth system must be then connected to the on board negative at the generating equipment. If it isnt then the earth wire does nothing and is electrically useless. On board generation doesn't really need a ground line a, similar to " double insulated " power tools, you cannot get a shock between earth ground and live. You can only get a shock if you insert yourself into the live and neutral circuit. What the ground wire achieves for on board supplies is it provides a method to trigger circuit breakers and RCBs, even though you can't get a shock from it.

Remember the big fault mode with conventional mains is that it is earth referenced generation, hence the planet provides an alternative return circuit, to ensure we have a low resistance path we use a neutral. But full faults currents can flow through a person merely by one side in contact with the earth and the other being in contact with the AC live wire. This fault mode doesn't exist in on board or any isolated mains production system.

Dave

ABYC still requires all on-board originating "sources", inverter, generator or isolation transformer to be grounded (green wire) to the ships DC grounding system. Neutral and ground are only connected at the source end..

If you don't ground AC & DC and then energize the DC system through a fault with AC, while you also have a failed grounding path via shore power you may not trip the protection without a path back to earth and metals can rise to dangerous voltages.

The ABYC requires RCD's or ELCI's now and still also requires AC/DC grounding on-board. Whether or not I am in agreement with it is still open, but regardless I still have to wire to current safety standards which include an ELCI and AC/DC grounding on-board...

I think were in agreement though my post and yours may have been confusing to each other. The standards are what they are, and I don't always agree with them. Sadly in the US I can't get hauled into court and say "but in Europe"....

[GeoffK]

How does all this relate to fresh water boats? Do you still consider the engine/prop or a grounding plate a path to earth?

Geoff

[Ike]

Geoff, Adherence to ABYC standards is just as vital on a fresh water boat as on a salt water boat, especially since there is a serious risk of shock hazard to people in the water near a boat that has an AC system with a ground fault. This risk is far higher for fresh water than salt water.

As has been pointed out several times here, the main reasons behind the standards are elimination of shock hazard. Eliminating galvanic corrosion and stray current corrosion are important but secondary to shock hazard. It is important to think in terms of five systems, AC, DC, grounding (the green wire to eliminate shock hazards), bonding (connecting metal fittings to limit galvanic corrosion) and lightning protection. Yes they are all related but when designing an electrical system they should be treated as separate systems.

Also, as was shown in one of the posts, there is a distinct difference between the term GROUND and the term GROUNDING. Ground is the physical reference of the system to earth, An ungrounded system has no connection to earth. GROUNDING refers to the wiring that provides an alternative path back to earth in case the ground fails. The GROUNDING wire is always green. The third green wire in an AC system is the Grounding wire. It never has current in it UNLESS there is something wrong. DC systems don't always have a third green wire, but can have one. This is usually referred to as the DC Grounding Buss. But I have only seen these on really large yachts with very complex systems.

For more see Basic Electricity New Boatbuilders Home Page - Basic Electricity Page 1 - Direct Current And AC Electrical

As for outboards, usually the DC system is grounded to the outboard. Obviously if the outboard is tilted up there is no ground to earth through the water. That does not mean that the connection to the O/B does not exist. if you have an electric start out board it is automatically connected to ground through the negative battery connection. If you do not have an electric start outboard there is no connection and you have an ungrounded system.

[goboatingnow]

Originally Posted by Maine Sail

ABYC still requires all on-board originating "sources", inverter, generator or isolation transformer to be grounded (green wire) to the ships DC grounding system. Neutral and ground are only connected at the source end..

If you don't ground AC & DC and then energize the DC system through a fault with AC, while you also have a failed grounding path via shore power you may not trip the protection without a path back to earth and metals can rise to dangerous voltages.

The ABYC requires RCD's or ELCI's now and still also requires AC/DC grounding on-board. Whether or not I am in agreement with it is still open, but regardless I still have to wire to current safety standards which include an ELCI and AC/DC grounding on-board...

I think were in agreement though my post and yours may have been confusing to each other. The standards are what they are, and I don't always agree with them. Sadly in the US I can't get hauled into court and say "but in Europe"....

I specifically pointed out the ABYC position , I merely mentioned the European by way of comparison

What I was addressing was your confusion over the purpose of dc interconnection and the uselessness of ground plates on seawater

Dave

[GeoffK]

Ike, thanks for the excellent review. The link to the New Boatbuilder's discussion is a good reference.

I hope you didn't think I was suggesting fresh water boats need less electrical protection. Though far from an expert, my reading suggests, as you say, quite the opposite. Because fresh water is such a poor conductor, if a swimmer comes close to a source of electricity he/she shows less resistance than the water. Thus the least resistant path for the current may, depending on the distances, be source to water to human to water to earth. The human may be paralyzed (tazed) and drown.

I have read about having a much larger ground plate for lightning protection in fresh water but that won't alter the plate to earth via human problem. I suspect the only safety addition would be a cable from plate to earth, not a very practical solution.

It sounds, from your discussion, we can install any number of electrical systems but should find a point in each where the potential is neutral with earth and connect this point to a ground plate that is as close to earth potential as possible. The connection must not be part of a path that carries current with normal function.

Geoff

[sdowney717]

death by 9 volt battery
enough current can flow when the skin is broken to generate a lethal current off low voltage batteries.

1999 Darwin Award: Resistance is Futile

http://community.discovery.com/eve/f...9?r=4951921039

[delmarrey]

Originally Posted by sdowney717

death by 9 volt battery
enough current can flow when the skin is broken to generate a lethal current off low voltage batteries.

1999 Darwin Award: Resistance is Futile

Death by 9 volt battery!!!!!!? - Topic

With all the millions of people on earth.........

As my electrical safety instructor said, "The reason we now have to teach the electrical safety course to all electricians at least twice per year is because some joe was bright enough to be the one person in the world who could figure out how to kill himself with a 9V battery."

Must have had one hell of a good heart, NOT! Nothing is idiot proof!

[sdowney717]

I know extremely unlikely to be the unlucky one. I have felt a 12 volt current and it is a different feeling than AC. I recall it to feel more of a burn like sensation.

[rtbates]

I'd keep the AC & DC grounds as far away from each other as possible! with your simple setup it shouldn't be hard. All dc devices run both +/gnd back to the battery or bus bars. Ac devices should have their ground referenced back to the
power companies box on the pole. KEEP AC GND AND NEUTRAL SEPARATE on the boat.

[goboatingnow]

Originally Posted by rtbates

I'd keep the AC & DC grounds as far away from each other as possible! with your simple setup it shouldn't be hard. All dc devices run both +/gnd back to the battery or bus bars. Ac devices should have their ground referenced back to the
power companies box on the pole. KEEP AC GND AND NEUTRAL SEPARATE on the boat.

Not if you have on board AC producing devices.

Also ABYC requires a interconnection between DC negative and AC protective earth

Dave

[cburger]

Why would it not be a better solution to have a secondary ground for the main AC panel as well as any AC producing devices going to a Dynaplate rater than comingling with the DC ground?

[s/v Jedi]

Guys please... ABYC requires nothing, they recommend.

Their whole grounding specs are confused. This is the result of them ignoring installation practices around the world for too long and then trying to catch up too quickly.

If you look at the EU codes for this, you can't mix those with ABYC. You have to take the whole package. And that is safe of course, even without the AC to DC interconnect.

I also always repeat this: think about using an isolation transformer! On small boats, think about not having a shore power connection at all.

ciao!
Nick.