isolation question

I am beginning to gather my winter projects and one of them is redoing the electric.

What I know about electric you could write on a 3x5 card but I am learning.

My STEEL boat has an isolation transformer but I’m a little confused about the way it’s wired.

My wiring diagram (factory) shows the shore power coming in at midships, running about 12’ to a plant/shore/off switch at the helm panel, then back to midships where the isolation transformer is. From what I can tell on the diagram the purpose of this is so the current from the generator also goes through the transformer.

Not what I was expecting from what I had read.

I thought the run from the shore inlet would be as short as possible and not get near any DC wiring. The plant/shore selector is directly under the DC panel and 40 years of rats nest wiring and then the run back to the IT is in a tray with all the other wires.

Does the current from the generator need to be isolated? Or should the shore power run through the IT first and then up to the selector, or should the selector be moved back to midships, or is it OK as is?


Which leads me to another question about mixing AC and DC. The isolation transformer puts out 120V and also 13V which goes to a switch just above that other selector and says “interior lighting AC/DC”. Its just an old chrome toggle switch. These lights are then controlled with light switches just like in your house and a couple of the lights are outside.

Can this possibly be safe? Is 13V AC dangerous? What happens if this switch fails and AC and DC are mixed? Do I need special bulbs for these? By the way this is factory and on the schematic.


There is also a second 30A shore power just for the Air conditioner that is not on an Isolation transformer. None of that wire seems to be near anything else so is that OK?

Thanks in advance

 

The location of the selector switch probably was considered to be more convenient.
It doesn't really matter how the cables are routed as long as their quality is still good. Because you mentioned "40 years of rats nest wiring" I put a question mark there; if the insulation is rubber it may have dried out and needs to be replaced.

In my opinion shore power should be treated as alien and kept as short as possible, but that is a matter of taste.

The generator does not need to isolated because it is part of the ship's wiring, so the danger of an offset voltage like from shore power does not exist. But possibly the 13V AC winding was the reason to wire it the way it is: you can now use AC power for the interior lighting when the generator is running.

13V AC is not dangerous; somewhere in the wiring there will be a fuse or circuit breaker that will blow/trip if the switch fails. If the switch is of the make-after-break type (it should be) the mechanical construction prevents a short circuit between AC and DC.
You do not need special bulbs but you cannot replace them or add LED lighting to this type of circuit.

The shore power wiring for the air conditioner is understandable because it would be a heavy burden on the isolation transformer. Make sure there is no ground wire connection between shore power and the air conditioner.

 

break the safety ground for the separate AC?
How about using a galvanic isolator?

I once got shocked touching the water output flow. I found in the box it had gotten wet and power was leaking into the outside metal of the compressor unit but not enough current flow to trip the breaker. So even though it had safety ground I still got a shock. I recall one of the wires the insulation was compromised enough that the water conducted current into the metal. the metal conducted current into the cooling water and the cooling water conducted current into the sea and back to shore power.

When trying to figure out how cruisair designed a ground, I figured out they use the copper pressure lines.

 

Yes, you understood that correctly. The A/C metal case is connected to ship's ground, like all other appliances. Should an isolation failure occur, there will be imbalance between shore neutral and phase and the GFI will instantly disconnect. The galvanic isolator can be installed to ease someone's mind, but doesn't contribute anything to safety on board.

 

CDK is right about the galvanic isolator. It does just what it says. It isolates your boat from DC current that can cause galvanic corrosion. It does nothing to AC. If your isolation transformer is wired correctly you don't need a galvanic isolator because the isolation transformer does the same thing.

The whole point of "isolation' is to prevent stray DC current from flowing in the Green grounding wire. Normally the green wire has no current in it, AC or DC. It is only there as a safety wire. If you get a fault and AC has no path back to ground at the power source it goes back to the power source through the green wire. But this wire is attached to the boat's ground, as is the DC negative. So if you have a DC fault you can get stray DC in the green wire. Without Isolation this can cause stray DC current on the metal hull and all metallic fittings in contact with the water. This results in corrosion. By adding isolation ( either a galvanic isolator or an isolation transformer, but not both) you stop the DC current flow. This prevents the corrosion.