chuck wrote:



So are there any benefits to not having a capacitor? Maybe.
There is no such thing as a galvanic AC current, I guess,
but there can be an AC electrolytic current. Research
suggests these currents may be even more damaging than DC
currents. So from a corrosion perspective, it would be good
to block them from getting into the boat's green wire. An
isolator without a capacitor would at least block the
lower-voltage AC, but would allow the higher-voltage AC to
pass (once the 1.5 volt threshold was exceeded).

So where is the benefit to adding the capacitor? None that I
can see.

The only ways to fully provide for onboard safety and also
eliminate galvanic and electrolytic currents from traveling
through the green wire are to use an isolation transformer,
or don't bring shore power aboard.

Usually, a simple galvanic isolator is sufficient.

Regards,

Chuck

The AC currents are currents are all ready in your green wire from leaky
equipment in your boat. These currents are making a complete circuit to
where the grounding wire is connected to neutral on shore. The question
is whether or not this happens through the grounding wire via the
capacitor in the isolator or through a metal fixture on your boat,
through the water, to ground, grounding rod, and then to neautral.
The down side is if you have a capacitor and your neigbor doesn't than
her AC currents might use your underwater parts and capacitor in your
isolator as the shortest path to the neutral/ground connection point on
shore. This is why ALL isolators should have a capacitor and I think
ABYC may require one. If they don't they should. Or you could just spend
the money and valuable space for an isolation transformer and not worry
about it.

Hope my point is better made than in the previous posting.

Eric

Hello Eric,

Thanks for clarifying.

I believe you are correct. You are talking about
high-resistance onboard leakages that generate currents too
small to be detected by the GFI circuit or the breakers. The
isolator diodes would probably not conduct under those
circumstances and a capacitor would help.

UL requires the GFI to trip at a 5 ma unbalance, so 24,000
ohms of leakage would trip it. Actually, the isolator diodes
would probably pass 5 ma in that circuit without a
capacitor. The capacitor would be necessary when the leakage
resistance was in the megohms and the currents in the
microamps.

Would rather not have that stuff flowing through my ground
connections through the water to adjacent boats, even at
those low current levels. This underscores the importance of
making sure you don't have dangerous leakages onboard in the
first place. Easy enough to check, but how many regularly
test their GFIs?

We sure agree on the isolation transformer, too.

Thanks again, Eric.

Chuck

On 2005-02-25 12:44:20 +1100, chuck said:

Hello Eric,

Thanks for clarifying.

I believe you are correct. You are talking about high-resistance
onboard leakages that generate currents too small to be detected by the
GFI circuit or the breakers. The isolator diodes would probably not
conduct under those circumstances and a capacitor would help.

UL requires the GFI to trip at a 5 ma unbalance, so 24,000 ohms of
leakage would trip it. Actually, the isolator diodes would probably
pass 5 ma in that circuit without a capacitor. The capacitor would be
necessary when the leakage resistance was in the megohms and the
currents in the microamps.

Would rather not have that stuff flowing through my ground connections
through the water to adjacent boats, even at those low current levels.
This underscores the importance of making sure you don't have dangerous
leakages onboard in the first place. Easy enough to check, but how many
regularly test their GFIs?

We sure agree on the isolation transformer, too.

Thanks again, Eric.

Chuck

Chuck,

I am puzzled by your UL 5 ma rating.

Here in Australia we have basically two ratings. The first is for most
domestic installations an is 30 ma. The second is for hazerdous
locations including hospitals where the rating is 10 ma. There are
other higher rating RCD (Residual Current Devices) which are used in
industrial contexts (eg 60 ma) but the majority of installations are
either 30 ma. or 10 ma.

BTW I am an EE as well as a licensed Electrical Contractor.

--
Regards,
John Proctor VK3JP, VKV6789
S/V Chagall

Hello John,

Thanks for the info. My first reaction is to suggest that we
use 5 ma because we Yanks are a less hardy lot and need that
extra protection.

But if you look at the link:

http://hypertextbook.com/facts/2000/JackHsu.shtml
Electric Current Needed to Kill a Human

you'll see that 60 - 70 ma. may be a lethal dose.

My guess is that the 5 ma was chosen to be below the general
human threshold of sensation. But I have no idea why
Australia has chosen so high a threshold.

And did I read somewhere that in Europe the residential
distribution lines are nowhere grounded? A two-wire system,
I seem to recall. You don't have that kind of system in
Australia, do you John?

Regards,

Chuck

John Proctor wrote:
On 2005-02-25 12:44:20 +1100, chuck said:

Hello Eric,

Thanks for clarifying.

I believe you are correct. You are talking about high-resistance
onboard leakages that generate currents too small to be detected by
the GFI circuit or the breakers. The isolator diodes would probably
not conduct under those circumstances and a capacitor would help.

UL requires the GFI to trip at a 5 ma unbalance, so 24,000 ohms of
leakage would trip it. Actually, the isolator diodes would probably
pass 5 ma in that circuit without a capacitor. The capacitor would be
necessary when the leakage resistance was in the megohms and the
currents in the microamps.

Would rather not have that stuff flowing through my ground connections
through the water to adjacent boats, even at those low current levels.
This underscores the importance of making sure you don't have
dangerous leakages onboard in the first place. Easy enough to check,
but how many regularly test their GFIs?

We sure agree on the isolation transformer, too.

Thanks again, Eric.

Chuck


Chuck,

I am puzzled by your UL 5 ma rating.

Here in Australia we have basically two ratings. The first is for most
domestic installations an is 30 ma. The second is for hazerdous
locations including hospitals where the rating is 10 ma. There are other
higher rating RCD (Residual Current Devices) which are used in
industrial contexts (eg 60 ma) but the majority of installations are
either 30 ma. or 10 ma.

BTW I am an EE as well as a licensed Electrical Contractor.