Some boats that require AC power but do not have the space or financial resources for a “real” marine generator use these small gasoline powered “suitcase” generators. Most famously, the line of these made by Honda are widely respected for being well made and reliable pieces of kit. But are they really suited for use on a boat?
There are some factors I’ll skip for this discussion because they have been discussed online ad nauseam. Questions like: Is the carbon monoxide they give off dangerous? (Yes!). Does the noise they make annoy other boats in the anchorage? (Yes!) Rather I’ll suggest that people using or recommending them do not really understand the electrical hazards that are involved. The issue with how these devices handle the safety ground is usually dismissed as not an issue, but it IS!
To understand the problem, we need to understand the purpose of the “safety ground” and the concept of the “source of power.”
The “green wire” is sometimes called the “ground wire”, or the “safety ground”, or “earthing wire.” In Europe, the term “Protective Earth” or “PE” is used. I'll stick with this term, because it is the clearest and least ambiguous. This wire's purpose is to carry any dangerous currents that escape from the normal flow back to the source of the power. When this happens, at low levels it would trigger an ELCI (Earth Leakage Current Interrupter) or GFI (Ground Fault Interrupter) switch in the circuit to interrupt the power. If a lot of current flows, it might also trigger a main circuit breaker.
For any electrical current to flow and either do work, (or do damage!) it has to have a route out from where it was generated, and then BACK. Without a route back, there is no circuit, no work can be done, and no damage or injury is possible.
These little generators are designed to be used on a remote job site where tools are DIRECTLY plugged into the generator. They have no connection at all on the “green wire” and yet they are safe without one. Why? Because there is no path from the handle of the tool, back to the “source of power.” There could be a direct connection from the hot wire to your hand, and nothing would happen because the electricity can not find a way to flow though you back to the generator. (Do not try this at home, just take my word for it!)
The problem starts when people plug these generators into the shore power inlet on their boats and treat them as if they were a standard shore power source with a normally functioning Protective Earth connection.
Let’s imagine there is a REAL shore power connection with a functioning PE connection, and some piece of electrical equipment develops a short circuit from the Hot wire to its external case. A circuit is completed back down the PE wire to the source of the power, and the ELCI or breaker trips and shuts down that circuit. Safe. Simple.
A very different situation occurs if the boat is plugged into one of these portable generators. Without the PE wire being connected to anything, when the short circuit described above occurs, there is no longer a circuit back to the source of power, so nothing happens. No breaker trips, everything might be working as expected, nobody gets shocked (yet!), everything looks OK.
EXCEPT... the entire PE circuit on the boat is now attached to the HOT side of the generator! Every case of every piece of electrical equipment. On most boats that includes the DC negative as well! Oh, and the bonding circuit too! Everything is HOT electrically, and will stay that way until that power finds a route back to the generator. This situation will continue because there is no sign of a problem—until that gap is bridged by someone or something. The result could be a fire, or a dangerous electric shock. Not only have you defeated the whole purpose of the Protective Earth wire, you have made the system MORE dangerous than if it didn't exist.
Sound like a good idea to you???