Re: Exp Bus Follow Up
>>I asked Control Vision to answer some queries about
>>their Exp Bus DC Management Unit (see http://www.
>>controlvision.com). They came back with answers
>>which I am am happy with.
>>Question: Under full load the EXp Bus could potentially be
>>carrying 40 Amps. Some would say that a bunch of wires
>>and connectors is going to be better than a printed
>>circuit board under these circumstances. What have you
>>done to ensure that the Exp Bus can handle this sort of load?
> We have used massive traces with heavy copper plating, and our board can
> take well in excess of 60 amps of DC current (tested!)
This is generally not a problem . . . I've designed a number of
system that put 40+ amp continuous loads on PCB traces. Actually,
Heavy current is EASIER to handle in thin wide traces than with
a similar cross section of round wire . . . heat rejection area
and surface contact is higher.
>>Question: Because the PCB could have some connectors or track exposed
>>is there a possibility that a short may occur resulting in catastrophic
>>failure?
> Again, we thought of that. The PC board is conformal coated with a
> resin, covering all traces and pads and solder connections. The
> terminals ARE exposed, as are all terminals in every spam can aircraft I
> have ever had the misfortune to venture under the instrument panel of...
>
> Really, if this is a concern, simply form an enclosure around the pc
> board with a piece of vinyl, or aluminum. We considered an enclosure,
> but as all homebuilts are different, this was deemed to be impractical.
A common question I see concerning any bus bar installation, "What happens
if the bus bar gets shorted?" I answer with, "Take a tool box and walk
out to any airplane of your choice and show me how you're going to get
the breaker bus grounded using only materials already present and in
close enough proximity to be a potential fault generator. Fact is,
there are no materials that fill the bill . . . exposed bus structures
behind the panel are generally quite free of such hazards or easily made
to be so . . .
> If you get the product, and are concerned that it won't
> perform safely, we will gladly return your money [although shipping
> charges to NZ and back would be your gamble!!!) Always remember that in
> an aircraft, you must be totally sure of the technology you are using, we
> understand that this is rule number 1!!!
I'm not concerned with SAFETY here . . . the potential failures I see here
will result in passive shut down of the affected circuit.
TOTALLY SURE is the operative phrase here. This product is mere months
in the markeplace and quite low in flight hours of experience compared
to breakers and/or fuses. . . .
>>Personally, I am far happier with solid state fuse technology as implemented
>>on the Exp Bus than with circuit breakers or fuses. I think these guys have
>>developed a unit which will considerably simplify electrical wiring
>>homebuilders and is probably much more reliable than hand wired looms.
I'm curious as to the source of the warm-fuzzy-feelings; the product
is new, the parts count is high, using electrical contacts of switches
(designed to support wires) as mechanical supports for PCB's is not
recommended by any switch manufacturer.
How will one control stresses on these contacts during installation?
Variations
in hole location, twist on the mounting nuts, switch to switch variations,
assembly variations when soldering to the board, etc ?. . . These may add up
to reduced switch life. When a switch needs to be replaced, you have to
drop the whole assembly with switches intact and replace one switch (the
replacement has to be exactly like it's adjacent brothers) while fixturing
it with sufficient precision to prevent new stresses on the assembly
when it's reinstalled.
I've no doubt that purveyors of this product BELIEVE they've got a good thing
going here but I can assure you that had the product received the benefit
of critical design review and/or extensive field testing, it would be built
much differently. There's nothing sinister going on here, they simply don't
KNOW any better.
. . . and last, I would challenge the idea that this product "simplifies"
anything . . the hardest part of building a switch panel is getting all the
holes in a straight line and then putting on decent looking lables. The
product in question doesn't address that issue at all. Beyond putting all
the switches in straight and making them look good, the use of fuseblocks
makes the rest of the task trivial and it's a whole lot less expensive
(approx
$10 per switched branch and $1.75 per unswitched branch). A 30-fuse,
6-switch
electrical distribution system can be installed for about $120 tops!
Bob . . .
AeroElectric Connection
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(o o)
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