Chuck,
I don't think you understand the problem. If there is a ground fault
detecting breaker some where in the distribution net, it clearly should
function whether or not we elect to use the green equipment line or not at
the service point. I agree that the green wire should provide a low
impedance path to earth. In fact , as I stated, that connection must be made
at the distribution transformer. The problem is that the low impedence path
provided at the transformer will NEVER be as low an impedance path to earth
as your hull is. Therefore energy leakage to earth anywhere within the net,
we are now talking about the whole marina, will route itself to your hull
simply because it is a better ground than the one provided at the
transformer.
Your statement that fault current does not flow through the green wire
cuircuit is flat incorrect. That is its sole purpose. The inverse is also
true, if there is no fault, no current will flow there. If we take the case
of the in-line AC line filter, you know, the capacitor type with the metal
case that bolts to equipment frames? Or granny's 25 year old vacuum cleaner
with worn out brushes and carbon dust all around the commutator? All that is
leakage current. Now, I concede that the earth point at the transformer is
also the neutral tranformer leg tie point and the leakage also returns to
the transformer at that point, but your hull has also created a huge ground
loop in the net and by default routes all earth reference current through
your hull.
Please also remember that your boat, if wired correctly, is a complete power
system AC and DC all properly tied together at your hull ground plate and
when hooking directly to shore power without an isolation transformer, you
are paralleling the shore based grid and the ground connection you make is
effectively an equalization bus and every code book I have ever read clearly
states that an equalization bus should never be longer than one meter. So
the correct advice is, if you don't want to waste your metal bits below the
waterline due to electrolosis and galvanic protection is your goal, never
direct wire to shore power. Instead, you should couple your energy
magnetically through the use of an isolation transformer and leave other's
faults to themselves.
Steve
Hello Steve,
I would caution strongly against using the water-earth path in lieu of the
green equipment grounding conductor. Ever.
The purpose of that conductor is to provide a low impedance path for the
purpose of clearing a ground fault.
In proper wiring, the impedance is no more than a few ohms and the fault
is cleared at a breaker. Fault current does NOT flow through the earth.
By contrast, the water-earth path may have a resistance of 25 ohms or more
for salt water, and tens of thousands of ohms or more for fresh water.
This path will NOT clear a ground fault. It will not trip a 20 A breaker.
Witness the 120 volt power cords dropped off a dock without tripping a
breaker!
If one has a problem with wiring impedances in the marina, deal with it by
getting the wiring inspected. Ask the marina operator when it was last
checked. Yell. Scream. Buy your own tester and tell the marina what you
measured. Tell the other boaters. Take names and publish on the Internet.
But don't throw out the safety net because even with corroded connections,
the wiring may still allow a ground fault to be cleared.
If there is a problem with a nearby source (boat or otherwise) then an
isolation transformer would be a good idea.
But even with an isolation transformer, always use approved wiring
practices. There is probably no more thoroughly reviewed standard than the
NEC. The ABYC standards for AC wiring are similarly solid.
And install GFCIs for the smaller ground fault currents that won't trip a
regular breaker but can electrocute.
Good luck.
Chuck
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