Wiring Musing, Safety to Neutral bonding - galvanic isolators and isolation transformers...

Several years ago I write the following in response to someone on the wooden boat forum asking questions around electrical wiring.  It seemed to get a good response, so thought I would perhaps repost it here.   The question is:  What are the issues around white wire and green wire handling on boats?   And do note:  The following really is an America's viewpoint, as much the rest of the world has different wiring standards.


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A galvanic isolator goes in the ground wire as it returns to shore. It in effect puts a little 'speed bump' in the path, and any 'voltages' less then about 1.2v will not be passed. For most 'galvanic' issues, this is more than sufficient to protect your boat: Say a neighbor boat has a old battery charger, one that 'electrically' is leaky and put a few millivolts (1/100) of a volt into the ground wire and his negative battery line, and hence the water. He will still have issues, but because the leakage is smaller than 1.2v, your boat will not become part of this return path though the shore power ground wire. In reality, this protects from 99% of issues you will find out there....

Now, lets say your neighbor boat has a catastrophic failure in this battery charger, and the Hot wire somehow gets connected to the Neg terminal. Let's further say that the boat was wired incorrectly, maybe he disconnected the 'green' wire back to shore because he was going through a lot of zincs... Or that some other failure happened and there was not sufficient current to trip the breaker, but now he is putting say 70v into the water. (Remember reading about people drowning when getting near a boat that had a failure, or was wired wrong and getting shocked..). In this case, your boat will become involved in the return current, as the voltage presented (70v) is greater than the 1.2v 'speed bump'. So, your boat in now the return path to the shore green wire ground point, and starts getting eaten up.

All of this is because there is a need for one 'ground' point for the AC power. When you are connected to shore power, that ground point is back on shore. Your green ground wire is carried back to shore where it is bonded to the white Neutral wire and provides you protection from a failed appliance on your boat, much the same way the Green ground wire in your house protects you. Also will add that a contributor here is the ABYC requirement to connecting the ships 'green safety' ground to the common ships DC ground point (typically the engine block). They want this because even if you do not have a hard wire between the two points, there is a kind of poor wire between them. Remember, the Green safety 'ground' wire is actually connected to earth back at a shore based common ground point. And your DC negative (typically through the motor) is sort of connected to the earth because it is sitting in water, which is sort of connected to earth. Problem is, this 'water' based connection to earth is not the greatest. It will pass enough current to kill you, but not enough to trip a circuit breaker. Hence the 'requirement' to have a good hard wire connecting the two points....

You could say, 'Well, I just will not connect Green Ground wire back to the shore power then'. You can do this, but it leaves you rather venerable from death if there is a bad short in that electric drill you are holding... OK, you say: "Well, I know a little. I will just connect the Green ground wire to my White Neutral wire at the boats breaker box. Just like my house it. That way I have protection, and am not involved in this whole Green Wire back to shore issue'. Well, that will keep you alive, but it puts you right back into the same galvanic issue you had before. Unlike your house where they trust you to make a ground point in your breaker box and that is the end of it, us boat people are not to be trusted. So they carry the green ground wire all the way back to shore and connect it to neutral wire there, establishing the common 'ground' point there. Even if you 'break' your green ground wire at your boat, you will still be connected to every other green ground wire in the marina because of this shore based common ground point. This time instead of the Green wire going all the way back to shore, it is the White wire that carries any 'galvanic' voltage. So, this is also a non winner.

What to do? Well, for 99% of the problems, the Speed Bump will work. Keep those relatively low level galvanic currents from using your boats green ground wire as a return path. For that last 1% you can install an isolation transformer.

An isolation transformer breaks all electrical connections back to shore: Both the Black Hot wire(s) and the white Neutral wire. it does this by using a magnetic field (transformer): One side is connected to shore which makes a magnetic field, the other side captures that magnetic field and puts it into a second set of wire which then can feed your boat. These sides are called Primary and Secondary respectively.

Marvelously things, isolation transformers. As in addition to disconnecting you from the shore wires, it will also allow you to change voltages! So, you can convert that lazy 208v into what your shipboard high power appliances (e.g., larger ACs) into the 220/240v they all want. Kill two birds with one stone :-)

But what about the green ground wire? With an isolation transformer, you are totally isolated from that shore based 'common ground' point, and you get to re-establish a new one of your own. You actually connect the ships green ground wire to the ships white neutral wire, creating a new (and isolated) ground point which is exclusively for your own personal use on your very own boat! Because it is totally isolated from the shore power wires, you do not have a return path to shore and hence are protected from both galvanic issues as well as your neighbors catastrophic failure. Note here, IF you were to 'add' a galvanic isolator to an isolation transformer, well, you have in effected shorted out any isolation and you are back to the 1.2v Speed Bump. So, here is a case where more (isolation transformer AND galvanic isolator) is not better....

BTW, when you are disconnected from the shore power and using your Generator, or Inverter, you do the same thing with regards to establishing a local ground point: inside the generator and Inverter they re-establish ground points so you are protected when using them as sources. This is for larger inverters, and is one of the key functions of the 'transfer' relay inside larger inverters, and the 'transfer' switch you switch to bring a generator on-line.

On the question of marina with 3-phase power and having never seen it at the dock, that is correct. Except for large yachts and ships, there is no need for 3-phase power to be brought out to the boat. What they do is bring in power to the marina on 3 'hot' lines, and a neutral. (1 hot wire for each 'phase). Between any hot wire and the neutral, you will measure 120v. So, if you slip is wired for 120v, they only bring one of those hot wires along with the neutral to your slip. To keep things balanced, your neighbor will get his slip wired to a different hot wire (everyone shares the same neutral), and the one next to him get the last hot wire of the three. Then it starts repeating. If you have 240v, then they will bring 2 of them to your slip. (and you get 208v across the two hots because of 'phasing' issues, while still getting 120v between one of the hots and neutral). Remember also that EVERYONE gets a green 'ground' wire brought to their slip. And all of these ground wires are taken back to the common 'ground point' back on shore. So, they bring 3-phase to your marine, but not all the way to the slip.

Your house is a little different: They put a transformer on the pole with the 'primary' side set up for what every high voltage is on that pole. The 'secondary' side has two 120v outputs, connected one after the other. From here comes three wires. The common point where the two 120v secondaries were connected together, this is the white Neutral, and then 2 black wires from the other respective sides of the two 120v windings. Hence, you get 120v between the neutral, and either of the black 'hot' wires, and because both of the secondaries are 'in phase', you get a true 240v (120 + 120) across the two hot wires. Your green ground wire is connected at your own circuit breaker box to the neutral to establish the 'Ground' point. Now, note that this looks just like the isolation transformer talked about above! And in fact that is what you are doing when you install an isolation transformer, you are creating your own little 'power' point just like your house is set up...

Well, this is long, and some of the fine details are missing, but I hope it helps. In short:

Galvanic Isolator (Speed Bump): Does a great job of protecting your boat from small stray voltages in your marine. Never use one with an isolation transformer.

Isolation Transformer: Does a Fabulous job of protecting your boat from those small tray voltages in your marine, as well as higher voltages from miss wiring or catastrophic failures. In addition, can solve any goofie voltage at the dock issues you have from either 3-phase power, or even long voltage drops due to long runs!

Finally, when you get a boat with Shore Power, a Generator, and perhaps an inverter, things can get really hairy to make sure all is connected to work, provide death level shock protection, and also protect your boat from stray 'galvanic' voltages. Here is where people who get paid well come in to play..