6 ms·

Arc flashes, for the uninitiated, are short-circuits caused by electricity flowing through through air (partially ionized) between two live conductors. The arc

Arc flashes, for the uninitiated, are short-circuits caused by electricity flowing through through air (partially ionized) between two live conductors. The arc of electricity causes an arc blast, which is a huge fiery shockwave that can (and does) blast through metal and all surroundings

HKH2 4y ago
Like a giant welder?
- mlindner 4y ago
  More than that: https://www.youtube.com/watch?v=6hpE5LYj-CY https://www.youtube.com/watch?v=6hpE5LYj-CY
- ta2234234242 4y ago
  More powerful than that. Think molten metal going everywhere or vaporizing into the air.
  - changoplatanero 4y ago
    What happens to the vaporized metal after things cool down? Does it become dust?
    - ars 4y ago
      It becomes a metal oxide - don't breathe it. Basically it's powered rock, rocks are made of metal oxides.
      - sebazzz 4y ago
        So basically the metal is going back to its original form. You might also call it accelerated rust.
    - hexane360 4y ago
      To be more specific, when GP says 'vaporized', they mean that part of it turns into plasma, and a lot more is sprayed out as molten droplets. All of it freezes on contact with solid surfaces, often the eyes and exposed body parts of the victims. Here's a good (SFW) video demonstrating an arc flash: https://youtu.be/bPifdBpcMRY https://youtu.be/bPifdBpcMRY
- atoav 4y ago
  You are forbidden by law (and mandated to report) such arcs in Europe. In Germany people who work in such environments have to have a few special certificates and PPE that looks like they are trying to defuse a bomb. Arc flashes like that can burn with 10000 to 15000 degrees celsius and if you are close by not only will they send you flying, you are probably dead before you hit the ground. And trying to extinguish such an arc can be quite a challenge.
  - vsnf 4y ago
    > will they send you flying What is the mechanism by which an object would be propelled when hit by such an arc?
    - ars 4y ago
      The heat of the arc rapidly expands air (like a lighting bolt), the heat of the arc also vaporizes the metal conductor, and the vaporized metal expands rapidly, and enormously. It's basically a small explosion. The hot metal then ignites in air adding even more heat to the explosion.
    - mediaman 4y ago
      There is an explosive heating of air that’s like a bomb going off.
    - schoen 4y ago
      Apparently mainly air pressure: > The pressure wave created by an arc flash explosion has the force of thousands of pounds per square inch. This is powerful enough to knock down or throw nearby workers and at the same time cause damage to the eardrums, lungs, brain and other vital organs. https://www.electricityforum.com/iep/arc-flash/electrical-arc-blast https://www.electricityforum.com/iep/arc-flash/electrical-ar... (I didn't know that, and just found that explanation while looking for an answer to your question.) A different risk is that electric current passing through the body could cause muscle convulsions, which could also cause someone to move or fall suddenly or erratically.
  - Semaphor 4y ago
jfoutz 4y ago
Yes. Like this, but not intentional - https://www.youtube.com/watch?v=GMbN9nb3qyk https://www.youtube.com/watch?v=GMbN9nb3qyk That alone looks very scary to me, indoors? accidentally? gah.
- moring 4y ago
  I don't get it. There is a huge arc in that video, but no shockwave. edit: It's explained further down the thread.
noja 4y ago
Is this at all related to use of DC-power in datacentres?
- andruby 4y ago
  Afaik DC or AC can cause similar arc flashes. Typically you do need high voltages to bridge larger gaps.
  - fzzzy 4y ago
    But ac will break the current at the alternating frequency and dc won't, so it is much harder to break a circuit with a high direct current. That's why monster movies have those huge knife switches.
n0mn 4y ago
What conditions can cause this to happen, and how does one prevent it?
- albrewer 4y ago
  Basically, the dielectric constant of air is not infinite. If the electrical potential is high enough between two points, air can conduct electricity. If you put a small bit of conductor near two legs of a high voltage system, and that conductor has a path to ground, you can get now get an arc between a voltage source and the conductor (e.g., someone with a screwdriver loosening a screw is too close to a high voltage leg). This initial arc ionizes the air, which makes a pretty great conductor, so the amount of current flowing through the air increases really quickly as if it's flowing through an open circuit. The arc is superheated, can burn through steel (or flesh) quickly, and expands. One of the hazards of working on high voltage is gasping in shock after an electrical arc - you inhale superheated plasma and it cooks your lungs instantly, so you suffocate. Anyway, to prevent this, you don't cut corners as an electrician - you have proper insulation on all your tools and body parts to prevent this sort thing. You isolate legs of high voltage circuits, or de-energize the circuit you're working on with proper lock-out tag-out procedures.
mc32 4y ago
The question is what caused it? One scenario is equipment failure. On-site person was not familiar, called for back-up, back ups did not don proper PPE nor follow proper protocol. Another scenario is the equipment failed in a manner not envisioned by the designers of the protocols and were really unlucky. In any case, this is really sad and bad and hope they recover.
- gaudat 4y ago
  It may be something simple as a shor tbetween the phases. In power engineering these are called fault conditions and the currents can run up to tens of thousands of amps before the circuit breaker acts. Not sure if you can imagine the magnitude but that is why the standard PPE in working with on-line low voltage (<1 kV) switchgear is a full body silver arc flash suit. Now imagine if the datacenter guys need to wear that suit before going down the racks...
  - mc32 4y ago
    There are unknowns to us. But, no I don't imagine regular detacenter folks walking around in PPE --unless they are going in to these segregated areas to address a known issue. Maybe it was totally unpredictable --like someone just walking in proximity to said equipment just as it was about to catastrophically fail, but in most accidents there was a known bad condition, techs go in to check things out and then go back in to bring in the certified electrician or whatever and in those instances people are supposed to don proper equipment and follow certain procedure typically involving cutting power away from the faulty equipment, though sometimes people cut the wrong power or altogether forget that step in the SOP. Sometimes the diagrams are outdated --CBF is old now, I think?

HKH2 4y ago

Like a giant welder?

mlindner 4y ago
More than that: https://www.youtube.com/watch?v=6hpE5LYj-CY https://www.youtube.com/watch?v=6hpE5LYj-CY
ta2234234242 4y ago
More powerful than that. Think molten metal going everywhere or vaporizing into the air.
- changoplatanero 4y ago
  What happens to the vaporized metal after things cool down? Does it become dust?
  - ars 4y ago
    It becomes a metal oxide - don't breathe it. Basically it's powered rock, rocks are made of metal oxides.
    - sebazzz 4y ago
      So basically the metal is going back to its original form. You might also call it accelerated rust.
  - hexane360 4y ago
    To be more specific, when GP says 'vaporized', they mean that part of it turns into plasma, and a lot more is sprayed out as molten droplets. All of it freezes on contact with solid surfaces, often the eyes and exposed body parts of the victims. Here's a good (SFW) video demonstrating an arc flash: https://youtu.be/bPifdBpcMRY https://youtu.be/bPifdBpcMRY
atoav 4y ago
You are forbidden by law (and mandated to report) such arcs in Europe. In Germany people who work in such environments have to have a few special certificates and PPE that looks like they are trying to defuse a bomb. Arc flashes like that can burn with 10000 to 15000 degrees celsius and if you are close by not only will they send you flying, you are probably dead before you hit the ground. And trying to extinguish such an arc can be quite a challenge.
- vsnf 4y ago
  > will they send you flying What is the mechanism by which an object would be propelled when hit by such an arc?
  - ars 4y ago
    The heat of the arc rapidly expands air (like a lighting bolt), the heat of the arc also vaporizes the metal conductor, and the vaporized metal expands rapidly, and enormously. It's basically a small explosion. The hot metal then ignites in air adding even more heat to the explosion.
  - mediaman 4y ago
    There is an explosive heating of air that’s like a bomb going off.
  - schoen 4y ago
    Apparently mainly air pressure: > The pressure wave created by an arc flash explosion has the force of thousands of pounds per square inch. This is powerful enough to knock down or throw nearby workers and at the same time cause damage to the eardrums, lungs, brain and other vital organs. https://www.electricityforum.com/iep/arc-flash/electrical-arc-blast https://www.electricityforum.com/iep/arc-flash/electrical-ar... (I didn't know that, and just found that explanation while looking for an answer to your question.) A different risk is that electric current passing through the body could cause muscle convulsions, which could also cause someone to move or fall suddenly or erratically.
- Semaphor 4y ago

jfoutz 4y ago

Yes. Like this, but not intentional - https://www.youtube.com/watch?v=GMbN9nb3qyk https://www.youtube.com/watch?v=GMbN9nb3qyk That alone looks very scary to me, indoors? accidentally? gah.

moring 4y ago
I don't get it. There is a huge arc in that video, but no shockwave. edit: It's explained further down the thread.

noja 4y ago

Is this at all related to use of DC-power in datacentres?

andruby 4y ago
Afaik DC or AC can cause similar arc flashes. Typically you do need high voltages to bridge larger gaps.
- fzzzy 4y ago
  But ac will break the current at the alternating frequency and dc won't, so it is much harder to break a circuit with a high direct current. That's why monster movies have those huge knife switches.

n0mn 4y ago

What conditions can cause this to happen, and how does one prevent it?

albrewer 4y ago
Basically, the dielectric constant of air is not infinite. If the electrical potential is high enough between two points, air can conduct electricity. If you put a small bit of conductor near two legs of a high voltage system, and that conductor has a path to ground, you can get now get an arc between a voltage source and the conductor (e.g., someone with a screwdriver loosening a screw is too close to a high voltage leg). This initial arc ionizes the air, which makes a pretty great conductor, so the amount of current flowing through the air increases really quickly as if it's flowing through an open circuit. The arc is superheated, can burn through steel (or flesh) quickly, and expands. One of the hazards of working on high voltage is gasping in shock after an electrical arc - you inhale superheated plasma and it cooks your lungs instantly, so you suffocate. Anyway, to prevent this, you don't cut corners as an electrician - you have proper insulation on all your tools and body parts to prevent this sort thing. You isolate legs of high voltage circuits, or de-energize the circuit you're working on with proper lock-out tag-out procedures.

mc32 4y ago

The question is what caused it? One scenario is equipment failure. On-site person was not familiar, called for back-up, back ups did not don proper PPE nor follow proper protocol. Another scenario is the equipment failed in a manner not envisioned by the designers of the protocols and were really unlucky. In any case, this is really sad and bad and hope they recover.

gaudat 4y ago
It may be something simple as a shor tbetween the phases. In power engineering these are called fault conditions and the currents can run up to tens of thousands of amps before the circuit breaker acts. Not sure if you can imagine the magnitude but that is why the standard PPE in working with on-line low voltage (<1 kV) switchgear is a full body silver arc flash suit. Now imagine if the datacenter guys need to wear that suit before going down the racks...
- mc32 4y ago
  There are unknowns to us. But, no I don't imagine regular detacenter folks walking around in PPE --unless they are going in to these segregated areas to address a known issue. Maybe it was totally unpredictable --like someone just walking in proximity to said equipment just as it was about to catastrophically fail, but in most accidents there was a known bad condition, techs go in to check things out and then go back in to bring in the certified electrician or whatever and in those instances people are supposed to don proper equipment and follow certain procedure typically involving cutting power away from the faulty equipment, though sometimes people cut the wrong power or altogether forget that step in the SOP. Sometimes the diagrams are outdated --CBF is old now, I think?