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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Bob Leonard on July 14, 2015, 08:53:17 AM
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Enough said. Always use caution.
https://www.youtube.com/watch?v=yo1diE6FZv4 (https://www.youtube.com/watch?v=yo1diE6FZv4)
https://www.youtube.com/watch?v=ynnCYnGzGjo (https://www.youtube.com/watch?v=ynnCYnGzGjo)
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Enough said. Always use caution.
https://www.youtube.com/watch?v=yo1diE6FZv4 (https://www.youtube.com/watch?v=yo1diE6FZv4)
https://www.youtube.com/watch?v=ynnCYnGzGjo (https://www.youtube.com/watch?v=ynnCYnGzGjo)
Just a warning, these are pretty graphic video and there's some obvious deaths. However, a surprising number of these shock victims survive what would almost certainly have killed ME. I would have probably died just from the falls.
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Two things-first notice how many rescuers also got hurt. It is easy to panic-but dangerous. I hope those that did something stupid (climbing a pole) were not let off easy-especially when someone got hurt trying to undo their stupidity.
Secondly, most people experience electricity in a home, office or motel room. When you start dealing with industrial/distribution there is a whole lot more energy available. The service on a factory I do a lot of work in (relatively small industrial) could power 150 + homes. When that kind of energy escapes unintentionally bad thing happen.
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Secondly, most people experience electricity in a home, office or motel room. When you start dealing with industrial/distribution there is a whole lot more energy available. The service on a factory I do a lot of work in (relatively small industrial) could power 150 + homes. When that kind of energy escapes unintentionally bad thing happen.
Yeah, my crew once shorted a 480-volt / 3-phase line to the conduit that was being fed by a 600 amp disconnect. It literally vaporized a section of the conduit and blew the elbow apart. I was glad to be a few hundred feet away from it and on the other side of a block wall, but it still sounded like a shotgun going off. The voltage doesn't scare me so much as the peak current available to generate an arc flash. That's some serious blast potential.
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I vapourised a set of wire cutters once in my grandmother's kitchen when we were doing a refit. I was cutting the cable to her hot water heater. I had turned off the power in preparation but then had to turn it back on for something and forgot to turn it off again.
It always surprises me how much damage can be done to cutters and screwdrivers before the relatively tiny fuse wire blows.
Steve.
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The voltage doesn't scare me so much as the peak current available to generate an arc flash. That's some serious blast potential.
The strange thing is that less peak current available can actually make things worse. The aforementioned facility has 3000 amp main fused switchgear. There is roughly 17, 000 amps available for a short circuit-and the fuses will carry something like 5 times their rating for 10 seconds-so the available fault current will take 7-8 seconds to blow a fuse-a VERY long 7-8 seconds!
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I vapourised a set of wire cutters once in my grandmother's kitchen when we were doing a refit. I was cutting the cable to her hot water heater. I had turned off the power in preparation but then had to turn it back on for something and forgot to turn it off again.
It always surprises me how much damage can be done to cutters and screwdrivers before the relatively tiny fuse wire blows.
Steve.
I serviced fire alarm panels for many years. I would often ask for the power to be disconnected as I usually didn't know where the disconnect was. Once assured the power was off I had a particular screwdriver that I would firmely seat in the 120 VAC screw on the termanal strip and then lean the screwdriver to one side until I contacted the cabinet. There were a few chunks missing from that screw driver.
Some would complain that I didn't trust them. I just showed them the screwdriver. Trust but varafy
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I serviced fire alarm panels for many years. I would often ask for the power to be disconnected as I usually didn't know where the disconnect was. Once assured the power was off I had a particular screwdriver that I would firmely seat in the 120 VAC screw on the termanal strip and then lean the screwdriver to one side until I contacted the cabinet. There were a few chunks missing from that screw driver.
My procedure when disconnecting power to a circuit is to first connect a means of identifying whether the circuit is dead or live (i.e., a radio, a lamp, and possibly a helper to tell me when things go off and on). I then disconnect the power and verify the circuit is dead. I then RECONNECT the power and verify that the circuit is live, then disconnect again (and verify it's dead). This helps guard against a simultaneous failure of the test scenario and serves as a double-verification that the circuit is indeed dead.
But before sacrificing your screwdriver, you *could* meter it. And even after metering, testing with the screwdriver as you do is a good idea. So is wearing PPE (personal protective equipment) so when the screwdriver shorts you aren't injured from flying molten metal.
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To reliably confirm that both conductors are cold.. a NCVT is the best tool for that job. One of the few applications I like it for. An outlet might not light the lamp or measure voltage with a VOM if only neutral is broken.
JR