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ekstra   ara
 Post subject: Larry's Arc Flash experience
PostPosted: Tue Apr 17, 2012 8:43 am 
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Joined: Tue Apr 17, 2012 8:19 am
Posts: 241
Location: Charlotte, NC
I have 2 stories:

When I was working in a plywood mill, another electrician was resetting a starter. Unbeknownst to him, the operator had jammed a stick into the start button because the motor kept stopping. When the electricina reset the overload (with the can open), the starter pulled in and exploded. Luckily he was wearing safety glasses. the flash just burned off his eyebrows and gave him 1st degree burns on his face.

I learned from this and a few years later I was restarting a 40Hp inverter that was just repaired. I had fixed the control board, and the OEM had replaced 2 transistors in the power bridge. He asked if it was right, and I said "You took it apart, weren't you paying attention?" He assured me it was right (first mistake).

I applied power and everything was fine. I turned up the speed pot on the front of the drive and the power section exploded, sending a ball of plasma straight out of the front of the inverter. Fortunately for me, it had been my practice since the exploding-starter incident to NEVER stand in front of anything when powering it up, especially the first time after a repair. He had put the flyback diodes in backwards.

Second story:
I was working on a motor tester for GE. They had complained that the voltage was erratic, so I was going to check on the variacs to see if the voltage was stable there. I called the manufacturer, since the tester was supplied with 6600 volts, and I wasn't going to intentionally put my fluke 77 anywhere near 6600 volts. He assured me that if you set the taps to 600, the voltage on the variacs would be 600 volts. The block diagram seemed to indicate otherwise, but he was the manufacturer, you would think HE would know.

He was wrong, the voltage WAS 6600 volts. As I was putting the probes across 2 variacs, I noticed a spark jump to one probe. Unfortunately, I didn't realize the significance, and the other probe was already too close anyway. The next thing I knew the power was out to the whole plant. The second probe only got to 1/4" away from the other variac, my meter display was black, there was a 1/4" divot in the variac, and it is a miracle that the insulation on the meter leads didnt fail. Sadly my meter was destroyed - but it was ONLY a meter and not me. I informed the tester manufacturer, and told them they owed me a new meter.


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