Thanks, Mark:
I think I have a good theoretical understanding of the issues. I have
some practical experience with electric systems on boats connected to
various shore systems around the world. You obviously have far more
practical experience.
So, I ask, what would you do with Fintry's grounds? Nothing from the
shore comes past the isloation transformers. Clearly the green wire is
grounded to the hull, no question there. On the DC side do we float
or ground the negatives? On the AC side, do we ground the neutral
(the center of the 240, as in common US single phase practice)?
Of course, when I say "ground", I mean at a single point, in so far as
it is possible. This would include insulating steel appliances from
the steel deck and taking other precautions against ground loops.
Issues:
On the DC side, I think I have everything isolated. It may be that
the engine starting circuits will be grounded negative, but the 24VDC
house loads can all be isolated.
Radios are always a question. I think the antenna ground on the SSB
is isolated from the negative power supply, but I'm not sure. If not,
the 12VDC system negative may have to be grounded to the hull, because
we certainly want the antenna counterpoise grounded (or can it be
capacitively coupled?)
On the AC side, I look at big ship practice and think that it makes a
lot of sense. Then I read of the dangers of floating systems and I
come back to earth, so to speak....
What would you do?
Details:
79', 150 ton steel hull and deckhouse, aluminum wheelhouse.
Five diesel prime movers:
New Cat 3406 main
Perkins 6-354 for hydraulics, pumps, alternator (old genset engine)
Two new Northern Lights 9kw 120/240VAC 60Hz 1ph gensets
Hatz single cylinder, rope start, for fire and bilge pump.
Four DC systems:
Engine starting -- one 24DC bank, isolated from everything else -- all
four engine alternators charge it while running. (Lest anyone worry
about single point failures, the 6-354 auxiliary has a spring
starter).
24VDC house system -- one big battery bank, charged by the Cat and the
Perkins (2x125A 24VDC Electrodyne brushless alternators on each
engine), as well as 240VDC universal chargers (paired for redundancy).
This system is mostly for boat operation -- electronics, nav lights.
This also drives two big inverters to power 120/240VAC house loads
while underway.
24VDC emergency system -- battery bank on upper deck, for spare nav
lights, electronics, emergency lights in the event of a main system
failure (fire in the engine room, etc.) Charged by battery charger and
by a diode off house system.
12VDC for radios -- You can't buy a VHF or SSB (except commercial
units at 4x prices) that runs off 24VDC, so we have to do both. I
love 24VDC, but hate the fact that as a practical matter, it means you
have to have both 12 and 24. Batteries on the upper deck (they're also
needed in emergency), charged by 240VAC charger. I chose this way,
because a 30A 24VDC-12VDC converter and spare for the SSB would be
more money and the 12VDC battery capacity would have to be there for
emergency use anyway.
120/240 VAC 1phase for hotel loads, major auxiliaries (pumps, air con,
etc.).
Isolation transformers on shore power. Whether shore power is 120 or
240, we'll use the transformers to create the usual US system --
hot-neutral-hot, 120/240.
In 50hz places we'll use 50hz wherever we can (lighting and air con,
for example) and supply 60hz from a universal battery charger taking
in 230VAC 50hz going to 24VDC back to 120/240 60hz. Except for bussing
both 50 and 60hz, the AC side is pretty conventional.
Jim Woodward
www.mvfintry.com
"Mark Browne" wrote in message . net...
Jim,
Thanks for an excellent and thoughtful post.
I frequently work with systems that have multiple nested grounding system.
We try to reconcile the competing interests of UL safety requirements, FM
intrinsically safety requirements, analog measuring circuits trying to
resolve 0.1 microvolt out of 10 millivolts riding on a 2.5 v differential
load cell output (multiple summed load cells in the system), and the onboard
communication radio to carry the measurements to a central processing
system. Sometimes these systems are installed on electrically powered
material moving equipments so we have to contend with high drive currents
coming off the same battery where we get our power. These drive systems
typically have their own stringent grounding rules to keep the motor
controller alive. To top it all off, in high humidity environments, low
level contaminants add additional and unintended current paths. We have
developed a set of rules and methodology that works, but it is hard to
explain why we do some of the things we do without getting into a lot of
seemingly useless minutia.
I did a miserable job of relating a sub-set of the rules we have to follow
into the boating environment; you provided a much more useful description.
Mark Browne