Galvanic Isolator Buying Guide: Stop Shore Power Corrosion
Quick answer: If your boat plugs into shore power and your anodes are wearing away fast, you probably need a galvanic isolator. It goes in the green grounding wire from the shore power inlet and blocks the small DC currents that cause galvanic corrosion, while still letting AC fault current pass safely to ground. Choose a fail-safe model rated at or above your shore power service (30A or 50A), or step up to an isolation transformer for the most complete protection.
When you plug into the dock, the green safety ground wire connects your boat's underwater metals, through the shore power system, to every other boat plugged into that dock. Different metals in salt water act like a battery, and that connection lets current flow between boats. The result is anodes that disappear in weeks and, once they are gone, corroded props, shafts, outdrives and thru-hulls. A galvanic isolator breaks that path for the small currents without removing the safety ground.
Step 1: Recognize the signs
- Anodes wearing out much faster than expected, often in a few weeks.
- Pitting or pink discoloration on bronze fittings and props.
- Corrosion on outdrives or shafts despite fresh anodes.
- Problems that started after moving to a new slip or marina.
These symptoms can also come from stray current or a wiring fault on your own boat, so have a marine electrician test the system if corrosion is severe.
Step 2: Understand how an isolator works
A galvanic isolator uses diodes in the shore ground wire. Diodes block current until voltage rises above a threshold. Galvanic voltages between boats are small, typically well under a volt, so the isolator blocks them. A real electrical fault produces much higher voltage, so the isolator conducts and the breaker trips as it should. Your safety ground stays intact.
Step 3: Isolator or isolation transformer?
| Feature | Galvanic isolator | Isolation transformer |
|---|---|---|
| Blocks galvanic current | Yes, at low voltage | Yes, completely |
| Protects against dock wiring problems | Partly | Yes, no direct connection to shore ground |
| Size and weight | Small, a few pounds | Large and heavy |
| Cost | Lower | Higher |
| Best for | Most boats on shore power | Liveaboards, larger yachts, problem marinas |
Step 4: Choose a fail-safe model
Look for an isolator designed as fail-safe and built to ABYC A-28. A fail-safe isolator is built to survive the fault current it may need to carry, so it does not fail open and leave the boat without a working ground. Models with a status monitor or test function let you confirm it is working.
Step 5: Size it to your service
- 30A shore power: a 30A rated isolator.
- 50A shore power: a 50A or 60A rated isolator.
- Two inlets: size for the total or follow the manufacturer's guidance for dual service.
Step 6: Install it correctly
The isolator goes in series in the green grounding conductor, between the shore power inlet and the boat's AC ground connection, with nothing bypassing it. Mount it in a dry, ventilated location. Because it is part of the AC safety system, have a qualified marine electrician install it. Keep your anodes up to date as well, since the isolator reduces galvanic loss between boats but does not replace anodes.
Common mistakes to avoid
- Disconnecting the green ground wire. Some owners cut it to stop corrosion. This removes a critical safety path and can make the boat dangerous. Use an isolator instead.
- Buying a non fail-safe unit. An isolator that fails open can leave the boat without a working ground connection.
- Undersizing for the service. A 30A isolator on a 50A system may not handle fault current.
- Assuming it fixes everything. An isolator does not fix stray current from wiring faults on your own boat. If anodes are still disappearing, have the boat tested.
Quick checklist
- Confirm your shore power service: 30A or 50A
- Choose a fail-safe isolator rated at or above that service
- Prefer a model with a status monitor or test function
- Plan a dry, ventilated mounting spot near the shore power inlet
- Have a marine electrician install and test it
- Keep inspecting and replacing anodes on schedule
- If corrosion continues, ask a marine electrician for a stray current test of your own wiring
Our picks
All in stock at Boat Supplies Direct.
30A shore power
ProMariner ProSafe FailSafe 30A Galvanic Isolator or the Newmar GI-30-FS Fail Safe 30A. For a potted, waterproof option, the Victron VDI-32A.
50A shore power
ProMariner ProSafe FailSafe 60A Galvanic Isolator or the Newmar GI-60-FS Fail Safe 60A.
Maximum protection
Victron Isolation Transformer 4500W for liveaboards and larger boats.
Frequently Asked Questions
What does a galvanic isolator do on a boat?
It blocks the small DC currents that flow through the shore power ground wire between boats and cause galvanic corrosion, while still letting AC fault current reach ground safely.
Do I need a galvanic isolator?
If your boat is plugged into shore power regularly, especially in salt water, a galvanic isolator is one of the best ways to protect underwater metals and make anodes last.
Why are my boat's anodes wearing out so fast?
Common causes include galvanic current through the shore power ground, stray current from a wiring fault, or the wrong anode material for your water. A galvanic isolator addresses the first. A marine electrician can test for the others.
Galvanic isolator vs isolation transformer, which is better?
An isolation transformer gives more complete protection but is larger, heavier and more expensive. A fail-safe galvanic isolator is the practical choice for most boats.
What size galvanic isolator do I need?
Match it to your shore power service: 30A for 30A service, and 50A or 60A for 50A service.
Does a galvanic isolator replace anodes?
No. Anodes still protect your underwater metals. The isolator just stops your anodes from being consumed by current from other boats on the dock.
This guide is general information. AC shore power wiring affects safety. Have a qualified marine electrician install and test galvanic isolators and isolation transformers.
