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From: Karl Denninger (no email)
Date: Sat Sep 29 2001 - 21:39:10 EDT
Shore power with an isolation transformer SHOULD NOT carry ground from shore
to the boat's ground.
Now I know some people will tell you to, but they're WRONG, and its
precisely for this reason that they're wrong! If you have ground brought
through you do NOT have an isolation transformer - you have a POLARIZATION
transformer, which has the SOLE job of preventing a reversed hot and neutral
on the dock.
In the marine environment, if you want my opinion, such an installation is
WORTHLESS.
Now ground MUST be carried through WITHOUT an isolation transformer.
WITH one you actually INCREASE risk by doing that; there is no way for
a current source ON the boat to get OFF, nor for one OFF the boat to get
ON with an isolation transformer.
Look at Nigel Calder's book, second edition, page 103. You will find the
wiring for both a 120VAC/30A and 240VAC/50A isolation transformer. The
latter will bring you 120 and 240VAC, split-phase (just like 2 120VAC 50A
cords will), and DOES NOT bring EITHER shore neutral OR ground past the
transformer.
This COMPLETELY removes the potential problem of a "loose neutral" (which
can blow up electronic devices) as well as the problem this guy is having.
A short from either hot to the water in such a boat will NOT produce a field
in the water (or on the dock ground), because the path back to the current
source is *ON BOARD*.
Further, there can NEVER be a current in EITHER the neutral OR ground on the
dock cable (excepting a transformer failure), because neither goes anywhere
beyond the transformer and the neutral is actually not connected to anything!
A variation of this uses the primary center tap on the isolation
transformer; this permits the use of a 120VAC/30A cord to provide BOTH
phases (but only at 15A on each phase) from a single 120VAC/30A feed.
Boats wired in this way can run every appliance (although not all at once!)
even if all you can get is a 120V/30A feed at a transient dock.
My boat is wired exactly like this.
To repeat - the shore ground SHOULD NOT have continuity to the boat's
ground, either AC or DC, in a properly set up isolation transformer
installation.
If it does its wired WRONG.
-- -- Karl Denninger () Internet Consultant & Kids Rights Activist http://www.denninger.net Cost-effective Consulting http://childrens-justice.org SIGN THE UPREPA PETITION TODAY On Sat, Sep 29, 2001 at 02:38:17PM -0400, Richard Goodwin wrote: > > The current was measured by unplugging the shore power cord > > and putting an ammeter between the ground pin (green wire) > > and the ground pin on the boat's receptacle. > > Then an isolation transformer won't fix that unless it also isolates the > shore ground from the boat ground. With almost half an amp flowing > through the ground circuits of your boat into the water, you have a very > bad situation. It can eat your boat, a diver in the water around your > boat, and maybe people in the boat too, depending on what they touch. > > One thing to check first: Measure the current and voltage from boat > ground to each of the hot/neutral/ground leads of the shore power cord > when it is unplugged. It could simply be that something is wired wrong, > in your cord, in the outlet on the dock, or further back. If that isn't > the problem, then an isolation xformer is one solution, but... > > An isolation xformer would help only if it also isolated the ground > connection between shore and boat. Otherwise you would have isolated > 120v hot and return, but the ground would still be pumping 450 ma > through your boat into the water. > > You might just as well cut the ground connection from the shore power > cord where it connects to your boat. Leave your boat ground system > connected to the outlets, so you do have a ground connection throughout > the boat, grounded to the engine and the sea. That way if you touch the > frame or safety ground of an appliance that is plugged in, and if you > touch the water or boat gound at the same time, you should be safe. > > You could also leave the shore power ground connected, but disconnect > the 120v system ground from your boat ground. That would put your 120v > safety ground at the white wire neutral potential, which is good, but it > would put it above the ground potential of the res of your boat, which > is a potential risk to anyone who touches a metal safety-grounded > appliance and any part of your boat's ground at the same time. > > This stuff is complicated when the power suppliers don't follow the > rules... > > Dick > ___________________________________________________________________________ > || The Live-Aboard List : send a "subscribe" or "unsubscribe" request || > || in body of message to: || ___________________________________________________________________________ || The Live-Aboard List : send a "subscribe" or "unsubscribe" request || || in body of message to: ||
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