Andina Marie wrote:
I can't agree, Steve.

The question involves safety not electrolysis. It is well understood
that there must be no connection between the grounds to eliminate the
electrolysis currents and that is the primary reason for installing a
transformer.

However considering the safety question, the concern is the (remote)
possibility of a short from the primary winding to the frame. I
disagree with your statement that a fault on either side of the
transformer will trip the circuit breaker - that is incorrect.

A short on either primary or secondary would trip the breaker, of
course, provided it has been sized properly.

If, as has been suggested, the transformer frame is connected to the
boat ground and you develop a fault from the primary winding to the
frame - and hence the boat ground - and hence the underwater metal, you
will NOT draw enough current to trip a 30 or 50 amp circuit breaker.
The underwater metal will be alive at a high AC voltage and
considerable current will be flowing to the water generating all sorts
of nasty gasses but there is no way the water is going to carry enough
current to trip the breaker. A swimmer in the vicinity is then at risk
of a lethal shock.

I agree with your analysis.

As Chuck pointed out, a GFI will disconnect as soon as it detects
leakage current to the water but GFI outlets on the dock are very rare
and GFI protection on the boat input side of an isolation transformer
is virtually non-existent.


It should be kept in mind that GFI protection onboard is both
inexpensive and relatively simple to install. I believe a strong case
can be made for using GFI protection even when the transformer frame is
connected to the shore power grounding conductor.

A short from the secondary side to frame, if the frame is connected to
either shore or boat ground is a far less critical situation since the
secondary side is floating so no lethal voltage would be present on the
underwater metal in either case.


Is that true? If the secondary wire that is connected to the boat's
ground shorts to the frame, which in turn is connected to the shore
power grounding conductor, then I would agree with your statement.
However, if the "hot" secondary wire shorts to the frame, the full
secondary voltage will be applied between the boat's underwater metal
and the shore power ground! Just what we are trying to prevent.

If the frame is connected to the boat's ground, then a short from the
secondary hot wire to the frame would simply trip a breaker.

Chuck

Regards,

Andina Marie Foster,