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Go Back   SailNet Community > On Board > Gear & Maintenance > DC/AC grounding
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Topic Review (Newest First)
04-03-2013 03:00 PM
sailvayu
Re: DC/AC grounding

"I should have said AC current can leak into the water" not "DC can flow between boats"."
Both these statements are true

"AC current can leak into the water via the DC grounding system when you bond them together IF the ground back to shore power fails."
This is true as well and what you want. As an example I was once shocked pretty good when working on an engine. I was holding a wrench and leaded back against the water heater and got a full 120 through me, I about wet my pants lol and lucky I jerked the wrench off the nut. The heater had a fault and the path to ground was through me. If a proper connection was made to the DC ground and then to the water I would not have gotten shocked. this could have been worse if say it went through my left arm.

Is it dangerous for swimmers? Well yes and no. This is very bad in fresh water but not so much in salt water. This is because the human body is basically a bag of salt water. In fresh water the current will want to flow through the easiest path and it flows better in salt water so we become a better conductor than the fresh water. We do not have as much salt water in us as the sea so in that case it does not want to flow through us. That is not to say nobody has gotten shocked in salt water but it is rare. You have to keep in mind what kills people in the water is not the current but that a small voltage will cause the muscles to contract and the person can no longer move and drowns sort of like being parallelized by a Tazer.

Capt. Wayne Canning, AMS
www.projectboatzen.com
04-03-2013 02:28 PM
BubbleheadMd
Re: DC/AC grounding

Quote:
Originally Posted by sailvayu View Post
Some boats have an isolation transformer and this is a good thing. But it adds expense and weight and few production boats have it. I do not know of any marinas that would do this as each pedestal would have to have one and they are not cheap.

I am not sure I understand this "DC current can flow between boats if the AC and DC systems become shorted together. How can this happen?"
I am not sure what you are trying to say here maybe you could explain it better. It is rare for the DC system to become energized with AC that would be a serious situation. Most AC shorts result in the ground becoming live but that is why we have a ground to give shorts a path to ground.

Capt. Wayne Canning, AMS
Project Boat Zen - Boat and Yacht Repair and Restoration
Ugh...I was squeezing this reply in between jobs at the office.

I should have said AC current can leak into the water" not "DC can flow between boats".

AC current can leak into the water via the DC grounding system when you bond them together IF the ground back to shore power fails.

Why do you bond the AC system to the DC system? So that if the shore power ground fails, stray current and exit into the water via the DC ground system instead of shocking the crew.

Is it dangerous for swimmers? Yes, but the ABYC feels that this is better than a shock hazard on the boat.
04-03-2013 02:11 PM
sailvayu
Re: DC/AC grounding

Some boats have an isolation transformer and this is a good thing. But it adds expense and weight and few production boats have it. I do not know of any marinas that would do this as each pedestal would have to have one and they are not cheap.

I am not sure I understand this "DC current can flow between boats if the AC and DC systems become shorted together. How can this happen?"
I am not sure what you are trying to say here maybe you could explain it better. It is rare for the DC system to become energized with AC that would be a serious situation. Most AC shorts result in the ground becoming live but that is why we have a ground to give shorts a path to ground.

Capt. Wayne Canning, AMS
Project Boat Zen - Boat and Yacht Repair and Restoration
04-03-2013 01:24 PM
BubbleheadMd
Re: DC/AC grounding

Quote:
Originally Posted by Captainmeme View Post
I'm confused. Maybe the image I have of shore power connection is wrong. Isn't there an isolation transformer between shore power and the boat load? And aren't the grounds between the two (shore power and boat load) isolated as in not connected? If this is the case then how does DC current flow between boats?
No, there is no isolation transformer between the dock pedestal and the boat.

DC current can flow between boats if the AC and DC systems become shorted together. How can this happen?

- Onboard battery charger malfunction or wiring fault (interfaces AC to DC)
- Inverter malfunction or wiring fault (Ditto)
- AC wiring run too close to DC wiring, chafing through.
04-03-2013 01:10 PM
Captainmeme
Re: DC/AC grounding

I'm confused. Maybe the image I have of shore power connection is wrong. Isn't there an isolation transformer between shore power and the boat load? And aren't the grounds between the two (shore power and boat load) isolated as in not connected? If this is the case then how does DC current flow between boats?
04-03-2013 01:00 PM
BubbleheadMd
Re: DC/AC grounding

Ok, the source of current leakage is irrelevant.

As you agree, a ground plate is the better solution because no matter who is leaking current into the water, his outboard engine lower unit and prop will get eaten up because they usually only have very small zincs on them.
04-03-2013 12:33 PM
sailvayu
Re: DC/AC grounding

99% of people with corrosion problems always think it is the other guy causing the problem but on 90% of the corrosion surveys I do the problem is located on the boat with the corrosion. I am not sure why everyone thinks it cannot be a problem on their boat? But I always say make sure your stuff is right before pointing fingers. And I agree I would rather see a ground plate installed as a permanent setup. Also another misconception is that AC causes most corrosion, this is wrong DC is to blame. If you think about this it makes sense as AC flows back and forth so it will not move metal atoms from one piece of metal to another while DC is a flow in one direction and can carry atoms in a steady flow removing from one mass and plating another. When you plug into shore power the problems occur from the ground wire allowing DC current to flow. Think of the boats in a slip as cells of a battery and the ground wire connecting them just like a battery. This is why a ground isolator helps, it blocks DC flow in the ground wire but will allow AC to flow if there is a short.

Capt. Wayne Canning, AMS
Project Boat Zen - Boat and Yacht Repair and Restoration
04-03-2013 12:07 PM
BubbleheadMd
Re: DC/AC grounding

Quote:
Originally Posted by Seascape View Post
Thanks for the replies. I suppose I could just ground to the outboard and keep it lowered in in the water when connected to shore power. I only connect to shore power if staying on the boat overnight. Otherwise I disconnect it when leaving the boat.
You'd better keep a close eye on your zincs if you use the outboard then. Outboard engine zincs are tiny, and will get eaten up quick if one of your neighbors is leaking current into the water.
I recommend using a ground plate instead of your O/B.

To the OP who is using an O/B, but still has the dead inboard engine in place:

Continue using the inboard engine as your ground point. The engine doesn't run, but it's still a large hunk of metal in contact with the water, so it makes and adequate ground point for the bonding between AC and DC systems.
04-03-2013 11:38 AM
sailvayu
Re: DC/AC grounding

That is the intent of the rule and it makes sense you need to provide a path to ground if there is a short. You cannot always assume the shore ground is intact. So yes connecting to the engine in the water or a ground bar on the bottom will work.

Capt. Wayne Canning, AMS
Project Boat Zen - Boat and Yacht Repair and Restoration
04-03-2013 10:39 AM
Seascape
Re: DC/AC grounding

Thanks for the replies. I suppose I could just ground to the outboard and keep it lowered in in the water when connected to shore power. I only connect to shore power if staying on the boat overnight. Otherwise I disconnect it when leaving the boat.
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