Andina Marie wrote:
The only function of the shore power ground wire when you have an
isolation transformer is to return short circuit current back to the
source so it will blow the dock circuit breaker. Although the
probability of an electrical breakdown on a transformer is minimal, the
metal frame and the copper shield should be connected to the shore
power ground to provide this return circuit. The transformer frame
should not be connected to anything on the boat and it should be
mounted on an insulated support in a metal boat.


To clarify, a short in the transformer primary (or secondary, for that
matter) should trip the boat's breaker. If it instead trips the dock
breaker there is a design or equipment problem. Although the
transformer's metal case could be connected to the shore power ground
wire, insulation of the transformer case is then required. Because
wiring in this way provides dubious benefits, I believe grounding the
metal case to the boat's ground system is a better approach. In terms of
safety, it seems preferable that the transformer case be at the same
potential as the metal cabinets of all other AC devices on the boat.

When purchasing an isolation transformer, getting one with adjustable
voltage taps on the input is worth the extra expense if you are
traveling to foreign ports. Our transformer can accept input from
about 80 volts to about 450 volts with different tap settings and in
addition to foreign adaptability, it has helped out on docks suffering
from low voltage.



While a tapped primary is generally a good idea, isolation transformers
with 450 volt primary taps and internal shields are not common in the US.

An alternative approach would be to use a "standard" isolation
transformer (untapped primary) and a separate autotransformer. The
latter does NOT provide isolation, but can be used on either side of the
isolation transformer to step voltages up or down as desired, and is
fairly inexpensive. Even a Variac (a continuously adjustable type of
autotransformer) can provide an inexpensive means of dealing with a
range of input voltages.

Unfortunately, if used on the shore side of the isolation transformer,
the metal case of a (non-adjustable) autotransformer would have to be
insulated. On the secondary side of the isolation transformer, the case
could be grounded as I recommended for the isolation transformer case.
Although it may be ok to apply 450 volts AC to the primary of an
isolation transformer designed for 120 volts, the manufacturer's input
voltage ratings should always be strictly observed. As a practical
matter, the autotransformer used at the isolation transformer secondary
will safely and conveniently correct under- and overvoltage conditions,
but it may not provide a universal solution.

The autotransformer (or tapped isolation transformer) can be a
double-edged sword. If it is set to raise an incoming line voltage from
80 to 120 volts, and in the middle of the night the line voltage rises
to 120 volts, there is a good chance that damage will occur to connected
appliances.

Chuck



wrote:

I have one with the earths in accordance with the sterling power
diagram. It was not very expensive - £175 for a 3.6kVA. When I was
installing the shore power its use was instantly demonstrated whilst
still on dry land in the builders shed. The building's neutral was
showing +12 compared to earth, once onboard after the ship side neutral
had been bonded to earth, the ship side neutral was of course at Ov.
The other major advantage of an IT is that you no longer have to worry
whether the shore supply itself is wired correctly - Live is Neutral
and vice versa.