When I was working for a large military contractor that manufactured high-powered radar systems, I was part of a design team that produced the frequency-generation hardware, which consisted of a 19-inch rack about eight inches high. There were many sub-modules within the rack, and most of the voltages were less that 30V DC. One exception was the use of 120V AC for some secondary circuits.
One day I received a call from the production technician. Apparently, the frequency generator was not working correctly. The plant was located on the property of a municipal airport. It was built during the World War II, when access to the airport was essential. When I arrived at the tech’s test station, he told me that the frequency generator was creating an oscillating burst at the rate of two cycles per second.
That was odd for a frequency generator that normally had internal frequencies in the range of 1,000MHz to 4,000 MHz. When I examined it, I could see that the generator was working normally except for the RF burst that occurred every two seconds. I thought I knew what the problem was, but to make sure, I needed to probe an internal circuit that was next to the 120V AC unit. I asked the tech if the 120V AC had an on/off switch that would allow us to shut it down during the test. He told me that it did. He turned the switch on the test panel and assured me that it was now in the "off" position.
I took a look, just to make sure. I could see that it was indeed "off." I proceeded to perform the necessary probing of the circuit. When my finger touched the terminal, which had the "off" 120V AC, I received a nasty shock. I asked the tech as to why the voltage was still "on." He replied, "Oh, the switch is broken. It doesn't work."
I then insisted I would do no further work on his problem until the switch was replaced. I told him that the cause of the two-second burst of oscillations was due to the airport’s radar sweeping across the test floor and upsetting the frequency generator, which had all of the shielding covers removed for the testing. Moral: never trust a switch for power removal. Test with a reliable tool to make sure the circuit is "dead," and never trust someone else to "assure" you that all is OK!
This entry was submitted by Lee R. Watkins and edited by Rob Spiegel.
Lee R. Watkins is a senior member of IEEE: P.E. He has a BSEE and has worked for Westinghouse, Honeywell, Martin Marietta, Motorola, and ON Semiconductor, and is currently a self-employed consultant. Lee has published several articles, as well as a book on analog filter theory and design.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
All you home improvement enthusiasts out there should take note. No matter how adept you are are understanding electrical currents, this is still always a danger with home repairs. My husband, who's not an engineer, but a pretty handy guy, has learned this lesson the hard way as I'm sure many of our readers have over the years. Broken switches and nasty shocks are definitely no fun.
Gosh, this reminds me of the Inspector Jacques Clouseau sketch from The Pink Panther Strikes Again:
Clouseau: Does your dog bite? Hotel Clerk: No. Clouseau: [bowing down to pet the dog] Nice doggie. [Dog barks and bites Clouseau in the hand] Clouseau: I thought you said your dog did not bite! Hotel Clerk: That is not my dog.
Makes me wonder how this tech got the job in the first place - he seems rather inept. At least the engineer who was shocked was not badly injured. Something like this could have really been ugly.
ANyone who is a tech should have learned this procedure by heart -
Turn it off, lock it out and then get your voltmeter and make sure it is really off.
Built, installed % service industrial equipment and that is the only way to avoid getting shocked. Generally the shocks don't hurt too bad, but the back of your head does tend to suffer from the reaction...
Seems you should have followed your own advice prior to learning the hard way! When it comes to anything above 48VAC, I check thrice!. Another lesson: I used to work for a hosiptial fixing these HUGE indistrial washers that used relay logic and stepper switches. No 5Volt logic here - all the ciruits were 120VAC and some were 208VAC. I had to check the panels hot in order to trace down the anomolly. One day my wedding ring got caught on a relay terminal just as it turned on. OUCH. From that day on, I always take off my wedding ring, watch band or anything metallic. Even with 5-Volt logic, you can draw some considerable current from these SMPS, easily frying traces and your finger. Word!
As a freshout designing high voltage circuits for military radar system, some of things I was taught was to work with one hand behind my back, no jewelry (especially if it dangled), don't reach over my circuits. There were many scary stories about carelessness that got people hurt or killed. Never got shocked never had a desire to either. I always remembered to put up my danger high voltage signs and didn't take anything for granted. While I don't design many circuit over 100K anymore I still carry and follow those rules with me today.
What's up with the guy knowing the switch was broken and not saying anything until you to got shocked. Some people just don't think. That's so not funny.
Oh yeah, by the way, that switch...... Did this tech go on to become an elected political official in charge of domestic financial policy? Oh yeah, by the way, the financial regulations that are supposed to prevent these economic meltdowns....
After taking a few 110 volt shocks, I invested in a circuit tester with lights and an audible alarm to check for a live circuit. It is worth its weight in gold.
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