Grounding conductor (GRN-YEL) and galvanic isolator

[jboatman]

I thought it would be a good idea to check my grounding conductor prior to installing a galvanic isolator. I do not believe the vessel (2002 SO37) has an isolator, at least not that I can see/find, but I'm glad I checked before installing one. What I found has me a little stumped, so I'm looking for thoughts.

When measuring from the grounding conductor (GRN-YEL) at the shore power inlet to the rudder post (or various instrument grounds) with a multimeter I get zero ohms, which indicates there is no isolator (diode stacks). However, if I reverse the mutlimeter leads I get open circuit. Hmm, seems like there is a diode in the circuit.

If I switch the meter to "diode test" I get 365 mV. A galvanic isolator would typically be 2 power diode drops, perhaps 900 mV or so.

Galvanic isolators tend to be rather large and relatively easy to find. I have not found one. But something is happening between shore power grounding conductor and vessel ground. There is no connection between neutral and grounding conductor (which is correct).

Boat is high and dry, so I'm pretty confident in my tests/measurements and I won't be installing the new isolator until I figure this out. Any thoughts or ideas would be welcome.

[Trevor]

Hello jboatman,

You certainly have an interesting finding. A couple of things maybe worth mentioning,

a. The mains ground should be directly connected to DC ground somewhere. I understand that may be through the Galvanic Isolator but in the absence of the Galvanic isolator a solid connection should be made.
b. Jeanneau does not (at least in later models ) adopt a "bonded system" for through hulls and metal components. In other words, the metal bits are not all bonded together and connected to a sacrificial anode. This is important only because you mentioned potential with respect to the rudder. The rudder will not be bonded by design. If may be conducting to ground by accident but not by design. A consequence of unbonded systems can be seen in the Hints and Tips article on a Jeanneau 45 that lost its rudder recently because of stray electrical current. Had the rudder been bonded that problem may have popped a fuse and not been so catastrophic. By the way, I can see good sense in both bonded and unbonded systems and don't have a really firm view either way. I only mention it as a matter of fact.
c. The negative side of the navigation network almost certainly goes to the DC ground.

I suggest trying to obtain a circuit diagram of the yacht and trace the mains and DC ground bond. I can see by your analysis you already know this stuff but thought it may help to at least discuss it a bit to get the grey matter working .

Interesting finding you have made and I am interested in what you resolve.

Kind regards,

Trevor