Boat Forum / Building / July 2006

Mystery solved- "OFP" is a typo- should be "GFP" - Ground Fault
Protection, says the technician at Charles.

Still haven't found any such marine 2 pole 30 amp breaker, though.

I think you should go buy "Boatowner's Mechanical and Electrical Manual" by
Nigel Calder.  His chapter on DC and AC power distribution covers all of
your questions.

There is some controversy on how to disconnect shore power, but for now,
note that the "common practice" is to disconnect the hot wire and the
current-carrying neutral (don't even consider a 2-wire system that uses the
bonding circuit as an AC neutral), but leave the (green) ground wire
connected.  On your boat, the AC and DC grounds SHOULD be connected to each
other and don't let anybody tell you otherwise.  Rather than follow bad
advice that's intended to reach the same goals, go buy a galvanic isolator
(aka 'zinc saver'), e.g.  http://www.boatersworld.com/product/196902571.htm
and keep those AC and DC ground circuits a) connected to each other on the
boat, and b) connected to the shore power ground.  I suspect (too lazy to
look it up right now) that the ABYC also requires that both the AC hot side
and current-carrying neutral are disconnected by the boat's disconnect
(breaker or switch).  This might be because all the neutrals are connected
together on the shore side and you could accidentally become a path to
ground if you touch it and the bonding circuit at the same time AND if the
neutral-to-ground connection on the shore side is faulty.  By breaking the
neutral at your boat, you reduce the amount of wiring that you might be
exposed to while on your boat, e.g. on your panel.  That said, note that
some folks believe that this risk is the smaller risk and that it's more
likely that you'd have bum wiring on your boat than bum wiring in the shore
side power distribution.  If so, then your boat's AC neutral provides an
alternate path to ground (additional safety onboard), noting again that the
shore side neutral and ground are connected to each other back at the shore
side transformer(s).  In non-marine practices, the entire world leaves the
AC neutral connected (always) and only disconnects the hot phases with
breakers and switches ...except for a couple of backwards places that insist
on being different (like France).  It's also important to note that Blue Sea
and other prime manufacturers sell only 2-pole circuit breakers for
disconnection of single phase AC shore power.  I'd swim WITH the fishies
rather than against and just go buy a Blue Sea 2-pole circuit breaker and
call it done.  And do follow my advice on grounds and the galvanic isolator.
Oh, and go buy Nigel's book... you only have to read about 20 pages to cover
the AC power distribution stuff.  You'll want to read his advice on properly
wiring and installing your bonding (grounding) circuit as well.  Good stuff,
Maynard.  Stay safe.  And don't go swimming near the docks.

Brian D