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

In article . net, chuck wrote:
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.

My understanding of shields in general, is that the shield should be connected to
the ground with the least noise.

greg




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.

Andina & Chuck,
This is not rocket science. There is NO safety issue by not using shore
safety earth. Shore safety earth ultimately connects with the distribution
transformer nuetral and tied to a ground stake at that transformer. That
ground stake is the reason for having an isolation transformer in the first
place. It will never be a closer reference to earth than your boat. Any
leakage current ANYWHERE in that distribution net will seek your hull to
earth, as it will offer the least resistive path to earth. (It is in the
water!)

The plan is to magnetically couple energy from the primary to the insolated
secondary with the secondary referenced by end tap (110 V) or center tap
(230 V) to the boats ground plate. There must not be any electrical
connection between your boat and shore power. If a fault would occur on
either side of the transformer, fault current will run quite nicely to the
ground plate tripping the feed circuit breaker no matter where it is.

The only reason to use the shore safety earth on the transformer case is if
the transformer is physically mounted on the dock, not in your boat.
Steve


"Andina Marie" wrote in message
oups.com...
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

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.

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.

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.

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.

Regards,

Andina Marie Foster,

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,

chuck wrote:
....snip

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

....snip
Chuck

I am following your discussion with much interest as I am planning to
install very soon an isolation transformer in my boat.
I add a further question: I have just learnt that a firm is going to
commercialize a Switch-Mode Isolation Transformer (they claim is the
first in the world!) and, if you are interested, just make a Google
search. My question is: is it indeed equivalent to the ol' faithful and
massive isolation transformers?

Thanks for your help
Daniel

The real problem, that no one has yet mentioned, is inrush charge current.
If the transformer is sized correctly for maximum shore load, the initial
inrush will most certainly pop the breaker at most marinas. The solution I
use works very well, if anyone is interested.

Tie in series with the primary coil of the transformer a standard light bulb
socket. Insert a 100 watt light bulb of the appropriate voltage and connect
a switch capable of handling the maximum current of the transformer in
parallel across the lamp socket. In use, open the switch and connect the
transformer to shore power, then close the switch before applying secondary
load. Initially the bulb will light and then fade out. Anytime after that
bright phase, it is safe to throw the switch shorting out the lamp.
Steve


"

Thanks for the info. I was not aware of the product and as you can
imagine, I have no experience with it. My comments are therefore
necessarily abstract.

The two primary advantages of a switch-mode device are lower weight
(less iron) and a built-in capability to regulate output voltage (and
perhaps adjust for different input voltages) automatically.

Potential disadvantages, however, are numerous. The standard isolation
transformer is perhaps the most reliable electrical component we have.
Switch-mode circuits tend to involve many electronic components, some of
which operate under life-shortening stresses. While an isolation
transformer could easily outlast a boat, I doubt the same could be said
of the vastly more complex switch-mode devices.

Without examining the actual circuit, it is impossible to discuss the
degree of isolation achieved or the potential isolation fault modes of
the device.

No mention is made in the specifications about the purity of the sine
wave output. For some appliances, this may not be critical; for others,
it is very important. Possible electronic noise from the switching
circuits could also be troublesome. While modern switch-mode circuits
can be designed to deal acceptably with these issues, it is difficult to
evaluate the product to which you refer since no mention of them is made
on the website.

Other considerations are susceptibility to noise and voltage transients
(e.g., lightning) on the shore power line and radio frequency
interference from onboard radios, battery chargers, fluorescent lamps,
and radar.

It is unclear how the ABYC and various EU standards will view such a device.

Sorry I can't offer anything more specific. Perhaps others have had some
experience with switch-mode isolation devices.

Good luck.

Chuck



Daniele Fua wrote:
chuck wrote:
...snip


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

...snip

Chuck


I am following your discussion with much interest as I am planning to
install very soon an isolation transformer in my boat.
I add a further question: I have just learnt that a firm is going to
commercialize a Switch-Mode Isolation Transformer (they claim is the
first in the world!) and, if you are interested, just make a Google
search. My question is: is it indeed equivalent to the ol' faithful and
massive isolation transformers?

Thanks for your help
Daniel

Good point, Steve. Has a lot to do with the trip curve of the
pedestal-mounted circuit breaker, but that's usually beyond the reach of
boaters to change.

A step up from your manual (and probably bullet-proof) solution might be
to use a simple 120 VAC, SPST relay with coil directly across the
transformer primary and a 100 watt light bulb in series with the
transformer hot wire. The relay's contacts would short the bulb when the
inrush current dissipates and the relay coil is energized. A shortcoming
is that the relay is energized whenever the transformer is in use.

Another step up would be to insert a manual SPDT switch so as to select
either the light/bulb relay circuit or a direct connection. That way the
light bulb/relay circuit could be reserved for those cases where inrush
is tripping shore power breakers.

Probably $15 worth of parts and 20 minutes to do the job.

It should be simple enough to design a better circuit that would
automatically drop the relay out after initial inrush.

Caution: these are lethal voltages and the wiring is best left to
professionals for those uncertain of their skills and understanding.

Chuck




Steve Lusardi wrote:
The real problem, that no one has yet mentioned, is inrush charge current.
If the transformer is sized correctly for maximum shore load, the initial
inrush will most certainly pop the breaker at most marinas. The solution I
use works very well, if anyone is interested.

Tie in series with the primary coil of the transformer a standard light bulb
socket. Insert a 100 watt light bulb of the appropriate voltage and connect
a switch capable of handling the maximum current of the transformer in
parallel across the lamp socket. In use, open the switch and connect the
transformer to shore power, then close the switch before applying secondary
load. Initially the bulb will light and then fade out. Anytime after that
bright phase, it is safe to throw the switch shorting out the lamp.
Steve


"

Chuck,
I have been down the route of automatic inrush control. The circuit I
designed was keyed off of secondary voltage in that when secondary voltage
was developed, would the relay short the lamp. This worked very well, but I
like the manual switch better. It is cheap clean and bulletproof. On the
switched transformers the problem is efficiency, reliability and noise. I do
use however a modified UPS for minor AC use and it works a treat. It is a
1400 KVA Smart UPS by APC, but instead of using it off internal batteries,
I run it off ship's batteries (24V). Normally it will not fire up without an
external AC source, but if you use the undocumented cold start procedure, it
will. It has an additional utility as well. When pluged into shore power and
is powered up, it will act as an intelligent battery charger, keeping the
ships batteries fresh.
Steve

"chuck" wrote in message
hlink.net...
Thanks for the info. I was not aware of the product and as you can
imagine, I have no experience with it. My comments are therefore
necessarily abstract.

The two primary advantages of a switch-mode device are lower weight (less
iron) and a built-in capability to regulate output voltage (and perhaps
adjust for different input voltages) automatically.

Potential disadvantages, however, are numerous. The standard isolation
transformer is perhaps the most reliable electrical component we have.
Switch-mode circuits tend to involve many electronic components, some of
which operate under life-shortening stresses. While an isolation
transformer could easily outlast a boat, I doubt the same could be said of
the vastly more complex switch-mode devices.

Without examining the actual circuit, it is impossible to discuss the
degree of isolation achieved or the potential isolation fault modes of the
device.

No mention is made in the specifications about the purity of the sine wave
output. For some appliances, this may not be critical; for others, it is
very important. Possible electronic noise from the switching circuits
could also be troublesome. While modern switch-mode circuits can be
designed to deal acceptably with these issues, it is difficult to evaluate
the product to which you refer since no mention of them is made on the
website.

Other considerations are susceptibility to noise and voltage transients
(e.g., lightning) on the shore power line and radio frequency interference
from onboard radios, battery chargers, fluorescent lamps, and radar.

It is unclear how the ABYC and various EU standards will view such a
device.

Sorry I can't offer anything more specific. Perhaps others have had some
experience with switch-mode isolation devices.

Good luck.

Chuck



Daniele Fua wrote:
chuck wrote:
...snip


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

...snip

Chuck


I am following your discussion with much interest as I am planning to
install very soon an isolation transformer in my boat.
I add a further question: I have just learnt that a firm is going to
commercialize a Switch-Mode Isolation Transformer (they claim is the
first in the world!) and, if you are interested, just make a Google
search. My question is: is it indeed equivalent to the ol' faithful and
massive isolation transformers?

Thanks for your help
Daniel