T&T: 120V green ground conductor

[Arild Jensen]

----- Original Message ----- 
From: "KevinR"

> A bit of clarification here -  ABYC E-11.17 requires that the safety 
> ground (green wire) of the  AC system ON THE BOAT be bonded to the "engine 
> negative terminal or its bus". However, to  prevent galvanic corrosion, a 
> galvanic isolator OR an isolation transformer is also needed.
> An alternate way of complying with the requirements if you have the space 
> and budget to do so,  is to add an isolation transformer to the circuit


REPLY
In my work  I have seen a few wooden boats nearly destroyed  because they 
were wired to conform to ABYC standards.  What most people do not  recognize 
is that  the standards was written to prevent shock hazard to people and 
sacrificing the boat was considered acceptable.
The intent of the standard is to GROUND  the AC electrical system because 
all the protection  like GFCI and circuit breakers rely on the current path 
provided by the green ground wire.   Bonding the engine block  is a 
convenience short cut.  This method is a quick way to connect to a large 
metal mass  in direct contact with  the sea water. But if a driver saver 
disk is installed in the shaft coupling then you hav ebroken this path to 
ground.  some of the newer  engines are now deisgned not to have the block 
as a DC current carrying  conductor in the start circuit. The end result  is 
an engine that is still isolated.  this negates the intent of th eABYC 
standad.

In wooden boats docked in waters  known to have stray current from shore 
power facilities  it is common to see  this current passing in ( via thru 
hul fitting) through one side of the hull;  follow the bonding wires and 
exit the hull on the other side  which is closer to the land mounted 
transformer. this will happen even if both a galvanic isolator or an 
isolating transformer is used as long as th efitings are bonded per ABYC. 
Not good!.
The wood around the thru hulls will soften as the lignin is destroyed by the 
passing current.

ABYC  standards were originally formulated for non conducting plastic boats. 
ABYC came into existence after the heyday of wooden boats had passed.  I 
doubt the technical committees  really gave much thought to the issue. Their 
focus was  people safety and shock prevention.
You can be compliant  to the standard and still get away  from  running 
bonding wires to every thru hull fiting.  You need a separate ground plate 
of approximately  one square yard or meter.  This will securely  ground the 
AC electrical system  If the boat is fitted with an inverter some additional 
wiring tricks  for the neutrals  and the green wire bonding bus  must be 
observed.

cheers
Arild