Fuses ratings

Bob Bruninga bruninga at usna.edu
Wed Jul 25 15:28:41 CDT 2012

Very good concept.  DC distribution bus with load control devices to spread
the power.  I like it....

But one thing that is driving this is the legacy of 12v DC devices
throughout these RV and boat systems, so they still have to back convert to
12v DC at each and every load.

A clean-slate approach would not do it that way.  They might do it at 120
VAC and still save even more in the copper because even the main bus can be
1/10th the size.  Then all of the load devices can be conventional 120 VAC
and can operate equally on shore power or battery power.  With inverters now
being 95% efficient, it makes no sense to distribute any power at low
voltage DC.  And with CFL lighting that is a big part of the load. Etc.    

Shucks, since 330 VDC even saves another factor of 10 in copper, that woiuld
be best except for the main reason of broken connections initiating plazma


-----Original Message-----
From: tacos-bounces+bruninga=nadn.navy.mil at amrad.org
[mailto:tacos-bounces+bruninga=nadn.navy.mil at amrad.org] On Behalf Of Mike
Sent: Wednesday, July 25, 2012 1:51 PM
To: Karl W4KRL
Cc: 'Tacos'
Subject: Re: Fuses ratings

fast and precise response is an advantage of the
"electronic fuses" now being used in "multiplexed
power systems".  the name is misleading because
it is the *control system* which is multiplexed.
the systems are based on the idea of a large DC
bus running the length of a boat or RV with taps
feeding electronic switching hubs. the drops for
switched loads come off those hubs instead of
doing the home-run back to a central distribution panel.
the system is controlled by a bus system - usually
some flavor of RS-485 - which has control panels
placed wherever they are wanted.

the power switching hubs can be programmed so that
each channel (varies from 2-16 in a hub) listens to
one or several switches (for N-way control), and
the overcurrent level and response curve for each
channel is likewise programmable up to some maximum
for the channel (usually enforced with a fuse, breaker,
or polyfuse).

some of the hubs have manual over-rides on circuits
so they can be switched even if there is a total
failure of the control system.

why add all this complexity just for switching
DC loads?  the first answer is "flexibility" to
move switches around and add additional control
stations very easily, but the *real* reason is
WEIGHT. a 60 foot bus of a 00 cable pair weights
appreciably less than all the home-run tails that
would be missing by just running to a nearby hub
on the centerline. This is a 12V or 24V system
and I-squared-R is *not* your friend and most of
those home-runs are (duplex or triplex) #12 or maybe #14
(and the wiring standards prohibit anything smaller than #16
for anything but very low current control signals).

in my boat, we had to freeze the design before
the buss-network switching systems were baked enough
to bet on, so we had to do the home-run thing mostly
(small sub-panel forward to take care of cabin hotel loads).
If we had it to do over and could use a buss-net system,
the electrician that did the wiring estimates that
we could save at least 3000 pounds of copper,
not to mention the *cost* of a ton-and-a-half of copper.

as you might guess, airliners were the first applications
of this technology and it has existed for some time in the
"why use lead when gold will do" world of commercial aviation.
recently with the explosive growth in power semiconductors
and the drop in cost of very cheap micro-controllers, it is
now easy to put a spring-head in every switch bank and
every power switching pod and take advantage of the savings.

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