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Subject: Re: TWL: Alternatives to isolation transformer?
From: Arild Jensen (elnav@XXX.XXX)
Date: Sun May 05 2002 - 11:27:21 EDT
Richard wrote:
> I have recently been strongly encouraged to install an isolation
> transformer on the boat.
<<< snip>>>>
> So, I'm wondering if there is any nifty device (sort of like a combination
> of reverse polarity alarm, galvanic isolator, and GFCI) that might provide
> most of the benefits of an isolation transformer, but be in a more modest
> price range. Or maybe just installing a simple device or two and being
> religious about checking the quality of shore power before plugging in
> would do most of the job.
REPLY
This topic is likely of interest to many people so I am posting to the list instead of a private reply.
While attending a seminar on stray current issues the two presenters were asked which was
better. A galvanic isolator or an isolation transformer.
Each said their favourite was better and they disagreed with each other.
Which only shows that even the experts do not agree and the issue is not clear cut.
It is true that a cheap transformer will produce hum. It is equally true that a quality transformer
is very quiet.
As to the issue of magnetic fields affecting organic tissue. Yes I have heard much discussion
on this, especially since I worked in a power utility in very close proximity to giant megawatt
tansformers. Often we even had to work inside energized switch gear.
Naturally the union was concerned about the issue of worker safety. I won't try to belittle the
issue, but would like to point out that engineering solutions exist, that will shield or minimize
electromagnetic fields. In any case the whole house or a boat is surrounded by power wiring and
thus extensive magnetic fields. If this is a concern, the better solution is to have a minimalist
DC only boat which totally avoids the issue of alternating low frequency magnetic fields.
An isolation transformer totally breaks any conductive path between the vessel wiring and the
shore side power wiring.
A galvanic isolator only breaks the DC path in the green wire. By design it has to allow AC
current to flow and therefore is not a preventative for stray AC leakage current problems.
The construction of the galvanic isolator puts two diodes in series back to back with another
pair of diodes. What this means is that it will only stop galvanic current caused by dissimilar
metals. If you have a DC wire leaking 12V into the bilge water or in contact with a grounded
metal fitting; this voltage is large enough to pass through the galvanic isolator diodes and back
to shore.
The galvanic isolator is not connected to either the white or black AC wire so reverse polarity
requires a separate detector. The most common approach is to wire a neon bulb across the
neutral and green wire. This does nothing to prevent damage if the white wire is connected to
120V and the circuit is not protected with a fuse or breaker.
In the case of a metal hull, there is no question that an isolation transformer is required for
safety. A wooden hull also presents a risk factor since the wood is often saturated with water
below the water line and extensive leakage current paths are possible.
For a fibreglass hull with a properly designed and installed electrical system, the galvanic
isolator and reverse polarity indicator offers a cost effective means of limiting damage and injury.
Neither transformer or galvanic isolator is a cure-all for leaky wiring, deteriorated insulation and
faulty equipment.
Bilge pumps and the associated float switches are the most likely culprit when it comes to stray
current from DC circuits. Next comes electrical components mounted directly to moisture
laden wood.
Home appliances where the green wire and white wire are connected internally is the worst
offenders for causing problem and stray current in AC circuits. Next in order of incidence
comes deteriorated insulation in non marine rated cords, cables and appliances.
I have also seen lamp cord used to wire both AC and CD circuits. Such wire degrades over time
and develop minute cracks which lead to stray current.
This is why a coast Guard inspected vessel must have the wiring meggered at specified
intervals. If the megger test shows too low a value, the wiring has to be replaced.
On a newbuild, both DC and AC wiring has to be meggered in order for the vessel to be certified.
regards
Arild
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