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  #11  
Old 04-03-2013
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Re: DC/AC grounding

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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.
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Old 04-03-2013
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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
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Old 04-03-2013
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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.
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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
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Last edited by sailvayu; 04-03-2013 at 02:03 PM.
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