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
"Jim Woodward" wrote in message
om...
Mark:
I don't like to be nit picking about teminology, but a lot of
electrical confusion can be avoided if we're consistent.
You don't need any ground in an electrical circuit. A plastic
flashlight is a simple example of something that doesn't have a
ground. The doorbell in your house is ungrounded even if supplied by
a transformer, the transformer isolates the doorbell wiring from the
ground. And so forth.
A "ground", or in British usage, an "earth", is just that, literally a
connection to the earth, which is a pretty good conductor because
there's lots of it.
So, strictly speaking, there isn't a ground unless there's a
connection to terra firma or the sea.
However, in many pieces of equipment -- autos and radios come to mind
as good examples -- one side of the power supply is attached to the
chassis. In auto and radio practice the chassis is used as the return
for all the current on that side (usually negative now, but positive
ground used to be common). Strictly speaking this is a "chassis
ground", not a "ground", but nobody's that strict.
On boats, there is usually a single point connection from ground to
the negative side of the DC supply and -- only when the supply is on
board, NOT with shore power -- the neutral side of the AC supply
(white wire in US practice). All current is through two wires, never
through the hull, mast, or anything else.
In the case of boats, many large vessels have ungrounded ("floating"
is the term of art) electrical systems. The engine starters, gauge
sensors, and alternators all have two wires going to them, both of
which are insulated from their metal bodies. Everything else is also
insulated and there are alarms to indicate ground faults -- a fault
connection from one side of a power supply to the hull. This is true
of both their DC systems and their AC, house supply, systems.
We're so used to engine starters being grounded on one side that we
forget that almost all electric motors except starters are insulated
from their metal cases -- insulated motors are the rule and automotive
starters are very much the exception.
(Now a little technical stuff) The reason that large vessels having
floating electrical systems is that it is easy to detect ground faults
when the system is floating. Ground faults are bad because, aside
from obvious safety issues, current will be conducted through the hull
leading to possible electro-chemical erosion. With a grounded system,
ground faults on the hot side are easy to detect -- a circuit breaker
blows. Ground faults on the other, neutral, side are very subtle and
hard to detect. You can detect them by careful measurements, but in a
marine environment careful measurements are almost impossible because
there's always a little leakage caused by moist salt air. So they just
sit there, conducting current, and damging the hull.
Jim Woodward
www.mvfintry.com
"Mark Browne" wrote in message
. net...
"basskisser" wrote in message
m...
K Smith wrote in message
...
CCred68046 wrote:
I have seen this grounding questions with metal boats before and I
dont
understand what the issue is. If the boat has a motor I can
almost
guarentee
its grounded somewhere to the hull weather its inboard or
outboard.
Would
someone actually go through the trouble and expense to isolate the
electrical
system from the hull? That would take a lot to do!!
The debate is long & I'll stay out of it for now, however you are
mistaken Cred about "almost guaranteeing" engine electrics are
grounded,
indeed most proper marine engine electrics are not.
How can an electrical circuit be made if there isn't any ground?
It has one - It just does not use the engine or drive components to
provide
the current return path; a separate "ground" wire carries the juice
back.
Look up "ground loops" on google. This type of wiring allows effective
assaults on this nasty problem. If you are trying to eliminate radio
interference problems or reduce electrically induced corrosion problems,
this can make a lot of sense.
There have been a few times that I wished that automotive components
were
built this way.
Mark Browne