No galvanic isolator: why surveyors flag it and what fitting one costs

Plug a boat into shore power and the green earth wire connects its propeller, shaft and through-hulls to the shore earth, and through it to every other boat plugged in on the dock. DEI Marine describes the result as a galvanic corrosion cell: dissimilar metals in seawater, joined by a wire. The least noble metal loses, and on a busy dock that's often your anodes first and your underwater metals after.

What an isolator does

A galvanic isolator sits in series with the shore earth wire. It blocks the small DC currents that drive galvanic corrosion and still passes AC fault current, so the earth does its safety job. The alternative is an isolation transformer, which separates the boat from shore power altogether and costs more.

Under the revised ABYC A-28, a fail-safe isolator has been confirmed by an independent laboratory to keep working, or leave a permanent earth path, if it fails, and needs no monitor. Units come rated 30 A, 50/60 A, 100 A and 200 A to match the shore supply.

How a surveyor checks it

Trace the green earth from the shore power inlet. An isolator is usually close behind the inlet; read its label for the rating and any fail-safe marking. No isolator and no transformer is the finding.

Then look for the evidence. Anodes that waste fast, owners renewing them more than once a season, and corrosion on underwater metals point to current through the shore earth. Worst is the earth wire disconnected to stop the wastage. That swaps a corrosion problem for an electrical safety one.

What fitting one costs

For a boat with two 30 A inlets, DEI Marine notes a single 50/60 A isolator can replace two 30 A units.

Photos to take

The shore power inlet and the wiring behind it, the isolator label or the gap where one should be, the anodes and any corroded underwater metals. With Axsail, say what you see as you take them and they go on the electrical finding.

Writing it up

State the shore supply, whether an isolator or transformer is fitted and its rating, whether the earth is connected, and the anode condition. If the earth is disconnected, say so first and grade it A. A missing isolator is a B on its own.