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From: (no name) (no email)
Date: Sun Sep 30 2001 - 13:38:37 EDT
In a message dated 09/30/2001 4:06:58 AM Atlantic Daylight Time,
writes:
> Norm asked if the current was AC or DC.
> Norm,
> The current was DC. Today we measured from the shorepower
> ground pins to the bay and found that there was 0.48 Volts
> at my friends dockbox. The shorepower was negative with
> respect to the bay.
> Then we went to my dockbox, 5 slips away, and repeated the
> procedure. This time we measured 0.36 Volts. The shorepower
> was POSITIVE with respect to the bay.
> I'm sure the BAY AIN'T THE PROBLEM!
> The guy across the dock from me, 15 feet away, is losing a
> "dinnerplate" zinc every 60 days!
> GALVANIC ISOLATORS ALL AROUND!
>
If I recall correctly, you would have to use a piece of platinum in the sea
to measure the voltage from a point to the sea, otherwise you would be
setting up a battery with the pole in the water being one electrode so the
voltage would vary with the material in the water.
Impressed cathodic protection systems use a platinum electrode as a reference
to adjust to the proper voltage.
I am under the impression that a DC current is required to have eletrolytic
corrosion, as in electroplating, and a transformer would block DC. As long
as there were no defects in your boat, using a transformer and not having any
other connection (no grounding lead) to the dock would reduce the possibility
of DC current flow to/from the dock or other boats.
I got my 7.5 KVA transformer from an advertiser in Boats and Harbors years
ago for less than half the price for a similar transformer at a local
electrical supply house. It was advertised as a unit to provide 240 vac from
a 120 line but it had two windings on the 240 vac side and several taps on
the 120 vac side to compensate for voltage to high or low. I use it mainly to
provide a balanced load to my 120/240 genset by connecting the 240 volt side
to the genset and feeding my 120 volt loads from the 120 volt side.
When it comes to very small currents and voltages things get more hazy.
There are plenty of places in our electrical systems where a little
corrosion, even the normal resistances of good wiring can cause things to not
go exactly by the book.
I wish I had more experience with dockside problems to offer, but I just have
had not any problems to require me to investigate. I have never used a
grounding wire from the grounding strip in my breaker box to the dock. And,
yes, I have been in the water around my boat and felt no electicity.
Norm
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