As I said above these are simple devices. When opened up you can see just how simple. Keep in mind that this particular model no longer meets the current ABYC safety standards and I will get to that in a moment. On the back wall we can see the two wires going to the gold plated studs. This is where your green grounding wire would be cut and connected to. Inside we can see the two diodes which allow current to flow in both directions but at the same time blocking voltages below 1.0 - 1.2V.
So why do I need to test my GI? Remember in the last paragraph when I mentioned that this particular GI no longer meets the current ABYC safety standards? Well,there is a very good reason for that.
Think back to when I described where a GI gets installed? It goes in the GREEN SAFETY GROUND WIRE! So now what happens if one or both of those diodes get blown? NO SAFETY GROUND....! There were far to many incidences of boats with GI's that completely lost their safety ground connection to shore due to failed diodes. A single lightning strike on a boat a few docks away could take out half the GI's in the whole marina and folks rarely even noticed.. Couple this with the fact that many more boats are not wired, on-board, to current safety standards where the AC green/grounding wire is bonded to the ships DC grounding bus and this means metallic parts of AC devices could become "hot" in the event of a fault. Hot cases and metallic parts of your boat spell the potential for electrocution or death to swimmers by electric shock drowning. The problem is that without the safety ground the fault protection devices will not trip as they should. This = BAD !
Below is a direct quote from Captain David Rifkin. He is perhaps the leading expert in marine electrical shock drowning deaths, bonding and corrosion.
"In the many boats I have tested with galvanic isolators, approximately 5% tested open circuited (meaning the boat did not have a connection to the marina grounding system) and the operators were completely unaware."
So 5% of the boats with GI's and an open circuited ground, does not sound like much but consider that with the millions of boats in the water there are potentially hundreds of thousands of GI's installed on boats. How many boats in your marina? 100? So if 5 of those boats are completely lacking a green safety ground wire do you want to go swimming in your marina??
So just what does meet the current standards for GI's? The ABYC standards for galvanic isolators have changed a few times. First it required "active monitoring" and remote idiot lights so an owner would know they had a working GI and still had their safety ground intact. This "active monitoring" added significantly to the cost of GI's and was a total PITA in terms of parasitic loads because it had to be "wired in" to more than just the green wire.
A number of years ago a company called Dairyland Electrical Industries, or DEI for short, invented/brought the "fail safe" galvanic isolator to the marine market. This advancement brought the simplicity back to the GI, did away with the idiot lights and related circuitry, but added a major new safety feature, "fail safe".
Fail safe means that these devices fail closed instead of open as a normal diode would. By failing closed you only lose galvanic protection but not your SAFETY GROUND wire....! According to Henry T., one of the owners of DEI, they have not had a single failure of a DEI Fail Safe galvanic isolator where it failed "open", even to lightning strikes. A pretty impressive feat considering the number of failures that occurred with the older technology, like the one you see here.
Today companies such as Guest, ProMariner & DEI all make ABYC compliant galvanic isolators with the fail safe technology. Yandina also makes a GI but it does not meet the current ABYC safety standards. I do mention them though because they are at least 100% honest that the product does not technically meet the standards. Kudos to Yandina for being honest in a world of BS marketing hype.. If you are a mooring sailor who rarely stays at a dock the Yandina GI can be a good value. If adding a GI, for regular dock side use, I would advise going with a "fail safe" product so you don't lose your safety ground wire.