Quote:
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Originally Posted by MikeDubreuil
Can anyone provide anything in the manual that explicitly mandates the use of copper wiring? If this is the rule it should be made clear in a team update.
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Mike,
In answer is no but perhaps there should be.
Here I go again…
Disclaimer and preemptive apology: I know you to be a very competent engineer and you are already aware of many of the points I’m about to make. I apologize as the tone of this (rather lengthy) dissertation will appear condescending but I offer it for those teams which have a limited engineering mentor base…
This response is part engineering, part legacy and part common sense..
Legacy:
Originally, we were only allowed to use the wire provided in the KOP. Then, FIRST allowed us to purchase additional wire which had to exactly match the KOP wire.
In 2003, FIRST supplied us with wire and modified the rule to "The wire supplied in the Kit is to be used to conduct electricity. The chassis of the robot is not be used to conduct electricity. You may use additional wire as long as it meets the gauge and insulation color requirements." All wire in the kit was copper...
There was a conscious effort at FIRST to reduce the size of the rule book. We, mentors and engineers, asked for this…
Today, FIRST no longer has hookup wire in the KOP and specifies only minimum AWG.
Engineering:
From the mid 1960’s though the early 1970’s, the NEC allowed aluminum wire to be used in residential housing. It seemed like a good idea at the time. Copper prices were becoming astronomical (or so it seemed back then).
A lot of houses burnt down…
1. Aluminum wire has a higher coefficient of thermal expansion than copper. As a result, the lug connections will loosen and fail easier than with copper.
2. Aluminum has a higher resistivity than copper. For a given insulation type, you need a larger conductor to service a load safely.
3. Aluminum is more flammable than copper.
4. Aluminum wire is subject to micro fretting and has an increased risk of arcing.
Back to burning houses… The U.S. Consumer Product Safety Commission estimates that homes with aluminum wiring (pre-1972) are 55% more likely to reach “fire hazard conditions”.
After 1972, aluminum alloy wires were introduced as well as special sealing compounds for aluminum. This mitigated but did not eliminate the expansion and fretting problems. Also, the NEC restricted the use of aluminum wire to “single purpose circuits” such as central air conditioners, electric ranges and main electrical services.
Lastly, manufacturers of electrical connections designed wire clamps to “spring” and compensate for the aluminum expansion. However, these aluminum rated connections are usually confined to the aforementioned “single purpose applications”.
As such, most general purpose clamp and crimp connectors are rated for use with copper wire only. This includes, to the best of my knowledge, all of the connections and connectors we are given in the KOP.
In order to safely use aluminum wiring on a robot, a team would need to reengineer the entire connection system. This task is outside of the abilities of most teams…
Common Sense:
FIRST has based its wire sizes (and much of its power distribution design) on automotive industry standards.
Companies like Ford, GM and Toyota have the ability to assign hundreds (thousands if you count Tier I suppliers) of engineers to design their power distribution systems.
If they could save a few dollars and a few pounds from each of millions of automobiles by switching to aluminum wiring harnesses, wouldn’t we be driving in them?
Bottom line:
Perhaps FIRST should specify “copper wire only” and perhaps they should add some words to the effect of “use proper engineering judgment”…
At any rate, I would recommend against attempting a switch to aluminum wire on a FIRST robot.
JMHO,
Mike