Your name wrote:

As a follow-up, I should point out that this is a somewhat controversial
issue, this business of AC vs. DC currents. Many students of galvanic
corrosion believe that AC can and does cause corrosion. If you choose to
believe this, then you will want an isolation device that limits both AC
and DC voltages.

Mercury uses a capacitor in their isolators because a sufficiently high
AC voltage will forward-bias the diodes and allow them to conduct DC,
even at low voltages. But bypassing the diodes for AC means that the full
AC voltage (if any) will pass through the isolator. The net effect of
this may be a benefit in isolating some DC voltages, but at the expense
of allowing AC to pass through unaffected!

If there is a chance that you will need to protect against AC as well as
DC with your galvanic isolator, you will be disappointed with a
capacitor. The best course then, is to use an isolation transformer. It
will provide complete isolation.

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Of course it can pass full AC thats the point. The whole idea is for a short in the
ungrounded
conductor (hot) to have a safe path to ground (hopefully tripping the breaker) The idea
behind
the isolator is to block DC currents caused when your aluminum outdrive and your
neihbors
stainless shaft become a battery because they are both connected electricaly and
emmerced in
an electrolyte. As far as small AC currents caused by leaky 48" shop lights, electric
motors etc.
you want the isolator to pass these. Its much better that these currents travel back
home via the
grounding conductor than through your through hull fitting into the water etc. This is
for the sake
of your boat and swimmers.