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Dimmer Rack Arc Flash

Discussion in 'Safety' started by MNicolai, Apr 11, 2018.

  1. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    For those who insist on servicing their racks without de-energizing, these lovely photos popped up on Reddit today.

    Supposedly someone tried to swap a fan without killing power to the rack. Guy's lucky to be alive.

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  2. venuetech

    venuetech Well-Known Member

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    So someone was trying to replace the fan? And the card was splattered from the arc above?
     
  3. JD

    JD Well-Known Member

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    Hot swapping module = not recommended.
    Changing out big awkward chunky metal fan inches from hot bus = insane.
     
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  4. EdSavoie

    EdSavoie Well-Known Member

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    Why anyone would change a metal fan that close to the hot bar while energized is beyond me...

    Bet you that guy won't ever do that again...
     
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  5. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    Think a lot of people don’t realize that this kind of incident is 1) a fraction of the disaster it could’ve been, and 2) if a live rack has all its modules stripped out or just one, the arc flash potential is the same.
     
  6. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    Optimistically he's young enough that his Mom's thoroughly tongue-lashed him, his Dad's whooped his sorry assets and he's managed to change into fresh diapers. Perhaps he's a candidate for nature's cleansing of the gene pool?
    Toodleoo!
    Ron Hebbard.
     
  7. Patch29

    Patch29 Member

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    Oh I think he was just trying to make some pyrotechnics. But I'm very glad that sad person is ok. As others said this could have been a lot worse.
     
  8. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    From a planning perspective, I wonder how convenient the disconnect was to the rack. If adjacent, obviously no excuse. If it takes a work order, several off master keys, and scheduling other people, still foolish but somehow more comprehensible. Over the years I've been criticized for including an aux bay for the dimmers, but this is why I think it's worth it.
     
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  9. STEVETERRY

    STEVETERRY Well-Known Member

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    Incomprehensibly stupid. Proves the old adage that idiots are diabolically clever at defeating warnings, cautions, and "Death May Occur" instructions. This falls into the same Darwinian group as the guy that picks up his rotary gas-powered lawn mower, uses it to trim a hedge, and chops his arm off.

    ST
     
  10. JD

    JD Well-Known Member

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    I've often thought that Breakers should have very easy access to turn off, but have very difficult access to turn back on. I don't think such a product exists (outside of large contactors) but it would be nice if there was a 3x400 (for example) that had a low voltage trip coil in it that could be remotely operated. In an application like a dimmer rack, you would flip the switch at the rack and the breaker would trip and could not be reset until the remote switch was turned back off. Such a switch could even be set up as an interlock. Can't fix stupid, but making it very easy to be safe might avoid some accidents.
     
  11. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    Exactly. No different than foh lighting positions that require some reckless ladder work to hang and focus.

    Really hard to believe that no one here has ever pushed beyond the "standard safety" limits of anything. I've sure pulled a module on a powered on dimmer rack, repatched cord and plug patch panels with hot circuits, and done some ladder work outside the normal bounds of OSHA and common sense. Hell, being in a canoe in big rollers is probably not the safest place to be. Not wearing a pfd (only in very calm and easily survivable temperature waters) is my choice.

    Note that if I was an employee, I would conform to my employer's rules. Likewise I would expect my employees - if I had any - to follow my rules - which would incorporate a lot. Good thing I work for myself.
     
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  12. epimetheus

    epimetheus Well-Known Member

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    Every major disconnect or circuit breaker manufacturer has lock out tag out devices for their products. This is common practice in the industrial world and should be in this case as well. The tech performing maintenance goes and shuts off the power, then hangs a lock and tag on the breaker or disconnect, preventing it from being switched back on without unlocking the LOTO padlock. We've enacted this practice for the company switches at my church.
     
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  13. JD

    JD Well-Known Member

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    I think the problem is that often this breaker is in place where access is difficult or a locked room. As a result, human nature is to break the rules and chance it. LOTO is fine IF the breaker is accessible and is not blocked by red tape or locked doors. A remote trip would be accessible and convenient and therefore more likely to be used. In addition, by preventing a reset, miscommunications (which is also human nature) would be eliminated. We can come up with all sorts of rules but humans are not perfect and therefor incline to break them. We put airbags in cars so that when humans make mistakes, they don't necessarily die. This is because we have learned that seat belt rules are often ignored.
     
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  14. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    I thought you suggested the reset be remote, but i would prefer the reset was adjacent to the equipment - the dimmer rack - so it could not not be reset out of sight of the rack.
     
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  15. JD

    JD Well-Known Member

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    Well, breakers are (of course) much like a mouse trap. It would only take a small solenoid inside to trip it and keep it in a disconnected state, but resetting would take a lot of push requiring a small gear motor.
    In any case, I am surprised no one (to my knowledge) has created such a product. Could be the only useful application (such as dimmer racks) are just too small a market. Still, I would think the same type of product would be useful in other applications such as HVAC.
    What keeps coming to mind are machine and and wood shops, where you have very easy access on shutting everything down via "panic" buttons.
     
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  16. FMEng

    FMEng Well-Known Member Fight Leukemia

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    Calling Steve Terry.... Why aren't dimmer racks treated by the NEC like motors, HVAC units, and other permanently connected equipment, where a disconnect switch is required to be within sight of the unit? It seems to me that his should be written into the code, if it isn't already. A simple, non-fusible, switch is all it would take. They call them "safety switches" for a reason.
     
  17. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    I'm not sure if there is an advantage to a motorized breaker over separate devices for ocpd and disconnecting, but you there are contactors available. Looked into them once to see if it would be a means to power and de-power circuits for LED fixtures.

    JD - I see why not just a big switch next to dimmer isn't satisfactory.
     
  18. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @FMEng Writing in support. A PAC constructed from the foundations up in downtown Burlington, Ontario opening in 2011 had two dimmer rack rooms, one for each theatre. The larger theatre's room housed four 48 slot / 96 dimmer racks but there was only sufficient wall space to accommodate three 400 Amp three phase isolation switches. The electrical consulting PENG was choosing to locate the fourth isolation switch in a busy secondary substation room at stage level two floors below. My immediate boss / employer was the A/V subcontractor and sternly advised me that this was none of my business and to keep out of it. I had show monitor video, audio and intercom locations in the room. I kept looking at the room and wondering why the architect had chosen to make the room with one corner chopped off at a jaunty angle rather than leaving the dimmer room rectangular like our audio rack room one floor directly below. I'm not good at watching dumb go by without a good reason. One day when the the electrical PEng was on site and had a free moment, I asked him if he'd have a problem with putting all four 400 Amp three pole isolation switches in the room with their racks. He had no problems and mentioned it would be his preference if the room was rectangular gaining sufficient wall space. Next time the architect was cruising through and had a moment, I asked him why my A/V rack room on the second floor was rectangular while the dimmer room directly above on the third floor had one side chopped in at a jaunty angle. Basically there was no good reason and no conscious thought behind the irregular shape of the room. As the block layers had yet to build up the walls in this area of the third floor and it was all just spray paint on the concrete slab, all it took to sort this was to put the right people in touch with each other and the yet to be built room was built rectangular, the bricky's were pleased to be building a standard 90 degree corner rather than two bizarre angles, the electrical PEng was pleased to locate all four of his 400 Amp isolation switches in the dimmer room immediately across from their four respective racks and the end users were delighted not to have to find their way to one of their isolation switches in the middle of the building's busy secondary electrical sub two floors below at stage level. Sometimes you can win if you point out a concern early enough, chat up all the right people at the right times and thrust them into communicating with one another.
    This nosy old blind geezer crawls back into his hole.
    Toodleoo!
    Ron Hebbard.
     
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  19. FMEng

    FMEng Well-Known Member Fight Leukemia

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    That's a cute story, Ron. These days, the general contractor would find a way to charge the client for a change order.
     
  20. chausman

    chausman Chase Premium Member Fight Leukemia

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    A giant contactor would be an interesting option. I'm fairly confident a quality local disconnect could be found cheaper and with fewer points of failure.

    Rather than one large contactor, I'd rather see each circuit switched individually. And ideally with an override on the contactor.
     

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