Early in my career, I had a job servicing office machines for a large national laboratory. These were largely mechanical calculators and adding machines. (Yes, it was before electronics took over those functions.)
One day, we got a call that an employee was experiencing mild shocks when he operated his adding machine. This wasn’t an uncommon complaint, since these machines had aggressive line filters with considerable capacitance between line and ground. A failure in the third wire ground (usually from a bad power cord) would allow sufficient capacitive leakage to produce minor shocks. Additionally, well-grounded equipment and a dry climate made for some severe static discharges in carpeted offices. So I grabbed a can of anti-static spray, a new power cord, my multimeter, and a line plug tester and headed out.
Imagine my surprise when I found a concrete floor, an outlet with a working ground, and a good power cord. Yet there was a definite tingle when I touched the adding machine. Connecting my meter between ground and the machine showed no voltage, but I could feel it. Finally, it clicked, and I measured between ground and the steel desk. The meter read around 50VDC, which just happened to be the on-hook voltage of the multiline phone that sat on the desk with the adding machine. Sure enough, a little investigation found the phone connected to a terminal block screwed to the side of the desk, and a stray wire from the phone cable was touching the desk. With a bit of tape and a call to the phone company, problem solved!
This entry was submitted by Michael Wolf and edited by Rob Spiegel.
Michael Wolf is a retired engineer who worked for Los Alamos National Laboratory, Amtech Corp., and Cell Robotics Inc. He holds 13 patents, and equipment in which he had a major design role has won numerous awards, including 4 R&D 100 awards, Photonics Circle of Excellence, and Medical Design Excellence.
Tell us your experience in solving a knotty engineering problem. Send to Rob Spiegel for Sherlock Ohms.
It's too bad expereince isn't understood and valued for what it is. Often younger engineers haven;'t learned all the hard lessons that a more experienced engineer has. And thus will go through the same mistakes that could have been avoided had a grey beard been present.
Reminds me of an old telephone troubleshooting story (probably apocryphal).
Rural farmer notices his dog barks BEFORE his phone rings. Consistently. Not thinking he has a clairvoyant dog, he ends up calling for phone repair. Technician confirms story with a test - rings up the farmhouse, dog barks, THEN after a few seconds the phone rings.
Circuit topology: Rural farm house on long run from C.O. One side of phone line connected to ground wire to ground rod inserted in sandy (drought that year) soil near the house. Dog tied up outside using metal chain clipped to collar and to ground rod.
Cause and effect: Ring current can't flow to ground because of dry soil. Ends up tingling the dog instead, who then pees on the ground. Freshly conductive ground rod then conducts enough current to ring the phone and quit annoying the dog.
Moral: any time you see a time-related delay in a problem or resolution, open your mind to physical effects that have a time constant in about that interval. Don't look for macro causes for problems that happen in a microsecond, and don't look for prop delay causes for problems that fix themselves in ten seconds.
Back in the 70's when I worked in what later became known as Biomedical Engineering ina Hosptal, we were very conscious of the need to make equipment used as safe as possible. One morning a doctor came to me saying his desk had just given him a painful shock. He explained that he only felt the shock when he touched both the water tap in the nearby sink and the desk at the same time.
My meter confirmed there was 110VAC between the metal desk and the water tap. I put on some gloves, moved the desk away from the wall and measured again. No volts now.
I found that there was an outlet on the all behind the desk. The Outlet had one half of what was left of the plug from a lamp stuck in it. The metal blade still inserted in the hot side of the outlet was protruding just enough to make contact with the heavy metal desk when it was pushed gainst the wall.
I extracted the metal blade with a pair of needle nose pliers [still wearing gloves.] and the desk was safe again. Thinking back, the desk could have electrocuted anyone who touched it.
Imagine the fun that would result when the 20Hz ringing voltage was sent down the line! Good thing the employee wasn't touching the adding machine when the phone rang! ♠
Maybe it's because it's the end of the day, but I love part 2 of your comment, Doug! Seriously, though. The example harkens back to a much simpler time. Today, calculators are so complex, you wouldn't even know where to start to fix something like this.
I love this story. It's really a throwback to another day and age.
The whole concept of accountants being shocked by their calculators is interesting. Maybe as our political leaders add up the costs of their proposals, an acual shock could be distributed, causing a second thought.
I agree. The experience level of engineers and designers is a very important and often underrated aspect of one's value to a company. Some jobs in technology fields don't put a lot of emphasis on hard won experience. That is simply a matter of economics. For example in Software, if a company is working on new technologies and designs, experience in that field is likely to be short to begin with. Only generalized experience is helpful perhaps in the senior positions if they exist at all.
Frequently a particular project might only need new young guys with the latest and greatest tech skills and long years of experience don't really matter or are not so economical to acquire. After all senior engineers and designers presumably cost more than the younger guys.
I lament the fact, and it's my firm belief, that engineers who've come of age in the past 20 years (i.e., they've grown up in a digital world) are completely flummoxed when it comes to diagnosing problems such as you describe here. If you've never had to find the source of, and stanch, 60-cycle hum in an analog recording set up, then you may have book learnin', but you're lacking some vital real-world skills. Or, I could just be a cranky old(er) guy...
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