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Dimmer Rack Arc Flash

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

  1. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @FMEng Readily understood. The last theatre I worked on from the foundations up, the dry wallers would want extras for patching and relocating back-boxes and the painters would want extras for touch-up painting and these were for contemplated change orders on a building that was still an unexcavated grassy field.
    Toodleoo!
    Ron Hebbard.
     
  2. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @chausman Contactors for motors are magnetically held automatically opening upon power failure and remaining open upon the resumption of power. Contactors designed for lighting use normally latch mechanically both closed and open requiring a momentary pulse to close and / or open them on the theory that they're not powering any hazardous moving machinery and with the assumption that you'll be pleased to have your lights automatically return to whichever state they were in at the moment of power failure. Even though I reside in Canada, I was with the AV and show control contractor for a project for the British company Tussauds built as part of the towering Italian casino with the indoor and outdoor moats and gondoliers in the heart of the Las Vegas strip. The electrical PEng had a proper lighting contactor powering each ETC dimmer rack. My only responsibility was to control them via our two Crestron project wide building control stations. From memory, this involved some seven contactors in 200 and 400 Amp three phase sizes powering seven ETC racks powering various differently themed rooms on Tussauds two floors of the project. This was installed in 1999.
    Toodleoo!
    Ron Hebbard.
     
    Last edited: Apr 14, 2018
  3. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    Well of course, that's what I do. Before LED, I had manual disconnects - usually 800 amp breakers - in aux bays for dimmers, but never expected dimmers be depowered except for service. In the transition to LED I simply looked into mains contactors, as a possible means for depowering the LED receptacles. The inconvenience of having to go to an electric room before and after every use for a manual switch was not useful.
     
  4. tjrobb

    tjrobb Active Member

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    Shunt trip breaker.
    They're used for elevator shutdowns for fire sprinkler discharge, and might work here. Press a momentary button, it trips; still have to find the breaker to actually turn it back on.
    The only caveat I can think of is excessive use causing wear on the breaker bits.
    Edit: I think they also make remote-close breakers, but likely only in much larger amperages.
     
  5. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @tjrobb When Hamilton, Ontario, Canada's 2,183 soft-seater initially opened in the fall of 1973, a key operated DPDT spring-loaded, 3 position rotary switch on the Strand IDM-Q console operated a 1200 Amp 3 pole motor operated main breaker in the DSR basement rack room. The last time I actually saw this breaker it was still in use powering 3 adjacent Strand CD80 racks. It was definitely noisy, if you activated it in the silence of the empty building at 2:00 a.m. with the booth windows open and the HVAC off your could just barely hear the mechanical thunk through the concrete slab and nearby spiral stairwell.
    Toodleoo!
    Ron Hebbard.
     
  6. chausman

    chausman Chase Premium Member Fight Leukemia

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    But without a way to re-energize the rack, it'd be just as likely to be ignored as finding the breaker to begin with.
     
  7. Dover

    Dover Active Member

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    Breakers larger than 600A are normally closed by a button that releases a charged spring and they can be ordered with a motor that will wind that spring. The only problem is price, a new 1200A breaker like Ron mentions will set you back over $14,000.
     
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  8. Jay Ashworth

    Jay Ashworth Well-Known Member

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    Wait. Where's the "and when my boss found out, i got fired" ending?
     
  9. Jay Ashworth

    Jay Ashworth Well-Known Member

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    Isn't a dimmer rack a "load for which the disconnecting means must be in sight"?

    In short, even if the *breaker* is somewhere else, must there not be an unfused disconnect in the rack room?
     
  10. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @Jay Ashworth Overall it's an installation I'm less than pleased with / proud of but in that sense I escaped unscathed and the end users were fairly pleased with what they got. Construction was slow in our area when their project went to tender. The GC's and trades 'low-balled' their bids hoping to win a contract and make their profits from change orders. Far too much of my time was spent cataloging, responding to, and following the progress of (from memory) more than 50 CCN's (Contemplated Change Notices) and those were only the CCN's that affected A/V. I spent an excessive amount of time playing secretary and insufficient time supervising our on-site installers.
    From the civic owners' perspective, they ended up with more theatre than they paid for.
    When the various bids came in, they were so low, three major items listed as optional upgrades were ALL included and they still had funds left over in their budget to purchase more accessory items they'd scarcely been dreaming of.
    Optional extras they found themselves able to include:
    - A freight elevator linking their basement with their truck dock and deck level which also served to schlepp touring wardrobe crates to their second floor chorus dressing and marshaling spaces.
    - A forestage / pit / additional seating area lifted by a GALA Gagnon Laforest (Spelling?) Spiralift installation.
    - A large second floor up stage room spanning the full width of their stage (Approximately 60') by roughly 30' with windows and black out drapes on its up stage wall. This had been only a dream for them but provided space to spike out full sets for rehearsing, civic orchestras to rehearse, and proved invaluable every year when amateur dance recital season explodes in our area. Among change notices which went ahead, the A/V subcontractor I was employed by added 8 or 10 overhead 70 volt speakers from the central monitor / paging system and a 2' by 2' bulkhead at each end of the room equipped with microphone and bi-sex line level XLR's, video, data, low impedance speaker receptacles along with orange iso-ground duplexes from their dedicated audio ground power transformer and panel.
    - One more item. They were able to purchase an SD9 and stage rack to power their various Meyer line array and point source cabinets,
    You win some, you lose some. Some you'd rather not have been involved with and some just happen.
    @Jay Ashworth back to your query: This was the project I was still trying to get wrapped up and escape from when I suffered an inoperable explosion deep within my skull and went blind literally overnight.
    Guard your health @Jay Ashworth
    Toodleoo!
    Ron Hebbard.
     
  11. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    If by "must there not be" you mean "code requires", I don't think so. Lots of racks around with no visible means of disconnection within sight. I do agree there probably should be but haven't heard the defense side. I do know that sales reps try to talk people out of the aux bay with the disconnect for the rack(s). Main breakers are an option on many panels, and dimmer panels are no different.

    I don't know what makes it different from say a motorized hoist, which has to have the disconnect within sight.
     
  12. FMEng

    FMEng Well-Known Member Fight Leukemia

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    The point I tried to make a few posts back is that the next revision of code should make the "disconnect within sight" a requirement.
     
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  13. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    Submit a code change proposal. It's easy - sll on line. That's the only way to be sure it's considered. Actions are more effective than just talk.
     
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  14. eadler

    eadler Member

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    I'm guessing that the motorized hoist requires a nearby disconnect in case of an error in normal (electrical) operation of the device which may cause a safety hazard (due to the nature of being a hoist) where the normal operation of a lighting dimmer system might just turn lights on and off. Also, those individual dimmers are required to have disconnects/breakers, are they not? It's essentially a fancy load center.

    That said, my dimmer rack has a breaker feeding it one step to the right and one half step forward (on the wall), with that breaker fed from a breaker on the building's MDP.
     
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  15. Dover

    Dover Active Member

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    The general logic of the code is that mechanical things that are serviced by non electricians need a local disconnect. Purely electrical equipment such as switch boards and transformers only require that the disconnect be "readily accessible" Part of the problem arises from the fact that the LOTO procedure on a winch to service the moving parts is quite a bit different than the procedure to work on the exposed electrical equipment inside the starter. True LOTO requires that you verify that the energy source you are trying to secure is actually secured. As far a mechanical item this involves verifying that the motor can no longer start. But when it comes to actual electrical work you must check for removal of all voltage with a meter while assuming the parts are still live. The "assuming it is still live" is where the problem comes in, if there is an arc flash hazard you must suit up in the appropriate level of gear to preform that test. Given that there are likely very few theater electricians that are trained and qualified to do that presents another problem that a disconnect alone can not solve.
    That being said there are voltage indicators that can be mounted in switch gear that can be used in place of the meter test but they have to be part of a documented safety program.

    Now it is doubtful that most people will actually go through all that just to change a dimmer module but that is the proper procedure. Most will just throw the disconnect and do what ever they need to do, that works just fine until the day one of the knives sticks and leaves a hot bus in the cabinet. Now you have a situation that is more dangerous than the original in so much as you now think everything is dead when before you knew it was live.

    So after saying all that, my point of view is that this is a building management and training issue rather than an inherent safety problem with the installation. If there is some one working on the dimmers that doesn't have the needed access and training to do the job safely than there are bigger problems than an extra disconnect can solve.


    Dover
     
  16. Ben Stiegler

    Ben Stiegler Active Member

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    Computer room power disconnects work this way ... big red pushbutton by the doorframe. Ez off, not so ez on.
     
  17. teqniqal

    teqniqal Well-Known Member

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    Regardless of the legal requirement for it, I always have a disconnect or circuit breaker installed adjacent to (or in the same room as) a dimmer panel / rack / bank.

    I take it one step further and also specify that each device like a Motor Control Center (MCC), Dimmer Bank (DB), Transformer, Panel Board, Road Show Panel, or other disconnecting means has a permanent tag identifying the location of the power source by Room name / Room number, the ID of the power source (panel board designation identifier), and if it is a specific Circuit Breaker, what that Circuit Breaker Number is. Present all the information, what they do with it is their choice.

    Also of concern is that the dimmer rack manufactures do not include a way to conveniently install a Panel Board Directory like you have in a conventional Circuit Breaker Panel Board. The NEC (NFPA 70) is specific about Panel Board Directories being typed, not hand written, and having descriptions of loads that are not generic. I have the installer treat the Dimmer Racks as if they are Circuit Breaker Panel Boards and create a directory that describes the loads not just by number, but also by location (1st Electric, Beam Catwalk, DSR Wall Pocket, etc.).

    Over-kill? Maybe, but I have yet to have an end user complain about it.

    IMHO, this should be a 'standard' way of constructing / renovating venues, not an optional way.
     
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  18. JD

    JD Well-Known Member

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    I am trying to think of any good reasons NOT to have a main disconnect breaker at the dimmer rack and I can only think of one. Throwing a main breaker off when under full load is not always consequence free. Although breakers are designed to handle the resulting arc, they are not fail-safe and can fail in a dramatic fashion. In addition, there may be cases where the breaker ends up being used as a switch on a regular basis. Breakers are not switches and should not be used as one. As such, I still like the remote-trip concept as the trip would would be initiated at a distance and therefore not endanger the person hitting the button.
     
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  19. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    I understand your point John but worry about the consequences If its difficult to reset, people will just hot swap the module instead of risking the delay of resetting at some remote point.

    They do make switch rated breakers but how often can you use a breaker to de-energize a panel without a switch rated breaker being required? How often are you likely to need to kill the power to a dimmer rack? I've been in a lot of school theatres where no one even knows where it is, so I doubt they are switching it off very often. Daily to swap modules, or annually to vac it out?

    For new build, its practically a non-issue. I haven't put a dimmer rack in for quite a while - only relay racks with main breakers.
     
  20. chausman

    chausman Chase Premium Member Fight Leukemia

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    It wouldn't necessarily need to be a breaker at the rack, would it? A disconnect designed to be switched under load should do just as well.
     
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