How to test a galvanic isolator that doesn't have a monitoring system

Galvanic isolators are a bit difficult to test.  Current ABYC standards require that galvanic isolators be self testing.  For testing older isolators without this feature there are a number of methods that can be used, the one that follows is one that I found most useful.

Unplug the boat from shore power before starting the test.  
Disconnect one lead of the isolator so that you are testing it only.  Get a digital multimeter set to the diode test function.  Put one lead on one side of the isolator and the other lead on the other side.

As the capacitor starts to conduct current  the reading should rise to approximately 0.9 volts.  Remove the test leads, short the two wires of the isolator together to discharge the capacitor and repeat the test with the test leads reversed. You should get the same answer.

Interpreting the readings:

  • If the reading is instantly 0.9 volts then the capacitor is defective or there is no capacitor.
  • If a voltage of 0.45 volts is observed one of the diodes is shorted.
  • If there is a reading of 0 volts then both diodes could be shorted.
  • If there is a reading in excess of 0.9 volts then one or both diodes are open (not conducting) in which case you should stop the test before the capacitor reaches 2.0 volts or you will damage it.

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