In early 1980s, I was a member of a team working on a nuclear magnetic resonance system, which required a very stable 100V 10A power supply for the magnetic field.
The stability requirement was a tough ~1ppm of noise and hum, and my colleague who was designing the power supply built a coarse 3-phase thyristor regulation, followed by a smoothing linear regulator.
To his big disappointment, those thyristors, even when abundantly over-dimensioned, kept misfiring, and the high inrush current into a large capacitor bank kept melting the semiconductors down, and causing in turn a short on the 3-phase mains, triggering the automatic circuit breakers, both on the PS rack, and the main ones, shutting the whole department off. It was happening several times a week, so it soon became rather annoying.
Finally to everyone's relief, my colleague has managed to reduce that inrush current, and also obtained some 70A rated thyristors, which cured the problem.
As the delivery day of the NMR system approached, we invited a professional photographer to take pictures for the documentation, and that's where I got my bad idea...
Demo day arrives
On the day before the demonstration for our customer, we were all busy with the last checks and preparations, and the PS designer was naturally rather nervous himself, he was sitting by the PS and an oscilloscope with the probe attached to the output voltage, the input AC coupled and the sensitivity maximized to show the line ripple, fine- tuning some trimpots, and watching carefully the changes on the output waveform.
I approached quietly behind him, with a photographic flash light fully charged, and as he reached once again for one of the trimpots I pressed the flash button. The flash light fired with a silent "PUFF".
For a few microseconds the time stood still. Then as in a slow-motion my colleague jumped up, the screwdriver which he was using to adjust the trimpots went left, the chair flew backwards, his glasses to the right and as he was falling on the floor he reached for the power switch and pulled it down.
The room burst into laughter.
After we all calmed down again my boss approached me and told me that my little joke could have had some very serious consequences if, by chance, my colleague dropped the screwdriver into the PS.
"I know, boss," I said, "so I gave him my plastic screwdriver last week!"
David, in my opinion engineering is by an large very humorless profession and i attribute it to the dangerous nature of the practical joke. Working with high current, voltages, heavy equipment etc need very careful handling otherwise disasters are just round the corners.
Sounds like you've been working with the wrong guys Himanshu!!
Yep, we sometimes work with dangerous stuff but how often do you hear of a practical joke going really wrong? I think most engineers know how far to go without putting someone in danger.
Hence the last line of the post:
"I know, boss," I said, "so I gave him my plastic screwdriver last week!"
Erik, was that the truth or was that just quick thinking on your part?
I agree with you David, as long as there is a measure of safety first displayed by the prankster, all is fun and is fair game, as far as I am concerned.
We have placed electrolytic caps in reverse (in purpose) for a technician to debug, and the moment of power-up is priceless for those in the know.
Once a colleague was repairing a measurement unit a customer nearly ruined. It took all morning and he was cursing a lot. He was really annoying.
When he was sure the repair was finished, he left for lunch.
In the mean time we placed a 0207 resistor underneath the unit, which was housed in a 19" 21 TE cabinet, wired it to a power supply well hidden on the next desk and adjusted the voltage. When our colleague returned, he continued - trying to bring the unit into service again.
You might imagine his reaction when he switched the unit on and - as the "auxiliary power supply" was activated at the same moment - a thin trail of smoke escaped from the overhauled unit. That was the time we defined new classes of parts: LET = Light Emitting Transistor, SER = Smoke Emitting Resistor, LER = LE Resistor (use a bit more power and you can remove the coating and watch the metal film glow).
Years ago I was working on the B1 bomber gyro platform and I was using a number of fans to cool the electronics box. One night another engineer brought a big blower to the empty bench next to mine and plugged it into the same outlet that ran the box. He had filled it with chad, little punches of paper from card stock used to program an old HP computer. The next morning, I turned the box on and instantly my world was filled with blown chad. I immediately shut it down and everyone thought it was pretty funny. It took me a half day to clean it up.
Then, I noticed that only a small part of the huge pile of chad actually blew out. Plotting revenge, early on Thursday afternoon I returned the blower to him, placing it on the empty bench next to his. He made some comment about any left over chad but I smiled and mentioned that he had no worries as it was all over at my bench. That brought about another round of good laughter.
I came in on Friday morning around 5 am. I plugged it into his bench and covered the power cord with some of the piles of papers on his desk. I also placed two "decoy" power cords next to the blower, thinking he might unplug them first.
True to form, at 8 am he turned on his bench and his world was filled with chad. What was even funnier, is that he didn't figure it out at first. Then when he did, he kept unplugging the wrong cords to shut down the blower. By the time he got the blower off, all of the chad was all over his desk. We both were called into the engineering manager's office and got a lecture on that one.
This joke was quite a bit louder and much more exciting, but not very funny. I was working with an electrician and we were in a utilities closet, which was behind a janitors closet, all concrete walls and ceiling. What we did not know was that as a gag, I guess, somebody had dropped some pieces of #10 bare copper wire into the conduit that fed the 3-phase 480 volt power to the main breaker in the panel we were looking at. When the electrician pushed on one wire to get a better view of the terminals, the chunk of #10 wire fell down across all 3 phases, and instantly evaporated, with considerable flash and bang. The acoustics in the small volume made the sound even louder. We never did figure out just who or when the trap was set, which is very fortunate for whoever did it. It was not very funny.
Years later I worked with another electrician who thought that it was quite funny to tie all three phases of the feed to a large motor, downstream of the starter contactor, together with about #20 wire, and then have the engineer, (me), see if I could spot the trouble when they went to start the 75 HP motor. Those watching were dismayed when I was not amused.
As per previous comments, playing jokes like this with 3-phase mains power is neither funny nor clever. The flash from evaporating copper wire can cause serious injury. You can get just as good results without such dangerous practices.
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