electrolysis

[Genesis]

OK, now I am getting closer to understanding this stuff. My isolation transformer case grounds to the bonding system and that is connected to the white wire on the secondary side only. The dc battery grounds are connected to the engines. The engines are connected to the mounts and those have wires to the bonding system. All electrical panels have case grounds that are tied to the bonding system. Doesn't all that mean that I will see continuity between my ground pin on the shore connector and the bonding system?

Ok, let's do this again:

On the SHORE CORD side (assuming 120V system):

1. BLACK wire (hot) on shore cord goes to primary on transformer.
2. WHITE wire on shore cord (grounded conductor) goes to primary on transformer.
3. GREEN wire on shore cord (safety ground) goes to SHIELD (NOT - NOT - NOT the transformer case!) around the primary windings. This protects against a short in the primary winding itself.

ALL shore wires STOP HERE.

On the SECONDARY side of the transformer, you have TWO lugs (120V transformer) for output, and you also have the physical CASE.

1. ONE of the output lugs goes to your electrical panel's HOT (black) side.
2. The OTHER output lug goes to the electrical panel's NEUTRAL bus (WHITE)
3. The NEUTRAL bus is ALSO connected at the transformer to the ground (GREEN wire) AND that is common to the bonding system (although the bonding wires MUST NEVER be used for this - separate wires, but interconnected.) This is called "bolting" the energy source and is done at the source - typically at the transformer output itself.

If you DISCONNECT the shore cord at the dock end and measure from ANY of the pins to your bonding system or the ground plug on any of your AC outlets, you should see OPEN (infinite resistance.)

If you measure WHILE THE CORD IS PLUGGED IN you will see continuity BECAUSE the ground on the shore cord IS IN FACT GROUND (water) and your on-board system has its own ground which is submerged in the water. HOWEVER, this will not produce electrolysis as there is no path BACK to complete the circuit (remember, a complete circuit requires TWO paths - out and back - for the electricity to flow)

I should also mention,(this will make Karl cringe for sure), The optional factory Air Conditioning has its own 30 amp 120volt connector. This does not go through the Isolation transformer and the Hatteras book says they can do that because they use two pole breakers throughout that air conditioning system.

This is moderately unsafe (it can get you killed as can a system with no isolation transformer in the system at all) but the only way it can cause electrolytic corrosion is if the AC system itself leaks current to its safety ground (green wire.)

Here is a copy of the page from Calder's book - this is the right way to do it for both a 120V and 240V isolation transformer system. (Yes, I know, the 120V picture is in a blowboat - we all have our faults)

Note that the shore ground is not connected to any part of the boat's grounding (which is tied to the bonding) system.

 

[Maynard Rupp]

Thats not possible. You cant have 2 seperate systems on the AC side without common grounds if they are coming from the same power source. If they both come from the dock power they have a common ground there to begin with plus the drawings I have from hatteras have grounds common.

I am sure he is refering to the boat side only; shore cords unplugged. All of my grounds appear to be connected to a main ground bus bar in the engine room electrical panel. The drawing also seems to indicate that this ground bus bar is connected to the bonding system. I will check that today.

 

[Trojan]

Boatsb, Not true. I have 2 separate systems on my 43. One panel and system runs the entire boat and ground system. The other panel and system is the A/Cs ONLY and is up from the boat ground. In my case the grounds are only common at the dock plugs. Under some condictions if you got across the 2 you could get 220 v,AC. And that's only because I don't have an isolation transformer yet.


BILL

 

[Genesis]

The ground bus is connected to the bond as is the negative side of the DC system. There is dispute over the wisdom of doing that in a system with an isolation transformer (it creates electrocution risks where they would not otherwise exist!) but this IS how the ABYC wants it done.

You control for THAT risk with GFIs.

However, protective ground from the shore side is NOT brought on board beyond the shield (again - not case!) of the isolation transformer.

 

[Maynard Rupp]

Thanks Karl...That is the way Unity is wired according to the drawing. The onle exception is what they are calling a case ground on the primary side. I will research that, as I sure do have continuity between the ground pins and the bonding system.

 

[Genesis]

Sounds like they connected the input ground pin to the transformer case, and since that is grounded (and thus connected to ship's bond) you have a connection that should not be there.

If there is no internal shield on the transformer (which is NOT connected to the case - that's where the shore ground SHOULD be connected) then you need to lift the shore ground wire at the transformer.

There is a balance of risks issue that comes into play if you have this circumstance. If the transformer does not have an internal shield then it is POSSIBLE for a short between the primary side and the case to energize the vessel and that MIGHT NOT trip the breaker on shore as the path back through the water would be of sufficient resistance to prevent it. That fault will also DESTROY your underwater metals VERY rapidly.

If there is no isolated internal shield on the transformer to connect shore ground to then I would proceed as follows:

1. Lift the ground in the shore cord so it is not connected to anything (on the boat side at the transformer or shore inlet)
2. Rig a GFI between the shore inlet and transformer to provide protection against electrocution in the event of a fault in the transformer that shorts the primary side to the case. The GFI will trip if there is an energy imbalance caused by this event.

This conforms with the NEC (a GFI is an acceptable alternative to a working safety ground under the NEC) but I have no idea what the ABYC says about it - nor would I care

 

[Boatsb]

I lalso have the 2 systems on 2 seperate panels but they share a common ground at the boat as everything is grounded and common at the panels. I do not have my wiring diagram with me but OI believe I remember the drawings showing all grounds being common as the ground for the DC and AC are on a single buss. If not the second AC panel would have no ground and that could be an issue.

 

[Genesis]

The second AC may be the cause of the problem here.

The fix for that is to lift the ground on the feed to that unit and fit a GFI in the line; this provides the same protection as a ground against shock and removes the consideration of bypassing your isolation transformer's protection against electrolysis.

 

[Trojan]

After all this I hope you have 240v ac. at your dock. Karl do you have a 120v ac. dwg available showing this? Maybe someone looking doesn't have 240v ac. .


BILL

 

[Genesis]

The picture shows both - a 120V and 240V system.

From Maynard's description it sounds like his is a 120V system, with a separate feed for his AC unit.

 

[Maynard Rupp]

OK I lifted all the wires off the case ground pin on the iso. amp. 2 of the three had continuity to my shoreline ground pin. Next I lifted every green wire from the ground bus including the battery grounds. ai also un-bonded that bar by removing the copper strap. I still have continuity to my ground pin. Bah!!!

 

[Genesis]

Your separate air conditioning shoreline is grounded and defeating your isolation transformer.....

 

[Boatsb]

There is an isolator for the ground only if you need. It will work and is quite a common solution to your problem.

 

[Maynard Rupp]

Your separate air conditioning shoreline is grounded and defeating your isolation transformer.....

That is what I thought. So far I have not been able to interupt the connection between either shore connector ground pin and bonding, I had to put the boat back together for the day as we are living on it. I even pulled the connector metal panel out of the cockpit to see if anything looked strange there. It really looks nice, uncorroded, and very original.

 

[Maynard Rupp]

There is an isolator for the ground only if you need. It will work and is quite a common solution to your problem.

I was reading about those. The one I saw online has 2 diodes in each direction. That sure should stop any small current flow to or from the bond I would think. They allow you to put both green wires from the shore ground pins through the isolator. I just fail to understand why the isolation amplifier doesn't accomplish the same thing. I think, as Karl says, the problem is the ground connection from the air conditioning plug is defeating that.

 

[Genesis]

Or the boat is wired wrong.

One of the two.

The separate line is PROBABLY responsible for this. If you're going to put in an isolator, do it only on the AC circuit.

 

[Boatsb]

I forget the actual name of the device and the manufacturer but it is a ground isolator for the AC power. I have it just after the shore power connections and it is where the ground on the dock line connects to the boat on one side of the unit and the boat ground on the other terminal. I believe its an isolation transformer for ground only. Anybody able to jostle the memory here? I have a bad case of CRS.

 

[SKYCHENEY]

I have two comments on this. One is that Calder's books are the best. I got one as a gift over the holidays and it really goes into great detail. The second comment is that the VC-17 that Maynard has on his boat has a very high copper content. I understand that you do have a stray current problem, but the dissimilar metals in the bottom paint and running gear may be making the zinc degradation even worse.

If I had the wiring that you have, Maynard, I would add a second isolation transformer to that A/C line. With only one transformer, you are really defeating the purpose of having them all together. I just don't understand why Hatteras would have only protected one incoming line like that.

 

[Avenger]

I forget the actual name of the device and the manufacturer but it is a ground isolator for the AC power. I have it just after the shore power connections and it is where the ground on the dock line connects to the boat on one side of the unit and the boat ground on the other terminal. I believe its an isolation transformer for ground only. Anybody able to jostle the memory here? I have a bad case of CRS.

I think the term you're looking for is "galvanic isolator". I also understand that there's still much debate as to their value.

 

[jim rosenthal]

I agree; if you are going to have an isolation Xformer on the incoming ship's service line, there should be one on the AC incoming line as well if it is separate. Some of our boats are wired so that AC comes on from shore as 220vac, and then splits into two 110vac lines, one of which runs AC units which are 110vac. Maynard, do you have two shore cords of 110vac or one of 220vac? Just curious.

Karl, you may have already covered this, but have the recommendations on how to wire a boat for incoming shore current changed a lot over the last ten or fifteen years, or are the practices the same? I have no idea.

The Calder diagram is very good, with the exception that when you look at it, you could easily think the shore green conductor is connected to the case, not the internal shield. As a matter of fact, if you had not made a point of it, I would not know it just from the diagram.

While we're on this subject, do all isolation Xformers weigh dozens of pounds, or are there any lighter ones available? Not only are they expensive, they weigh a lot.