How To

Testing a Galvanic Isolator

Perform this test on galvanic isolators to ensure their proper operation.

By Capt. Vincent Daniello

Updated: April 19, 2016

A galvanic isolator allows significant current to pass through the shore power ground, but it blocks minor voltages common in marina ground wires to protect your boat’s metal from stray-current corrosion. “A lightning strike, not just on your boat but nearby in the marina, can pass massive current that causes that isolator’s diodes to fail,” says Wayne Kelsoe, the recently retired senior electrical engineer at Blue Sea Systems. Older isolators might fail “open” so they dangerously disconnect the boat’s ground from shore. Newer isolators — installed on new boats since 2011 — fail “closed” for safety, but that leaves the boat vulnerable to corrosion.

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To test a galvanic isolator, flip off dock and boat main breakers, disconnect the dockside end of the shore cord, and bring it close to the boat’s electrical distribution panel. Using the diode-check function of a multimeter, connect the black meter lead to the shore cord ground (the bent prong on a 30-amp cord), and connect the red meter lead to the boat’s green-wire ground bus. Voltage should slowly climb to about 1 volt and stop (your meter might read around 1000 millivolts) — that’s the maximum voltage that galvanic isolators block. Capt. Vincent Daniello

Now touch that shore cord ground prong to the green ground bus (or use a wire between). This discharges the galvanic isolator’s internal capacitor. Capt. Vincent Daniello

Repeat the test with the black meter lead on the boat’s ground and the red lead on the shore cord ground. It should again climb slowly and stop around 1 volt. Capt. Vincent Daniello

50-amp Cord Galvanic Isolator Diode Check
With dock and boat breakers turned off, bring the dockside end of the shore power cord close to the boat’s electrical panel. Set your multimeter to its diode check function.

Connect the black lead to the shore cord ground (the outer metal jacket on a 50-amp cord), and the red lead to the boat’s green-wire ground bus. Voltage should climb to about 1 volt. Capt. Vincent Daniello

Touch the shore cord ground to the boat’s ground bus to discharge the galvanic isolator’s internal capacitor. Capt. Vincent Daniello

Repeat the test, this time with the red meter lead connected to the shore cord ground and the black lead connected to the boat’s ground bus. Again, voltage should stabilize around 1 volt. Capt. Vincent Daniello