One of the questions I get asked fairly often concerns galvanic isolators — those small but important devices that sit in the safety grounding connection between your boat and the shore power system. Specifically, people want to know whether their galvanic isolator is fail safe. It is a question worth taking seriously, because the answer has real consequences for both the safety of anyone in the water near your boat and the long-term survival of your underwater metals.
What a Galvanic Isolator Actually Does
When you plug into shore power at a marina, you connect your boat's AC grounding system to the dock's grounding system. That connection is necessary for electrical safety — it is what allows a breaker to trip if something goes wrong aboard. But it also creates a path for galvanic current to flow between your boat and every other boat plugged into shore power on the same dock. Over time, that current eats away at your propeller, shaft, hull fittings, and anything else made of metal below the waterline.
A galvanic isolator interrupts that low-level DC galvanic current while still allowing the safety grounding connection to function for AC fault protection. It does this using pairs of diodes — semiconductor devices that block small voltages but pass higher ones. In practice, virtually all galvanic isolators use two sets of counterpose diodes, and they have done so since the earliest versions of these devices came onto the market decades ago.
The Critical Difference Between "Fail Safe" and "Not Fail Safe"
Here is where the details matter enormously, and where many boat owners are operating with a false sense of security. A device with two sets of diodes can fail in either direction — the diodes can fail open or fail shorted. If they fail open, you lose the safety grounding connection to the dock entirely. Your boat's grounding system is no longer connected to the shore earth. That is not safe, and — critically — the device will give you no indication whatsoever that this has happened. You are simply unprotected, and you would have no way of knowing.
This is exactly the failure mode that drove a revision to the ABYC standards in the early 2000s. The initial response was to require an alarm that would trigger if the isolator failed. That turned out to be difficult and expensive to implement reliably, so the standards moved to a more elegant solution: require that the device be designed to fail shorted rather than open. In other words, if something goes wrong with the diodes, the circuit remains closed and the safety grounding connection to the dock is preserved. You may then have a corrosion problem — galvanic current can now flow freely — but you do not have a safety hazard.
Devices that meet this standard are required to be labelled as such. If you go to your galvanic isolator and it carries the words "fail safe" on it — typically alongside a reference to the relevant ABYC standard — then you have a compliant device. If it does not carry that label, it was almost certainly manufactured before these requirements came into force, and it should be replaced.
What You Need to Do Right Now
The first step is straightforward: go and look at your galvanic isolator. Find where it is installed in the shore power system and check the label. If it says "fail safe" and references ABYC compliance, you have a device that meets the current standard. If it does not, the answer is equally straightforward — replace it. These are not expensive pieces of equipment, and the cost is trivial compared to the safety risk of operating without a reliable grounding connection to the dock.
There is one more thing worth knowing even if you do have a fail safe device. Current fail safe galvanic isolators will not alarm or alert you when they have failed shorted. The safety grounding connection is preserved, but the device is no longer functioning as a galvanic isolator — it is, in effect, simply a piece of wire. You need to test it periodically to confirm it is still doing its job. Testing is not complicated, but it does require knowing what you are looking for and how to interpret the results.
Going Deeper on Shore Power Safety
The galvanic isolator is just one piece of a larger picture when it comes to shore power and electrical safety aboard. Understanding how the grounding system works, how galvanic and stray current corrosion develops, and how to test your protective devices properly are all topics I cover in detail in the Advanced Marine Electrics course. If you have any doubt about the state of your boat's electrical system — and a question like this one suggests you are already thinking carefully about it — that course will give you the knowledge to evaluate what you have and make informed decisions about what needs attention.
