Ask PYS: Power

Ed, on May 14, 2012 asked:

I just re-read your excellent article on galvanic isolators. I was installing a "Smart Plug" on my PT 38 trawler and was horrified to find that I had one of those boats where someone in the past had simply not installed the ground wire. From your article, do I understand correctly that the isolator simply is installed in the ground line (ie. ground line from boat to isolator , then isolator to the ground connector of the plug)? You also mentioned having "fail-safe" isolators. How would I know if the isolator fails in the future, assuming failure is a possibility?

It certainly is distressing to realize that the AC grounding wire, a critical safety component on any boat, is missing. You’re right - the AC grounding wire (green colour) is intercepted right at or very near the shore power receptacle, and both severed wire ends are connected to the galvanic isolator. When all AC sources of power are disconnected, you can test a GI for failure by simply performing an ohm's test. A failed GI ohm's reading should show some resistance, but very little. Only a diode test in one direction can show if a GI is working properly.