I work in the wire and cable design and manufacturing business, and the industry continues to produce interesting analytical challenges. A few years ago, I made a routine request of a junior technician to collect a random sample of a new cable design from the manufacturing floor. I wanted to confirm that the as-made product was meeting the designed/expected capacitance requirements.
A short time later, he called me out to the lab, sounding quite concerned. I reviewed his findings, and I too was concerned to see a capacitance value much higher than expected. It was way above any acceptable manufacturing or design tolerance.
I first asked the tech to confirm what the dimensions were of the sample, just in case he had been given a piece of the wrong construction (wrong wire and/or insulation thickness). No, he assured me, the conductor was correct. He also noted that the insulation diameter was compliant with designed/specified values, and that the insulation was of the correct material. This ruled out the possibility of an incorrect sample.
Well, when all else fails, check the obvious. Glancing at the coiled sample on his bench, I asked, "How long a sample did you test?" He responded that the sample was 10 feet long. This length is frequently used for electrical/electronic testing in order to produce higher-quality data. It's more accurate than evaluating a single foot, yet with 10 feet you're not wasting too much material in the testing process.
"OK then," I asked, "How do you know the sample is 10 feet?"
His answer? "I asked the operator to give me a 10-foot piece."
And that's the piece he had tested, without actually checking the length. As you've already guessed, it turned out that the sample was a very generous 10-foot length. The faulty length completely explained the anomalous test data.
This entry was submitted by Peter M. Blackford and edited by Rob Spiegel.
Peter Blackford graduated from Worcester Polytechnic Institute in 1970 and has been working in the electrical/electronic wire and cable industry since 1978.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
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