AC/DC Grounding Question

[Roland5048]

Recently had to lower my breaker panel to access stanchion bolts. Noticed a green wire running from the neg. pole on the main DC breaker to the neg. pole on the AC main breaker. Is this correct? Has been like this for many years with no problem that I can tell. I am only hooked into shore power about two days a month to top off the batts. One thing I have noticed is when I am plugged into shore power and the charger is in charge mode, there is considerable static from the dc stereo. Have searched the archives and also have Calder's book on elec. systems but have not been able to discern a definitive answer. Is there a simple correct / not correct answer to this? Thank you, all.

 

[dlochner]

Well, that is almost correct.

The AC ground (green) should be connected to the DC ground (black) at the grounding point on the engine. As it is wired now, if there is an AC short to ground the whole DC negative system would be energized. If it is connected to the motor ground the leaked current will go out the prop shaft and directly to ground.

The static is probably due to the charger, especially if it is an old style charger with a transformer. Your batteries will be happier and your stereo quieter if you invest in a modern 3 stage charger.

 

[Roland5048]

The AC ground (green) should be connected to the DC ground (black) at the grounding point on the engine.

Thank-you, @dlochner. Does that mean that the AC ground should be connected right to the engine or any other ground point that connects to the engine? I hope I'm making sense, here.

Will check out my charger tomorrow. Fairly new but not sure if 2 or 3 stage.

 

[Stu Jackson]

Does that mean that the AC ground should be connected right to the engine or any other ground point that connects to the engine? I hope I'm making sense, here.

Yes, makes sense. Think of it this way: different end of same wire.

Noticed a green wire running from the neg. pole on the main DC breaker to the neg. pole on the AC main breaker. Is this correct?

There are no negative poles on breakers, they switch HOT legs. Double pole AC breakers switch hot and neutral. Grounds should never be interrupted.

 

[Roland5048]

This would, in effect, isolate AC from DC if I'm reading this right. If I run a wire from AC ground to the engine, the run would be about eight feet. Any issue with gage ?

 

[dlochner]

Thank-you, @dlochner. Does that mean that the AC ground should be connected right to the engine or any other ground point that connects to the engine? I hope I'm making sense, here.

Will check out my charger tomorrow. Fairly new but not sure if 2 or 3 stage.

Think about it this way, electrons are really lazy, they want to get home (ground) the quickest and easiest way possible. Sort of like leaving work late on a Friday afternoon. The shortest most direct route is the best route. Tied directly to the motor ground is the most direct route.

Tied to the ground bus near the panel potentially leaves lots of options for travel, electrons are stupid, keep it simple.

There is also a difference between being connected to a circuit and being part of a circuit. When something is part of the circuit (ground, hot, neutral, positive or negative) the current can flow through it. When it is just connected to the circuit, current does not go through it. This is why birds can sit on an electric transmission wire safely, but squirrels get fried when they bridge the connections on a transformer or across an insulator.

 

[Davidasailor26]

Calder says the purpose of connecting AC and DC grounds is to provide a safe path to AC ground for any AC faults that contact the DC circuitry. Based on that, is it necessarily preferable to have the ground point at the engine? It seems like the AC hot to DC fault could occur anywhere in the boat with equal likelihood, so would the engine be any electrically “closer” to that fault than a DC ground bus behind the panel?

Calder’s grounding diagram shows the AC ground connected to a ground bus, not the engine.

My understanding is that the ground wire should be the same gauge as all the other ground wires, able to carry the full AC current if needed.

 

[mermike]

I'd suggest a ground bus as David says. There's no way to attach all your grounds to a single bolt head (typically a starter bolt) without exceeding the number of lugs (4) or messing with the torque value of the bolt. Attaching grounds to multiple locations on the engine or anywhere else is also a no no. The AC system ground should be tied to the DC ground at one point only. A bus bar makes that a lot easier.

 

[dlochner]

Calder says the purpose of connecting AC and DC grounds is to provide a safe path to AC ground for any AC faults that contact the DC circuitry. Based on that, is it necessarily preferable to have the ground point at the engine? It seems like the AC hot to DC fault could occur anywhere in the boat with equal likelihood, so would the engine be any electrically “closer” to that fault than a DC ground bus behind the panel?

Calder’s grounding diagram shows the AC ground connected to a ground bus, not the engine.

My understanding is that the ground wire should be the same gauge as all the other ground wires, able to carry the full AC current if needed.

Calder is correct about the reason for grounding AC and DC together. If there is only one DC ground bus and that bus has a large enough cable to carry the DC load and goes directly to the engine block, then that would be equivalent to having it all connected at the engine block. I think it is better avoid a lot of connections and alternate path ways for the stray electricity to flow, for example through the negative bilge pump wire, the current could go from the fault, to a bus bar, to the failed bilge pump wire instead of from the fault to bus to AC ground or out prop shaft to earth.

Some boats, mine for exampled, have multiple ground busses that then connect to a either a main bus or directly to the engine. The multiple busses are there to reduce wiring runs. For example the bilge pumps, propane solinoid, and some LED indicator lights all share a small buss and one wire goes to the main bus instead of 4 or 5.

The ground wire should be large enough to carry the maximum current. In a boat with 30 amp service, 10 gauge would be appropriate.

 

[dlochner]

Attaching grounds to multiple locations on the engine or anywhere else is also a no no. The AC system ground should be tied to the DC ground at one point only.

Having multiple connection points on the engine this puts the engine in the circuit and not attached or connected to the circuit. Yanmar did this to my alternator.

 

[mermike]

They connected the alternator ground to a different point on the engine than the starter ground? So you're using the engine as a conductor? That doesn't sound like a good idea.

 

[dlochner]

They connected the alternator ground to a different point on the engine than the starter ground? So you're using the engine as a conductor? That doesn't sound like a good idea.

The starter is case grounded, there is no separate ground for it. The Alternator feeds the main DC + bus and then to the batteries. The alternator ground goes to an attachment point on the opposite side of the engine block from the DC ground point. It is close and the circuit is flows across the back of engine which is better than a case ground. Still, it is on the. list to fix.

Case grounding the starter is not as big a deal as the starter is only energized for a few seconds.

 

[mermike]

If your alternator is grounded to the block, how does current return to the battery?

 

[JamesG161]

They connected the alternator ground to a different point on the engine than the starter ground? So you're using the engine as a conductor? That doesn't sound like a good idea.

Case grounding the starter is not as big a deal as the starter is only energized for a few seconds.

My engines starters has small wire to engine ground, plus the normal power wires.
The same for the Alternators. That includes the Genset.

My engine ground wire is #10 green/yellow striped.

My boat uses the Keel to ground the boat, #4 wire, to the water [water acts like an "earth" Ground]
_____
However we are mixing wiring sizes and uses for Amps flows, I think.

1) Ground reference for our boat electrical needs
2) Grounding for Lightning protection.
3) Grounding to protect submerged metals for Zinc Anode protection

I know most modern boats are built to ABYC standards for all 3 needs, I listed.

Different boats , different methods to achieve overall boat grounding.
Jim...

 

[dlochner]

If your alternator is grounded to the block, how does current return to the battery?

Through the ground bus and then to the block.

On the aft starbord side of the engine block there are 2 threaded connection points for the engine ground. On the Port side there is one smaller threaded connection point for the alternator ground. This is near the starter.

I should probably move the Alternator DC- from the block to the DC- Bus. Another spring project.

"Really honey, the boat work will be done someday," he said to his patient wife.

 

[Davidasailor26]

Calder is correct about the reason for grounding AC and DC together. If there is only one DC ground bus and that bus has a large enough cable to carry the DC load and goes directly to the engine block, then that would be equivalent to having it all connected at the engine block. I think it is better avoid a lot of connections and alternate path ways for the stray electricity to flow, for example through the negative bilge pump wire, the current could go from the fault, to a bus bar, to the failed bilge pump wire instead of from the fault to bus to AC ground or out prop shaft to earth.

I think I agree with everything you’re saying about DC grounding. Remember though that the OP is asking about tying the AC ground into the DC circuit. The considerations for where that AC ground point should happen are, I think, different.

Consider this case: An AC wire chafes and contacts a DC grounded fridge compressor. (This is the type of fault that the AC ground connection is meant to make safe.) The DC negative circuit is now carrying AC hot voltage, which is looking for a way to get back to earth ground. If the AC ground wire is connected to the engine block then the AC current needs to go through the fridge ground wire back to the panel, then to the DC bus, then to the engine before finding the AC ground wire. Each of those jumps represents an opportunity for a failed connection or for alternate current paths if there is corrosion. For example if the engine ground connection isn’t great some of the current might go from the ground bus through a bonding wire to a seacock, through the water, then up the prop shaft to the engine where the AC ground wire is. If the AC ground wire is tied to the DC bus then the faulted AC current goes there and doesn’t need to risk taking those extra hops.

In reality it’s probably a mostly academic argument. As long as there’s a good enough connection across the DC ground plane to carry 30 amps for a brief period of time the current will find its way back to ground a pop the AC breaker, which is the intended effect.

 

[dlochner]

I think I agree with everything you’re saying about DC grounding. Remember though that the OP is asking about tying the AC ground into the DC circuit. The considerations for where that AC ground point should happen are, I think, different.

Consider this case: An AC wire chafes and contacts a DC grounded fridge compressor. (This is the type of fault that the AC ground connection is meant to make safe.) The DC negative circuit is now carrying AC hot voltage, which is looking for a way to get back to earth ground. If the AC ground wire is connected to the engine block then the AC current needs to go through the fridge ground wire back to the panel, then to the DC bus, then to the engine before finding the AC ground wire. Each of those jumps represents an opportunity for a failed connection or for alternate current paths if there is corrosion. For example if the engine ground connection isn’t great some of the current might go from the ground bus through a bonding wire to a seacock, through the water, then up the prop shaft to the engine where the AC ground wire is. If the AC ground wire is tied to the DC bus then the faulted AC current goes there and doesn’t need to risk taking those extra hops.

In reality it’s probably a mostly academic argument. As long as there’s a good enough connection across the DC ground plane to carry 30 amps for a brief period of time the current will find its way back to ground a pop the AC breaker, which is the intended effect.

There is a phenomena called ground looping in AC wiring that I really don't fully understand. It is caused by having multiple AC ground points. One common place it happens is with bar bands. When they plug in their equipment multiple ground paths are established the result being interference in the electronics causing a loud hum. I'm not sure it really applies here or not.

Anyway, we agree, the fewer places the fault can jump to other places the better.

 

[Roland5048]

Thank you all for the replies. Let's see if I have this straight. Since I don't have a bus bar, the shortest route to ground would be to attach the AC ground right to the engine giving any stray electrons a quick exit ramp. Since the DC side is already grounded to the engine (3YM20), leaving the AC ground wire connected to the DC ground on the main breaker should accomplish the same thing, however as @dlochner put it, "the fewer places the fault can jump to other places the better". I get that. Since I'm not running refrigeration or AC, there are very few places for those wayward electrons to go except out the DC ground. Am I looking for a solution to a problem that really doesn't exist? Maybe I have too much time on my hands.

 

[dlochner]

Am I looking for a solution to a problem that really doesn't exist? Maybe I have too much time on my hands.

Perhaps sailing more would take care of that time problem.

Is there only one negative bus bar behind the panel? Or are there others lurking around? If the AC ground goes directly to the DC- bus bar and is next to the the cable leading to the engine block and it goes. directly to the engine ground, you're probably OK. But, I'm a knowledgeable sailor, not an ABYC certified marine electrician.

In reality, if AC is seldom used on the boat then the probability of something happening is pretty small. That's the upside. The downside is the consequences can be serious.

 

[FastOlson]

Data point: our 1988 boat has a stock AC ground bus bar behind the main AC distribution panel. Also, there is a stock DC bus bar behind that panel. There is no electrical connection between those bus bars that I have found.
We do have a high amperage Neg. bus bar located in the engine compartment that now gathers the "big wires" that used to go to a lug on the bell housing -- I added that bar.

I do not believe that the two "grounds" are connected, and have seen conflicting advice over the years on whether this is a good or potentially bad idea. I am open to suggestions and/or possible derision.