A galvanic isolator sits in the shore power grounding conductor and is frequently misunderstood as something that cuts that conductor. It does not. It blocks the small direct-current potentials that drive galvanic corrosion between your boat and everything else connected to the dock, while remaining able to conduct alternating fault current so the protective earth path still works when it is needed.
The problem it solves
Shore power brings a grounding conductor aboard, and that conductor connects your boat’s underwater metals, through the bonding system, to every other boat on the same dock circuit. In seawater that makes one enormous galvanic cell. Your anodes end up protecting the neighbor’s outdrive, and a boat with poor protection quietly consumes the anodes of every boat nearby. Disconnecting the grounding conductor would solve the corrosion neatly and would also remove the fault path that keeps people alive, which is why it is never an acceptable answer. That shared connection is also why a neighboring boat with a wiring fault can affect your underwater metals.
How the device manages both jobs
Galvanic potentials between dissimilar metals are small. A galvanic isolator uses semiconductor devices that do not conduct until the voltage across them exceeds a threshold above those galvanic levels. Below that threshold the corrosion current is blocked. Above it, during a genuine alternating current fault where the voltage is far higher, the devices conduct readily and carry fault current back to shore so the breaker operates. Capacitance across the devices also helps pass alternating current. The result is a component that is effectively open to slow corrosion currents and effectively closed to fault currents. The threshold has to sit above galvanic potentials and well below anything that would appear during a fault, which is a narrower window than it sounds.
Fail-safe design and why it matters
A device in the grounding path must not be able to fail in a way that leaves the path open. Modern isolators are built with redundant diode arrangements specifically so that the expected failure mode is a short circuit, which restores a solid ground connection, rather than an open circuit. Many also include monitoring that reports the isolator’s status to a panel indicator. When choosing one, look for compliance with the applicable ABYC recommended practice and for a current rating at least equal to the shore power circuit protection, since it must carry fault current without destroying itself. Status monitoring is worth the extra cost, because an isolator is otherwise a component nobody has any reason to think about.
Where an isolation transformer fits instead
An isolation transformer takes the concept much further by magnetically coupling shore power to the boat with no metallic connection at all. The boat’s neutral and grounding conductors are then derived aboard, which eliminates galvanic coupling entirely and also addresses poor or incorrectly wired dock power. It costs more, weighs considerably more and requires careful installation of the bonding arrangement on the secondary side, but for vessels kept in salt water on shore power permanently it is the more thorough solution. The choice between the two is an engineering decision, not a preference. An isolation transformer also breaks the neutral connection to shore, which changes how the onboard system must be bonded.
Installing and verifying it correctly
The isolator goes in the shore power grounding conductor as close to the inlet as practical, before any branch connection, and the conductor must not be connected to the boat’s bonding system upstream of the device. Installation errors that bypass the isolator are common and invisible. Because the device is silent when working and silent when failed open, it needs testing rather than assumption: use a monitoring model, or have the isolator tested with the procedure the manufacturer specifies. Anodes disappearing faster than expected on a boat with an isolator fitted is a reason to test it. Any yard work that disturbs the inlet wiring is a good moment to confirm the device is still connected as designed.
What to check before you act
- Confirm the isolator is rated at least for the shore power circuit protection.
- Install it in the grounding conductor close to the inlet with no bypass.
- Choose a fail-safe design and, ideally, one with status monitoring.
- Test it to the manufacturer’s procedure rather than assuming it works.
Common questions
Does a galvanic isolator make the boat less safe?
A correctly specified, fail-safe isolator does not. It passes fault current at the voltages that occur during a fault, so the protective ground path remains effective.
Do I still need anodes with an isolator fitted?
Yes. The isolator stops galvanic coupling to the dock; it does not stop corrosion between the dissimilar metals on your own vessel. Anodes remain essential.
The practical takeaway
Fit a fail-safe isolator rated for your shore power protection, install it in the grounding conductor with nothing bypassing it, and test it periodically. If your boat lives on shore power in salt water, price an isolation transformer as well, because it solves a larger set of problems at once.
References and further reading
- OSHA: grounding requirements and the function of equipment grounding conductors
- NFPA: grounding and bonding provisions in the National Electrical Code
- CDC and NIOSH: shock hazards and the role of protective grounding
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
