Galvanic Isolator

[SvenG]

Originally Posted by DotDun

Thanks again for the thought provoking conversation.

I'll second that. While there have been disagreements along the way they have been civil and pretty respectful. This is in sharp contrast to threads in another CF subforum where posters are mugged as soon as they discuss something other than sales literature.

While we plan to become less dependent upon the shorepower umbilical and therefore in less need of any isolator, the thread has still been of real value. Senta II is bonded throughout. As long as we hang a good and well connected sacrificial zinc over the side that may (see below) all be fine and good, but otherwise it is a real danger.

I certainly wouldn't want the bonding to help a lightening strike blow out all our through-hulls, so the only reason for keeping it is to minimize galvanic damage. Given that a stray current from a nearby shore-connected electrical source could set up a potential across the bonded through-hulls that overpowers the zinc, I think it is time to cut the bonding.

I still have to confirm that the potential could overpower the zinc, but if it weren't for this thread I would not have even wondered about it.



-Sven

[osirissail]

Originally Posted by DotDun

Osirissail,
After more thinking about scenarios that could cause trouble with my installation, I came up with one that could be bad.
With my current configuration, shorepower ground disconnected at inlet, if my boat was on the hard and connected to shorepower AND there was a transformer fault (primary to shield), the boat ground is now hot. There would be the possibility of 30ma for 100ms for someone that touched the ground plate on the hull. This would not happen with the boat in the water as the water would provide enough ground to trip the ELCI. . . . .

That jogged my memory about grounding the boat when you are on the "hard." I have seen many stored boats on the hard have their electronics "wiped" by a nearby lightning strike because they were not "grounded" as they would be if they were in the water.
- - In the same vein, some protection might be a good idea for yourself and any workers/kibbitzers who might inadvertently touch or come in contact with your grounding plates when the boat is out of the water.
- - The way I solve both those problems is to visit Home Depot or any electric supply house and get a copper/steel grounding rod/spike. I then sledge hammer the rod/spike into the ground nearby to an access to the boat so I can run a fat piece of scrap electric cable into the engine room and hook it up to my ship's ground. I suppose you could drill and tap a hole into the ship's grounding plate and attach the wire there also. But I just find some "throw away" wire cable in the dempsey dumpster and use that to attach to the ground rod/spike and boat's grounding system.
- - After loosing 3 battery chargers to nearby lightning static discharges, this system of using the ground rod/spike stopped all future problems

[osirissail]

Originally Posted by SvenG

. . . I still have to confirm that the potential could overpower the zinc, but if it weren't for this thread I would not have even wondered about it.
-Sven

I don't think it is a problem of "overpowering" the zinc but instead a problem with the zinc "disappearing" - being rapidly consumed - by the stray current coming down the faulty shore power system. Once the zincs are gone, then the stray currents start to "eat" away the lowest blade on your propeller. And that can get expensive.

- - Food for thought - If there are stray currents coming down the shore power ground wire (most likely from other leaky boats next to you, and/or a bad earth ground up at the main power supply source on land) then what will the ELCI/RCB do? Will it prevent you from being connected to the shore power?
- - Supposedly the ELCI/RCB monitors the "hot leads" versus the "neutral/return" lead and if the two do not match the system trips. What monitors the earth/ground lead of the shore power for the presence of stray currents?
- - The classical "galvanic isolator" only was involved in the shore power ground lead and separated your boat's side from the shore power side by the use of diodes.

[SvenG]

Originally Posted by osirissail

I don't think it is a problem of "overpowering" the zinc"

It doesn't have to be, but I think it could be. If the potential is large enough (assumes a bunch of stuff, orientation, spacing and potential) it could push up-hill so the zinc is the cathode in an externally powered system. I'm not saying that it will happen and I'm not saying that it could happen under normal circumstances, but I am going to look into if it could reasonably be expected to happen in a worst case scenario.

Good food for thought all around.

Thanks,



-Sven