EXP Bus - Nuckolls review of the design and advertisment
Some months ago, a reader sent me a copy of the advertisement
for the EXP-Bus. I read the article and added my own critical
design review. See a thread elsewere on this site that sumarizes
some discussions on the topic including a response from the
gentleman who designed and sells EXP-Bus . . . .
>ARE YOU WIRING UP A HOMEBUILT AIRCRAFT?
>if so, then you should investigate our new product, the EXP-BUS.
>Thisproduct can save a lot of time and money wiring up a project. . . .
Compared to what? You still have to cut holes for a PREDETERMINED
number of switches arranged in PREDERTERMINED order. Little chance
for customization. The assembly suggested DOES eliminate the need
for fabricating a breaker panel. So that time and space is saved.
However, fuseblocks and toggle/rocker switches can be used to
fabricate an easily customized power distribution and control
system for about $10 per switched circuit (landing lights,
nav lights, etc) and $0.50 per unswitched circuit (turn coordinator,
>HOW DO "SOLID STATE FUSES" WORK?
>The purpose of the limiter is identical to that of the breaker, to shut
>off whenever too much current is being drawn from a particular circuit.
>Technically, the devices used are very non-linear thermistors with a
>positive temperature coefficient (PTC). When excess current is drawn, the
>PTC device heats up and becomes a poor conductor of electricity. This
>shuts down the offending circuit. These components are UL recognized, and
>manufactured by a major US component manufacturer.
Generally true . . .
>Suppose that a wire leading to a nav light is chafing against a grounded
>metal part in a wing, such as a rib. If the light is on, and the
>insulation abrades away, the wire will intermittently or permanently short
>to ground. With a fuse, the overcurrent will blow the element in the
>fuse, before the wire can get hot and start a fire (although the spark
>could ignite fuel vapors before the fuse blows).
True of any form of circuit protection whether fuse, breaker OR
>With a breaker, the
>bimetallic element in the breaker heats up (more slowly than the fuse) and
>trips the breaker. With the solid state device, the PTC device gets hot,
>increasing it's resistance, and shutting down current flow to the nav
>lights. Actually, about 1/30th of an amp continues to flow, which is not
>enough current to heat up any wiring. The voltage drop across the PTC
>device keeps the device hot (about 100 degrees C), and the device stays
>"tripped". The load presented to the circuit by the nav lights will keep
>the device tripped even if the short is intermittant.
There are some operational considerations with self-reseting devices.
An intermittant fault gets you a popped breaker or fuse; an
immediate indication of system difficulties.
>The devices used are made to perform this function, and can do this over
>and over, thousands of times without damage. To reset the device, power
>is removed from the circuit for about 10 seconds (by switching off the nav
>light) , the device cools and switches back on, and the circuit is
>restored. If the short still exists, the device will immediatly trip
An important issue here is, "how often does a breaker or fuse
get cycled in an airplane?" The answer is, "almost never." This
being the case, automatic resetability is a non-convenience.
>We demonstrated this scenario literally hundreds of times at Oshkosh this
>year. These devices are used in automibiles and also in military
>electronics manufactured by Control Vision. No smoke and no mirrors used
>here. Using these devices, we have produced a PC board with switches
>mounted on it that replaces up to 16 fuses and circuit breakers in a small
>aircraft. Because these devices are quite inexpensive, we are able to
>offer the entire assembly for $249,. slightly less than the cost of the
>individual circuit breakers it replaces.
There are breakers and then there are breakers. You can buy
thermal breakers for $3 to $30 apiece depending on size
and relative "quality." $3 breakers will protect 16 circuit
for about $50. Of the 16 circuits, perhaps 5 will have switches
for another $35 bringing the total hardware costs to $85.
Switch panels have to be fabricated no matter what. Breaker
panels can be replaced with fuseblocks which pushes the labor
close to zero. If one assumes that $20 Klixon breakers are
the standard of comparison, then yes $249 IS less than $320.
>This is not a scam, we are offering this product with a 90 day money back
>guarantee, and a 1 year warranty. A builder can save time,
I cannot see how . . .
Depends . . . if the customer intends to use EXP-Bus to replace
traditional breaker panel, I agree. However compared to using
fuseblocks, the EXP-Bus is more expensive . . .
Again, compared with clasic breaker panel/switch panel
installations, yes. . . . but not compared with fuse-blocks
and contemporary switch installations.
Vague . . . what troubles? Popped breakers and fuses on a
finished airplane are extremely rare and to my way of thinking,
the positive indication afforded by fuses and breakers is preferable
to the masking and intermittant condition with self re-setting
current limiters. . . .
Again, compared to what? Fuse blocks and switches are the lightest,
lowest cost, and fastest installation you're going to find. Further,
it's easiest to maintain in terms of individual switch replacment
and if necessary one can replace the entire bus bar and protection
system in 10 minutes with a screwdriver and needle nose pliers for
a cost of about $32. Try that on your panel mounted breaker patch!
There are several products of this genre on the market. They
all feature higher cost, longer installation times, greater
weight and higher parts counts than the fuseblock and switch
configuration of power distribution and control. Both types I
saw at OSH were not well thought out with respect to vibration
resistance. Both dropped wire segments directly into soldered
pads of etched circuit boards with no mechanical support. Parts
counts on both products was very high. One version soldered
one edge of an etched circuit board to a row of toggle switch
connections . . . a real nightmare for replacement of a single
switch . . . With all due respect to these folk and their
entrepreneurship, I cannot recommend these products as cost
or performance effective in amateur-built airplanes.
Bob . . .
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Comments and alternative views welcome!