A question on isolation transformers and their ground path. I have
seen 2 different wiring diagrams. One has the ground from the copper
screen inside the transformer going to shore ground and the other has
it going to ship earth.

My instinct tells me that the former is correct - if there is a problem
inside the transformer the current path is back to shore. If the shore
supply has a RCD/GFCI then it should be detected and the shore side
supply breaker should trip. If there is no shore RCD/GFCI, then at
least the hull potential is not being raised.

With the latter the current path is to hull - which I think raises the
potential of the hull - not a good idea. If there is no GFCI/RCD on
the shore supply breaker, the leakage may not be detected. The ship's
RCD/GFCI will not detect it either as it is installed downstream of the
transformer.

Former at
http://www.victronenergy.com/upload/...ransformer.pdf

Latter at http://www.sterling-power.com - products - isolation
transformer

Comments/Thoughts???

Should be
Former at http://www.sterling-power.com - products - isolation
transformer

Latter at
http://www.victronenergy.com/upload/...ransformer.pdf

wrote:
Should be
Former at http://www.sterling-power.com - products - isolation
transformer

The above diagram follows the ABYC's recommended connection. The shield
goes only to the green shore cable grounding wire. The transformer's
metal case is connected to the boat's AC ground.

Charlie Wing's Handbook of Wiring conforms with the ABYC recommendations.


Latter at
http://www.victronenergy.com/upload/...ransformer.pdf


It is not clear what is represented in this circuit. The shield appears
to be connected to the boat's AC ground system and the shore cable
grounding wire appears in the circuit to be left unconnected. I have not
encountered this and do not recommend it. If you are using a Sterling
transformer, you might email them raising the issue.

Good luck!

Chuck
S/V Sans Serif

Very few people use isolation transformers. Everybody should, but ignorance
prevents them from doing so. If you use one, never connect the safety earth
from shore or you might as well not have an isolation transformer. This
isolates your vessel earth. Afterall, the reason to use these is to protect
your hull from leakage current from Granny's decrepid 30 year old vacuum
cleaner on the other end of the marina.
Steve

wrote in message
oups.com...
A question on isolation transformers and their ground path. I have
seen 2 different wiring diagrams. One has the ground from the copper
screen inside the transformer going to shore ground and the other has
it going to ship earth.

My instinct tells me that the former is correct - if there is a problem
inside the transformer the current path is back to shore. If the shore
supply has a RCD/GFCI then it should be detected and the shore side
supply breaker should trip. If there is no shore RCD/GFCI, then at
least the hull potential is not being raised.

With the latter the current path is to hull - which I think raises the
potential of the hull - not a good idea. If there is no GFCI/RCD on
the shore supply breaker, the leakage may not be detected. The ship's
RCD/GFCI will not detect it either as it is installed downstream of the
transformer.

Former at
http://www.victronenergy.com/upload/...ransformer.pdf

Latter at http://www.sterling-power.com - products - isolation
transformer

Comments/Thoughts???

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.

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.

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.


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.

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.

As I recall from training many years ago, the shield is a noise remedy.
It seems it should connect to the vessel's signal ground /
counterpoise through a radio frequency capacitor, to cancel and block
noise from the radio circuits, via the ship's neutral connector. The
transformer case and frame should be connected to shoreside "Earth",
and neutral at the power pole / entrance.

This presumes that the shield is not connected to the frame of the
transformer. If it were, I would try it both sides and disconnected, to
satisfy noise and galvanic requirements while at the dock.

Yes and no. The type of shield you are thinking about was indeed
intended to reduce noise, and it typically was grounded on the side most
likely to be noisy. The Faraday shields in those "non-marine" isolation
transformers are designed to handle signal-level currents.

Marine-type isolation transformers (as contemplated by the ABYC) utilize
shields designed to handle the full current rating of the transformer.
Presumably the presence of that shield would cause primary leakage
currents to flow through the shield to the shore grounding wire (thus
tripping a GFCI breaker) rather than through the transformer core to the
secondary. The shield is intended to provide belt and suspenders safety,
rather than noise reduction, although it undoubtedly offers up some of
that as well.

Chuck





Terry K wrote:
As I recall from training many years ago, the shield is a noise remedy.
It seems it should connect to the vessel's signal ground /
counterpoise through a radio frequency capacitor, to cancel and block
noise from the radio circuits, via the ship's neutral connector. The
transformer case and frame should be connected to shoreside "Earth",
and neutral at the power pole / entrance.

This presumes that the shield is not connected to the frame of the
transformer. If it were, I would try it both sides and disconnected, to
satisfy noise and galvanic requirements while at the dock.

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