Thanks for your comments.

I understand your concern. Let's look in more detail at the 120 volt,
single phase system.

Case I.
Transformer case connected to shore power grounding conductor.
ABYC requires transformer case to be fully insulated with ventilation.
ABYC requires no ground fault protection.

Case II.
Transformer case connected to boat AC grounding conductor (i.e., water).
ABYC requires ground fault protection at the main shore power disconnect
circuit breaker.
No insulation or ventilation is required and the transformer case is at
the same potential as all of the other grounded metal on the boat.

Under case I, a direct short from the hot primary wire to the
transformer case will trip the boat's breaker (hopefully) or the dock
breaker.

Under case I, a ground fault (leakage) of say 15 amps on a 20 amp
circuit breaker can occur and continue indefinitely. This could be
leakage from a hot primary wire to the case, to the grounded (neutral)
primary wire, or to the secondary. Definitely not a desirable state of
affairs, but one that grounding the transformer case to the shore
grounding conductor does not and cannot prevent (unless the marina
wiring has provided ground fault protection devices at their
distribution box; quite rare).

Under case II, a direct short from the hot primary wire to the
transformer case (i.e., to the boat's grounding conductor) will trip the
primary circuit's ground fault protection device once the current
exceeds 5 milliamperes or so. Similarly for leakage currents in excess
of 5 milliamperes but less than the circuit breaker trip current.

[In either case I or II, leakage current between the hot primary wire
and the neutral (grounded) primary wire will result in heat, lower
output voltage, and without thermal overload protection, potentially
serious consequences if the breaker doesn't trip. Has no real bearing on
what we are discussing and ground fault protection will not help.
Fortunately, transformers are among the most reliable electrical
components.]

I have omitted any discussion of the shield, which ABYC requires to
carry the full current rating of the transformer. Depending on the
manufacturing geometry, that shield may preclude a current path from
primary to secondary or from the primary to the case, thus rendering the
short and leakage scenarios very improbable.

Ignoring the role of the shield, I believe Case II is a better solution,
even if ground fault protection is added to Case I (which I strongly
recommend).

My current ABYC standards are not at hand and I am using an older
edition, but I believe they are similar to standards now in effect.

Does this provide better clarification?

Chuck


Andina Marie wrote:
Chuck,

I don't agree with your grounding advise.

If you ground the frame of the transformer to the boat ground and there
is an internal short from the shore power primary winding to the frame,
there is no return path for the current so you will not trip the supply
breaker.

In addition, you now have the boat ground, including underwater metal,
live at 110 or 220 volts which can electrocute persons in the water or
boarding from a metal dock.

Andina Marie