# Dimmer Rack Arc Flash



## MNicolai

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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## venuetech

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


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## JD

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


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## EdSavoie

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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## MNicolai

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.


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## RonHebbard

EdSavoie said:


> 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...


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.


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## Patch29

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.


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## BillConnerFASTC

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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## STEVETERRY

BillConnerFASTC said:


> 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.



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


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## JD

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.


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## BillConnerFASTC

JD said:


> Can't fix stupid, but making it very easy to be safe might avoid some accidents.



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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## epimetheus

JD said:


> 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.



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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## JD

epimetheus said:


> 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.


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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## BillConnerFASTC

JD said:


> 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.


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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## JD

BillConnerFASTC said:


> 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.


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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## FMEng

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.


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## BillConnerFASTC

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.


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## RonHebbard

FMEng said:


> 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.*


 @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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## FMEng

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


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## chausman

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.


BillConnerFASTC said:


> 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.



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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## RonHebbard

FMEng said:


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


 @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.


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## RonHebbard

chausman said:


> 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.


 @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.


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## BillConnerFASTC

chausman said:


> 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.


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.


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## tjrobb

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.


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## RonHebbard

tjrobb said:


> 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.


 @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.


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## chausman

tjrobb said:


> 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.



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.


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## Dover

tjrobb said:


> 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.



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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## Jay Ashworth

RonHebbard said:


> @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.



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


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## Jay Ashworth

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?


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## RonHebbard

Jay Ashworth said:


> *Wait. Where's the "and when my boss found out, i got fired" ending?*


 @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.


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## BillConnerFASTC

Jay Ashworth said:


> 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?



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.


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## FMEng

BillConnerFASTC said:


> 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.



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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## BillConnerFASTC

FMEng said:


> 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.


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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## eadler

BillConnerFASTC said:


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


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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## Dover

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


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## Ben Stiegler

JD said:


> 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.


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


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## teqniqal

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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## JD

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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## BillConnerFASTC

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.


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## chausman

JD said:


> 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.



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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## EdSavoie

Barring a bad design, aren't almost all dimmers specifically designed to hot swap modules? (Provided you throw the module's breaker of course.)


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## RonHebbard

EdSavoie said:


> Barring a bad design, aren't almost all dimmers specifically designed to hot swap modules? (Provided you throw the module's breaker of course.)


 @EdSavoie Even if you rudely hot swap modules under full load, in theory, many dimmer modules are designed such that the shorter length control contacts break before and make after the physically longer primary and load contacts thus causing the dual-SCR's, TRIACS or IGBT's to cease conduction the moment the next zero crossing point comes along and prior to the heavy power contacts arcing under load. 
*It's a good theory for a starting pointl.* 
Toodleoo!
Ron Hebbard.


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## derekleffew

EdSavoie said:


> Barring a bad design, aren't almost all dimmers specifically designed to hot swap modules? (Provided you throw the module's breaker of course.)


Yes; BUT...

starksk said:


> TheGuruat12 said:
> 
> 
> 
> Another quick question, are the original Sensor D20s hot-swappable, or do I need to power down the rack?
> 
> 
> 
> Because of NFPA 70E, and its regulations on arc-flash potential, *we [ETC] cannot recommend hot-swapping any dimmer module while the rack is powered on*. We _strongly recommend_ that you power off the rack before swapping modules to prevent the potential for an arc-flash incident.
Click to expand...

@starksk is speaking for ETC. The modules were/are designed to be hot-swappable, but again the old adage applies: Just because one CAN, doesn't mean one SHOULD.


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## EdSavoie

I suppose this falls in the same category as moving the lift while up in the air. You shouldn't, but really, you do because the alternative is prohibitively long.


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## derekleffew

I do not, sir!


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## JD

The thing about hot-swap has less to do with the potential of damaging the module. It is more about what the user may expose once the module is pulled, such as large copper buss bars that are running off a 400 to 600 amp breaker. In someways, some of the older dimmer systems actually were designed safer. For example, still have one of those 150 pound EDI 12x2.4k SCRimmers back in my shed. The modules can be hot swapped. When you pull the module you are not exposed to any hazard. The rear of the module draw has what is basically a female stage pin connector. If you pulled a module it would become fully disconnected before you could make contact with the components in it.


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## EdSavoie

Yep, CD-80 Is the same. You'd have to be beyond stupid to get yourself killed by one.

It would take reaching your arm into the newly formed module slot, and sticking your pinky into the live socket. 

Although doing something astoundingly stupid like changing the fan while energi- Oh....


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## RonHebbard

derekleffew said:


> *I do not,* sir!


 @derekleffew @EdSavoie Certainly *never* in public *on Control Booth! * 
Toodleoo!  
Ron Hebbard.


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## Jay Ashworth

Where, teq, by "tag" you mean "sign large enough to read as you run out the door to that room"?


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