A New Stage!

[venuetech]

I perhaps should have said she refused to take responsibility for the event and was able to convince the principal that it was the student who was at fault, not the instructor. The students were all very capable young folk. But the teacher had almost no technical knowledge. Artistic yes.

 

[BillConnerFASTC]

Drop crescent wrench in CD80 rack, end up with chrome plated bus bars.

 

[BillConnerFASTC]

Well as a student I rode arbors to and from the loading bridge, never used personal fall protection anywhere, and worked with celastic (?) and acetone with bare hands, among other now long banned activity and tasks. My fault or the teachers? Who knows. As a college teacher a year or so out of grad school a student was working on a prop that included a light, switch, and plug. I didn't look at it. he plugged it in and all good. Flipped the switch - it was wired in parallel with the lamp - instantaneously the switch disintegrated in all directions. No physical injuries or property damage - besides the 49 cent switch. Paraphrasing Peter Benchley from Jaws, the phrase for the year was "just when you thought it was safe to come back to the theatre..." My fault for thinking a 20 year old could wire a simple circuit? I don't know. I don't think I would let a student clean a dimmer rack, but probably more out of concern for damage to the dimmers than injury to the student if you asked me why I said no to the students offer.

 

[EdSavoie]

I don't think it's inconceivable for a student to pull out the card cage and drop a screw by the back.

Student reaches in for the screw with one hand Student leverages other hand on the frame where the cage was and accidentally touches one of the phases while reaching for the screw.

I don't think there would be much student left...


From that perspective, yes. It is more than possible for it to be an issue. Given the knowledge and supervision available on site, would it happen? No, not really.

 

[RonHebbard]

Well as a student I rode arbors to and from the loading bridge, never used personal fall protection anywhere, and worked with celastic (?) and acetone with bare hands, among other now long banned activity and tasks. My fault or the teachers? Who knows. As a college teacher a year or so out of grad school a student was working on a prop that included a light, switch, and plug. I didn't look at it. he plugged it in and all good. Flipped the switch - it was wired in parallel with the lamp - instantaneously the switch disintegrated in all directions. No physical injuries or property damage - besides the 49 cent switch. Paraphrasing Peter Benchley from Jaws, the phrase for the year was "just when you thought it was safe to come back to the theatre..." My fault for thinking a 20 year old could wire a simple circuit? I don't know. I don't think I would let a student clean a dimmer rack, but probably more out of concern for damage to the dimmers than injury to the student if you asked me why I said no to the students offer.

But you're alive today to portage your canoe and train others so all's well that ends well so long as you continue to be careful.
Toodleoo!
Ron Hebbard.

 

[Malabaristo]

I perhaps should have said she refused to take responsibility for the event and was able to convince the principal that it was the student who was at fault, not the instructor. The students were all very capable young folk. But the teacher had almost no technical knowledge. Artistic yes.

Sorry if my comment wasn't clear: I was very much criticizing the teacher's attitude, and not your description of it.

I'm a strong believer in pushing students outside their comfort zones by giving them challenging tasks and letting them learn from their mistakes. I also strongly believe that it's my responsibility as the adult in the room to ensure--to the best of my abilities--that those mistakes don't involve too much potential for serious injury or significant damage. That means I have to know as much as possible about what the risks are and how to mitigate them. Anyone who doesn't feel similarly should not be allowed any responsibility in a setting as dangerous as a theatre.

 

[tjrobb]

I worked at a hotel with several Unison racks. There were spaces where the lights must stay on (no sunlight, 24/7 facility). I found that putting dimmer modules into a rack live while standing as far to the side as I can (modules off, mains on) makes a good yoga pose. It was the only conceivable way for me to be at least partway out of a potential blast. Other than that rack, screw 'em, I drop the mains; your aesthetic isn't worth my life.

 

[TJCornish]

I don't think it's inconceivable for a student to pull out the card cage and drop a screw by the back.

Student reaches in for the screw with one hand Student leverages other hand on the frame where the cage was and accidentally touches one of the phases while reaching for the screw.

I don't think there would be much student left...

While there's no doubt reaching into a live dimmer rack is dangerous, it's worth pointing out the difference between a shock and an arc flash. A metallic object - even one as small as a single strand of wire from a flexible cord - has a very small resistance. This can create an arc, where if enough current is available, the air between the bus bars ionizes and several thousand amps of current (depending on the distance from distribution and other factors, but a number several to many times higher than the rating of the breaker in the panel or device in question) turns everything metallic into an exploding fireball. In this case, there is a good chance that not much would be left of our hypothetical student.

The scenario you describe is different. Humans have a moderately high resistance, dependent on skin moisture level and the distance between point of entry and point of exit. This resistance is high enough that no arc is possible, and therefore there is no chance of an arc flash. As to the survivability of our student touching a bus bar - unless this is a high voltage panel, touching the bus bar will deliver no more energy to our poor student than if said student touched the hot wire of a receptacle. There's no doubt that a 120v shock across the heart can be deadly, but death is far from a sure thing, and even if death occurs, our student - though dead - would be fully intact.

 

[EdSavoie]

Yes, you are right that the student in that scenario would be intact in the scenario I gave. (Though the skin of the hands might be a tad crispy right?)

If that same student dropped a wrench between case and busbar, then the result might be a rather melty arc flash, correct?

 

[TJCornish]

Yes, you are right that the student in that scenario would be intact in the scenario I gave. (Though the skin of the hands might be a tad crispy right?)

If that same student dropped a wrench between case and busbar, then the result might be a rather melty arc flash, correct?

I grabbed a volt meter and did a resistance measurement between pinched fingers on each hand. The lowest value I got including licking my fingers was about 300,000Ω. Doing the math using Ohm's Law, this corresponds to a 0.4mA current flowing through my body at 120v, which works out to 0.048 watts. That's not remotely enough energy to cause burns. Using the 300Ω value this article indicates as the internal resistance of our body which would be a worst case scenario, that means 400mA would flow through the body, which would be 48 watts of power. Over time and a small enough surface area that could eventually cause burns I suppose, but it is very unlikely for a person who was standing up when shocked to remain in the current path for more than a fraction of a second, so it seems unlikely enough heating could be done to cause burns. 400mA of current is more than enough to be a big problem though - GFCI breakers trip at 5mA, which is considered reasonably safe.

Yes, a wrench between anything hot in the rack and anything grounded would very likely cause an arc flash. For comparison, Wikipedia's Arc Flash article gives an example of a 480v arc flash with 20,000 amps available for 10 cycles as 1,600,000 joules worth of energy, equivalent to 0.8 pounds of TNT blowing up literally in your face, assuming you're not wearing an arc flash suit. A 120/208v panel would be somewhat less than that, but for comparison, the worst case shock example above is about 8 joules of energy (assuming the same 1/6 second timeframe).

Both arc flashes and shocks are bad news and both can be deadly, but they are very different scenarios.

Edit: For what it's worth, I find YouTube at least as scary as any horror movie. There are a number of electrical deaths caught on camera for your viewing if you have a strong stomach and wish to be motivated to be more careful.

The scariest picture I've seen in recent memory is the remains of an AV tech's arm after he disconnected a camlok connector that was still under load. Be careful out there - appendages don't grow back, folks.

 

[EdSavoie]

Well, there's my learning for the day!


Yes, I've seen some of the videos for arc flash, not pretty...

 

[RonHebbard]

Well, there's my learning for the day!


Yes, I've seen some of the videos for arc flash, not pretty...

You'd be better in the long run to make that your learning for life although I know what you meant by your figure of speech.
Toodleoo!
Ron Hebbard.

 

[Dionysus]

Well, there's my learning for the day!


Yes, I've seen some of the videos for arc flash, not pretty...

I've SEEN arc flash (also have my arc flash certification), trust me NOT PRETTY.