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(no email)
Date: Fri Nov 21 2003 - 22:54:53 EST
Each case should be decided upon investigation of the circumstances. What
AC equipment is on board? Is there a GFI? At how many places does the AC
system come in close vicinity to the DC, as in an inverter or a battery
charger? Is there an isolation transformer?What is the quality of the
isolation between the electrical systems on these devices?
The advantages of isolating the systems is electrolysis currents from a
differing dock ground potential are prevented from entering the underwater
parts of the boat so electrolysis is avoided and an electrolysis isolator or
isolation transformer are not required to prevent it. This results
primarily in a cost saving and secondary, the added safety of not having
electrolysis eat critical parts of the boat.
The disadvantages are that if a short develops from the AC system to the DC
system, it can render the underwater parts, or the rigging if it is bonded
to the DC ground, or even the whole boat if it is metal, alive at a
dangerous voltage. This could be a hazard to persons boarding the boat,
particularly from a metal dock, or to swimmers in the vicinity. There is
also a risk to those on board if at some location they could simultaneously
touch the boat ground (engine block) and the metal cabinet of an AC device
(shore power charger).
If those interconnecting components are of good quality, and a GFI is
installed, the risk is quite minimal however the consequences can be life
threatening. The construction of the boat plus the quantity and type
equipment is a significant factor.
It is a risk/"reward" decision that should be made by a qualified engineer
and offered to the owner to decide.
The ABYC cannot recommend a solution that involves even the slightest risk
of an accident so they err on the side of least litigation and require them
to be bonded. In a practical world, this can be expensive for protection
equipment and/or deterioration of underwater metal.
On our boat (steel hull) we have an isolation transformer with input taps
that can accept anything from 80 volts to about 450 volts input, and put out
120/120 volts 50 amps. Since the probability of a short from the shore
power AC to the hull or DC ground exists only in the short run from the
shore power receptacle to the transformer, and in the transformer windings
themselves, we regard the risk as being so minimal we don't ever connect a
neutral or ground to the dock. Our input is two wires only. On the
secondary, where there is no electrolysis continuity, our neutral is
grounded to the hull and the DC system to provide a path to blow circuit
breakers if there is a short between the systems.
Andina Foster,
> Andina and Arild:
>
> If you were writing the recommendations for wiring a boat what would you
> recommend??
> Would you specify that the AC green wire be connected to the DC ground??
> What other changes would you make to the existing recommendations ?
> I am not trying to put you on the spot but I would really be interested in
> what you both think is the correct way to wire a boat.
>
> Frank Burrows
>
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