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Galvanic Isolators

My sailboat has two shore power connects, one for regular house, and the other for air conditioner. Can I use just one galvanic isolator or do I need to order two? Do I order a 30 amp for one 20 amp shore and one 50 amp for a 30 amp shore or do you match 30 to 30?

Tom Wood responds:
For full disclosure, let me begin by saying that I am not a big fan of galvanic isolators. They are nothing more than a set of diodes placed in the ground wire of a three-wire AC line as close to the inlet as possible. The theory is that these diodes can open and close the circuit at a preset electrical potential, usually about one volt. This, again in theory, isolates the AC ground from the DC ground, eliminating the possibility of stray electric current from eating up zincs, shafts, props, and thru-hulls unless there is a major need for grounding, in which case the diodes open the ground circuit.

The reality is that many boats have numerous connections between the AC and DC grounds with generator, battery charger, or inverter installations, rendering the galvanic isolator useless. The systems make a proper lightning protection system impossible. And if the diodes fail one of two things happen: if they fail in the closed position, it's as if the galvanic isolator wasn't there at all, but if they fail in the open position, the risk of electrical shock becomes very real.

If you feel that you absolutely have to have them, you must have a separate galvanic isolator on each incoming power inlet. Each must be sized to be at least 25 percent larger than the maximum capacity of the master circuit breaker on the line, so a 30-amp line should have a galvanic isolator rated at a minimum of 37.5 ampsóa 40-amp unit would be the best choice but a 50 would do the same thing. A 30-amp unit would work on a circuit with a 20-amp master breaker.


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