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Engineers: Have You Ever Burned Off Your Fingertip?

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Old Man
User Rank
Iron
High Voltage
Old Man   7/19/2012 9:55:10 AM
One of my first jobs I was tasked to build and debug a complex high voltage TWT switching power supply.  The voltages I was working with were +20kV to -20kV and some supplies were as much as 2kW, more than enough to kill you.  I asked my boss what safety proceedures were in place and he told me, "Don't die." (this was a very small company)  As cautious as I was I would recieve mild to moderate shocks most days.  To help prevent the moderate shocks, a technician and I developed the 'wand of power', basically a long plastic stick with a ground wire attached to the end, and used it to pre drain the capacitors before working on a board.  This job was hair raising, electrifying and definately memorable!

Larry M
User Rank
Platinum
Spark coil sparks indeed
Larry M   7/19/2012 10:14:26 AM
In my early teens someone gave me a spark coil from a Model-T Ford. This device contains a transformer and mechanical vibrator in an elegant wooden box with dovetailed joints. The vibrator chops the incoming 6 volts DC. The resultant square wave feeds a step-up transformer to produce about 10KV.

I decided to make a Jacob's Ladder by attaching two bare wires configured in a V-shape to the high-voltage terminals. The trick to this is adjusting the spacing so that the arc will jump across the V-gap at the closest point. Localized heating of the air causes the arc to be blown upward across the widening gap.

After three or four power off-on tries at getting the spacing right and having it too far (no arc) or too close (insufficient heating). I decided to try moving the wires while it was on.  Had I picked up a plastic-handled screwdriver from the bench, all would have been fine, but none was at hand. Instead I picked up a plastic-handled hunting knife which was on thw workbench, without noticing the rivets which passed through the plastic handles and blade.

When I touched the hot electrode and received the shock, my arm jerked upward over my head and I released the knife. It flew the 20' length of the workshop and stuck (Jim Bowie style) in a wooden bookcase at the end of the room. I couldn't have thrown it better had I tried.

It was a good thing I was working alone at the time.

OLD_CURMUDGEON
User Rank
Platinum
Burnt fingertips
OLD_CURMUDGEON   7/19/2012 10:45:33 AM
In the early years of my engineering career, I was charged w/ the design of several high power rf (tube) amplifiers.  More than once did I smell the unique odor of "rf-cured bacon" extending from my hands.  After a while, I played it more safely, using an NE-51H neon bulb securely taped to the end of a plastic wand about 3 feet long.  Much safer tool to investigate "hot spots" on the chassis. 

Specific incidents?  Too numerous to mention & too long ago to account accurately.

sensor pro
User Rank
Gold
Re: Burned fingers? Sure!
sensor pro   7/19/2012 11:26:32 AM
while working on Argon Opthalmic laser tubes I also had some problems with my wedding ring. Numerous shorts and also beal diflections. All this until my lead engineer recommended one of two solutions: remove the ring or get divorced.

Well, I'm still married.

warren@fourward.com
User Rank
Platinum
Re: Spark coil sparks indeed
warren@fourward.com   7/19/2012 1:07:16 PM
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That reminds me of my high school physics teacher.  He brought in a Jacobs Ladder and had it working on the table while he talked.  While explaining things he took a metal ballpoint pen and suavely touched the spark in the middle.  By my calculations, if the transformer driving it was putting out 20kV the middle of the spark was 10kV.  He got sick leave for three days after it knocked him very painfully backwards into the chalk board.

warren@fourward.com
User Rank
Platinum
This is fun!
warren@fourward.com   7/19/2012 1:18:43 PM
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I use copper beryllium flat spring material as a spring set to move lenses.  I have to cut it to length then drill holes in the corners in a fixture I made.  While holding the fixture and drilling with a high speed Dremel tool the drill bit went through multiple layers of copper beryllium and through my left index finger.  Then the drill bit came out of the Dremel.  I stood there with a drill bit through my finger and my finger nail with equal amounts of drill bit on either side.

I drove myself to the hospital emergency room and held up my finger and told them, "I think I have a problem."  They took me into a room and the Dr. just unscrewed the drill.  I could have done that!

But for the next week I was pulling out little curlicues of spring material out of my finger.  My finger should have been X-rayed, and I should have kept my finger away from the business end of a drill!

Larry M
User Rank
Platinum
Re: Spark coil sparks indeed
Larry M   7/19/2012 1:29:30 PM
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Warren wrote:  While explaining things he took a metal ballpoint pen and suavely touched the spark in the middle.

Interesting that he had the electrodes exposed. I've built several Jacob's Ladders over the years, usually from discarded 10KV neon sign transformers. They work best if they are in an enclosure vented at the bottom, since they rely on convection airflow to move the arc up from the bottom of the V to where there's really an impressive width.

I've demonstrated this principle by standing above the enclosure and blowing down to hold the arc at an arbitrary spot.

A Jacob's Ladder without an enclosure would only work well if the air was very still.

warren@fourward.com
User Rank
Platinum
Re: Spark coil sparks indeed
warren@fourward.com   7/19/2012 2:04:39 PM
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I failed to mention that this was in 1966 when the air was still and there were jobs to be had.


Bob Salter
User Rank
Iron
Shock & Burn
Bob Salter   7/19/2012 2:36:43 PM
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I watched a young man, a new hire, operate a drill press for his first job at my plant.  He claimed to be experienced, and after turning on the drill and pulling on the feed lever, nothing was happening. After about half a minute, he let the drill bit retract and was somewhat a little confused and amazed that the end of the drill was a bright red color. Before I could say anything, he quickly touched the end of the bit with his thumb and index finger. The smoke that came off was definately not cutting oil evaporating. I explained that in any metal removing operation, if the tool is not advancing and no chips are being produced, you are definately not cutting. When he asked why this drill did not cut, I asked him if he turned the switch to FWD or REV.

Tim
User Rank
Platinum
Re: Lessen in heat transfer
Tim   7/19/2012 10:31:32 PM
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Missed that one.  It should be "lesson"

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