# More Phase Questions



## Shawncfer (Jun 10, 2010)

Okay okay so I know I've asked a lot of electricity questions lately. And I swear I'll stop pretty soon.

Just got three more for you right now 

Okay. So First, can Arc flash happen just from you hooking up a dimmer rack to a disconnect? Or do you have to be doing repairs on it or something else for Arc flash to occur?

Second, Lets pretend youre on tour and you're using three phase on your tour. But then, you get to a building that has single phase only! How do you hook up to the disconnect? I mean if your rack can handle single phase. And if your rack cant handle single phase, do you just tell them you cant do the show or what?

Third, Let's pretend youre on tour and you're using single phase on tour. But then you find out your venue has three phase. So I know it's possible. But do you hook up two phases and nuetral and ground and leave the third phase un-used, or one phase, nuetral, ground and two phases un-used? And if you have one of the phases that you're not using, and it's not connected, is it bad if your disconnect is on but you have nothing plugged into one of the legs?

So a little more than three questions, sorry.


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## photoatdv (Jun 10, 2010)

As for number 1, if for some reason what you were connecting the dimmers to was hot, then yes. This is why you DON'T do it hot.


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## Shawncfer (Jun 10, 2010)

photoatdv said:


> This is why you DON'T do it hot.



In other words, dont have the power on when I'm conencting?


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## SteveB (Jun 10, 2010)

Shawncfer said:


> Second, Lets pretend youre on tour
> 
> But do you hook up two phases and nuetral and ground and leave the third phase un-used, or one phase, nuetral, ground and two phases un-used? And if you have one of the phases that you're not using, and it's not connected, is it bad if your disconnect is on but you have nothing plugged into one of the legs?
> 
> So a little more than three questions, sorry.



In theory, when you prepare to tour, you first determine the venues you'll be playing and find out their technical capabilities. If you find a venue that cannot support the event, you deal with that before you set out - I.E. bring it to the attention of whomever is booking the event, promoter, etc... 

Some dimming systems can operate on single phase feed, but not the larger 48 to 96 dimmer sized systems. The control electronics on these systems will not work if they don't see 3 phase, so no amount of game playing with feeders is going to work. If you anticipate going to a venue without 3 phase, perhaps a different dimming system, such as a distributed pack system that can run on 20 amp Edison outlets, would be more appropriate. 

And, NO, you do not do live tie-ins.


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## Shawncfer (Jun 10, 2010)

So then how do you use a rack that can only use single phase on a three phase disconnect? And according to photoatdv, arc flash won't happen if the disconnect is off when hooking up the legs right? I mean its always off when nothings connected anyways, but an arc flash won't happen if I'm just connecting it like normal?


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## STEVETERRY (Jun 10, 2010)

Shawncfer said:


> Okay okay so I know I've asked a lot of electricity questions lately. And I swear I'll stop pretty soon.
> 
> Just got three more for you right now
> 
> ...



As to your second question: this is a fairly common scenario, and one that is anticipated by NEC section 520.53 (O)(1), which states:

*Neutral Terminal.* In portable switchboard equipment designed for use with 3-phase, 4-wire with ground supply, the supply neutral terminal, its associated busbar, or equivalent wiring, or both, shall have an ampacity equal to at least twice the ampacity of the largest ungrounded supply terminal.
_Exception: Where portable switchboard equipment is specifically constructed and identified to be internally converted in the field, in an approved manner, from use with a balanced 3-phase, 4-wire with ground supply to a balanced single-phase, 3-wire with ground supply, the supply neutral terminal and its associated busbar, equivalent wiring, or both, shall have an ampacity equal to at least that of the largest ungrounded single-phase supply terminal._

What this means is that if a portable dimmer rack or pack does not have an internal method of switching between three-phase and single-phase (as most large racks do not), it must have a 200% rated neutral terminal. 

Why? Because it is assumed that the road electrician, when confronted with a single-phase service for his three phase rack is not going to say "Gee, Mr. Merrick, we can't do a show tonight--we only have single-phase". He or she is going to take the two most lightly loaded phases of the dimmer rack and connect them to L1 of the single-phase service, while the third leg will get connected to L2 of the service. This is a classic "show must go on" event. This is fine, as long as the rack neutral terminal can handle the overcurrent caused by the imbalance. Any Listed rack will be able to do so.

For your third question, it is possible to connect a single-phase, 3-wire plus ground rack to a three phase service. You would simply not connect anything to the third phase of the service, as you would only have an L1 and L2 conductor on the dimmer rack. You would connect the neutral normally, as well as the ground.

As to your first question, there is a difference between _arcing_ and _arc-flash_. Arcing can occur when a connector is made or broken under load. An arc-flash is the result of an arc being drawn between phase and ground, between two or more phases, or phase and neutral, where the full fault current of the source flows into the arc--which can be tens of thousands of amps or more. Both arcs and arc-flashes are bad, but the latter is catastrophic. An arc-flash has a number of causes that are outside the scope of this discussion. As an iron-clad safety rule, portable dimming equipment should never be connected or disconnected with the source energized.

Finally, please see the Glossary entry Qualified Person, per the NEC. Realize that the NEC section 520-53 (P)states:

*Qualified Personnel.* The routing of portable supply conductors, the making and breaking of supply connectors and other supply connections, and the energization and de-energization of supply services shall be performed by qualified personnel, and portable switchboards shall be so marked, indicating this requirement in a permanent and conspicuous manner.

_Exception: A portable switchboard shall be permitted to be connected to a permanently installed supply receptacle by other than qualified personnel, provided that the supply receptacle is protected for its rated ampacity by an overcurrent device of not greater than 150 amperes, and where the receptacle, interconnection, and switchboard comply with all of the following: 
(a) Employ listed multipole connectors suitable for the purpose for every supply interconnection 
(b) Prevent access to all supply connections by the general public 
(c) Employ listed extra-hard usage multiconductor cords or cables with an ampacity suitable for the type of load and not less than the ampere rating of the connectors._

ST


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## SteveB (Jun 10, 2010)

STEVETERRY said:


> Steve
> 
> Could you clarify that Sensor CEM's and CEM+'s will operate correctly on single phase (2 hots) feeders ?, as I was under the impression that the 24 and 48 module racks will not operate in that mode, due to the CEM's wanting to see 3 phase, and that the CEM itself operates off one of the phase feeds and that if you do indeed need to operate in 1 phase mode if allowed), make sure the CEM is getting powered.


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## JD (Jun 10, 2010)

In the 80s we did a lot of small venues using a 86k EDI dimmer system. It consisted of 3 twelve channel 2.4k per chn. packs. Each of the packs was wired to run off of two legs. We simply carried two distros. The single phase distro simply supplied the packs in an AB AB AB layout. The three phase distro supplied the packs as AB BC CA. The packs themselves (Scrimmers) were laid out A 1,2,5,6, 9,10 / B 3,4,7,8,11,12. Depending on what we found, we simply grabbed the correct distro off the truck for that night. The old Scrimmers had a phase ramp generator on each 2 channel module, so it didn't really care one way or the other.

Load balance was a bit important as we rarely got the ampacity specified on the tech rider. (Often two 100a legs and a four leaf clover. System load was 48k so many scenes were modified on the fly.)


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## STEVETERRY (Jun 10, 2010)

SteveB said:


> STEVETERRY said:
> 
> 
> > Steve
> ...


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## derekleffew (Jun 10, 2010)

STEVETERRY said:


> Both Sensor CEM Classic and CEM+ control modules do not care about the phase angle of the three phases feeding them. ...


 Then why must the CEM be told whether the pack is being fed with single or 3Ø power?


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## STEVETERRY (Jun 10, 2010)

derekleffew said:


> Then why must the CEM be told whether the pack is being fed with single or 3Ø power?



In order to map the right zero-crossing circuit to the right dimmer.

Think of an SR6 layout for 3-phase and single phase

Dimmer............3 phase............Single Phase

1,2.................A.....................L1
3,4.................A.....................L1
5,6.................B.....................L1
7,8.................B.....................L2
9,10................C....................L2
11,12..............C.................... L2

So, in single-phase mode, one half of the dimmers are mapped to each zero-crossing circuit, while in 3-phase mode one third are mapped to each zero crossing circuit. This is done in software.

Note that in the unbalanced scenario where the racks stays in 3-phase mode but has A+B connected to L1 and C connected to L2--from my original post--this re-mapping is not needed because the right zero-crossing circuit is always on the right dimmer.

ST


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## Shawncfer (Jun 10, 2010)

STEVETERRY said:


> As to your second question: this is a fairly common scenario, and one that is anticipated by NEC section 520.53 (O)(1), which states:
> 
> *Neutral Terminal.* In portable switchboard equipment designed for use with 3-phase, 4-wire with ground supply, the supply neutral terminal, its associated busbar, or equivalent wiring, or both, shall have an ampacity equal to at least twice the ampacity of the largest ungrounded supply terminal.
> _Exception: Where portable switchboard equipment is specifically constructed and identified to be internally converted in the field, in an approved manner, from use with a balanced 3-phase, 4-wire with ground supply to a balanced single-phase, 3-wire with ground supply, the supply neutral terminal and its associated busbar, equivalent wiring, or both, shall have an ampacity equal to at least that of the largest ungrounded single-phase supply terminal._
> ...



Im not quite sure I understand.

And in single phase, are there two legs, a nuetral and a ground? Or just one leg?


STEVETERRY said:


> Qualified Personnel. The routing of portable supply conductors, the making and breaking of supply connectors and other supply connections, and the energization and de-energization of supply services shall be performed by qualified personnel, and portable switchboards shall be so marked, indicating this requirement in a permanent and conspicuous manner.



So if Im doing lighting for a band on tour, but I dont have enough expereince to be ETCP certified, then I cant hook a rack to a disconnect?


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## derekleffew (Jun 10, 2010)

STEVETERRY said:


> ... Note that in the unbalanced scenario where the racks stays in 3-phase mode but have A+B connected to L1 and C connected to L2--from my original post--this re-mapping is not needed because the right zero-crossing circuit is always on the right dimmer.



Sorry, I'm still not getting it. What's this zero crossing circuit of which you speak (and why does it matter)? Only dimmers 7,8 are different.


> Think of an SR6 layout for 3-phase, single phase, and "unbalanced" (3Ø rack on 1Ø service)
> 
> Dimmer..........3Ø..............Single Phase...."unbalanced"
> 1,2.................A.....................L1...............L1
> ...


Hypothetically, given a Sensor SP6 pack, 12x2.4kW dimmers, set up for 3Ø operation,
when encountering a single phase or "split phase" service,
the user has two options:

A) Re-phase the pack by _<following the manufacturer's directions>_ AND re-configure the CEM. Black cam into L1, red unused, and blue into L2.
OR,
B) "Cheat" with your "unbalanced" method by Cam-lok tee-ing the black and red together on L1 and the blue by itself on L2; and do nothing to the CEM.

Provided one had enough single phase power available, why would a user choose one method over the other?



> Members of ControlBooth are ALWAYS encouraged to keep safety as a top priority. Forums are monitored by the CB Staff, but may not be able to react immediately. We reserve the right to edit or delete posts which we feel are unsafe, or discuss unsafe practices or materials. ControlBooth shall not be responsible for damage, injury or death resulting in any information posted on ControlBooth. When in doubt, always consult a qualified professional in person.


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## derekleffew (Jun 10, 2010)

Shawncfer said:


> ... So if Im doing lighting for a band on tour, but I dont have enough expereince to be ETCP certified, then I cant hook a rack to a disconnect?


Usually, you hand your Tails or the male ends of the Cam-Lok cables to the House Electrician or other venue personnel, and (s)he does the Tie-in. Next, you verify that the stagehands have run the Feeder appropriately. Mis-matched Cam-Lok colors should be a huge warning flag. Next, with all of your equipment off, you ask that the service be energized. Once it's hot, you meter the power to ensure the voltage is correct. (Always meter the power, every time! Trust no one. No one should be offended seeing you check his/her work.) Then and only then, do you turn on your touring equipment.


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## STEVETERRY (Jun 10, 2010)

derekleffew said:


> Sorry, I'm still not getting it. What's this zero crossing circuit of which you speak (and why does it matter)? Only dimmers 7,8 are different.
> 
> Hypothetically, given a Sensor SP6 pack, 2x2.4KW dimmers, set up for 3Ø operation,
> when encountering a single phase or "split phase" service,
> ...



There are effectively three dimming engines in a three-phase control module, each synced to the zero-crossing point of the three phases. Dimmers on each phase must be "told" which dimming engine to operate from. That's the 3-phase/single-phase setup in a CEM.

Don't forget that the NEC does not require a 200% neutral in cases where the rack is designed to be converted from single to three phase. In that case, the neutral only has to be as large as the largest phase conductor in single phase mode.

So, the "cheating" mode (as you call it) should not be used if the rack can be converted. If the rack cannot be converted, there is no choice, but the neutral will be 200%.

ST


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## STEVETERRY (Jun 10, 2010)

derekleffew said:


> Sorry, I'm still not getting it. What's this zero crossing circuit of which you speak (and why does it matter)? Only dimmers 7,8 are different.



Glossary entry Dimmer, zero crossing referenced covers the zero crossing circuit.

Actually, dimmers 5,6,7,8 move as the B phase dimmers are split between L1 and L2 (A and C when in 3-phase mode).

ST


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## JD (Jun 10, 2010)

The zero cross detector (on digital control mods) starts a countdown that ends and restarts at the next cross point. The firing circuit calculates what the count should be when it fires the SSR. 

If the wrong cross detector were selected for a specific dimmer mod, that dimmer would be on at 1/3 or 2/3 brightness with the dmx on that channel being at 0. 

In true single phase, the setting would not matter as the cross point would be the same on all of the hot legs. However, in the real world, "single phase" may actually be two phase legs off of a 3 phase source. In this case, the cross detectors must be routed to the proper dimmer mods.


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## Shawncfer (Jun 10, 2010)

derekleffew said:


> Once it's hot, you meter the power to ensure the voltage is correct. (Always meter the power, every time! Trust no one. No one should be offended seeing you check his/her work.) Then and only then, do you turn on your touring equipment.



Okay, I know how to meter it. I just want to make sure Im understanding you correctly. So with the disconnect hot, are my cables not connected to the dimmers yet? Or distribution or whatever? How do I check voltage on the legs if theyre connected?

And Im still wondering. In single phase, are there two legs, a nuetral, and a ground? Or just one leg?


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## MNicolai (Jun 10, 2010)

When connecting a distro to a company switch (or similar power source), you want the cables from the distro to the company switch to be connected properly before turning on the power source. Arcs, arc-flashes, and potential for other bad situations is reduced when the conductors are mated while they are not carrying electricity. When you flip the switch, power then should safely flow as far as your distro.

After the distro, it is dependent on your particular situation. Cables like camloks should always be connected before they are turned on, be it into a distro, a portable dimmer rack, a generator, or anything else you may be connecting these cables to. It's dangerous to mate connectors with so much raw power available.

Single-phase systems have two hots, a neutral, and a ground. Each hot is 180deg apart from the other, and conventionally at 120v (but can vary). A voltmeter measuring the potential difference between either hot and the neutral should read 120v, however the potential difference between both phases is 240v. Most single-phase systems are used in houses as 120v legs. However, the advantage of being able to tap into 240v is useful for motors and large appliances such as stoves and laundry machines.

Three-phase systems on a wye-configured 120/208v transformer have five wires. L1, L2, L3, neutral, and ground. The neutral and ground are bonded together, therefore having a potential difference of 0v between them. Each hot wire to neutral will read 120v, while between phases a voltmeter will read 208v.


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## Shawncfer (Jun 10, 2010)

Okay so I wanna ask some questions with pictures

So power comes out a disconnect and into something like this right?


From there, the Camlocks daisy chain off to a dimmer rack correct? And then if there are two dimmer racks can the cam locks daisy chain off one rack into the other?

In the picture above theres a row of plus and it has A plug coming out but only one, and I have no idea where it goes. And the top row has one plug comming out, but once again I have no idea where it goes or what its for.

Then the two rows go out and connect to the sound equipment like this right?


What kind of cables are those?
Or the other two that I dont know what they're for?


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## starksk (Jun 10, 2010)

Shawncfer said:


> Okay so I wanna ask some questions with pictures
> 
> So power comes out a disconnect and into something like this right?
> 
> ...



First of all, these pictures all look like sound equipment and power distribution. 

While it is possible that the distro pictured in the top picture is also daisy-chained to the lighting rig, it is unlikely as sound guys don't like the noise that lighting can create on the line.

The top rows are edison power that sound guys use to power accessories that need to share the same ground (for noise reduction purposes). The second row that has the single connector attached looks to be a powercon connector that is used to power some powered speakers. 

The third and fourth rows look like they are twist-lock connectors, possibly running to the amp and equipment racks that are pictured in the second picture but looking at the the gender, they are either reversed, or drawing power not providing power... I will leave that to the sound boys (and girls) to answer.

The next row looks like those used by snakes that run to consoles, but again I have been away from the sound side of the industry for too long to remember exactly what use those connections serve.

Finally at the bottom are the Cams that provide and also appear to be passing the feed power through the rack. Those are the things that are the most similar to a lighting touring rack. The cable that feed those connections are usually called feeder and are quite thick to be rated for the appropriate feed and amperage needed for the equipment.

_____________

~Kirk


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## Shawncfer (Jun 10, 2010)

What does power supply for moving lights look like? Like does anyone have any pictures?

And does power go to a distro, and cam locks daisy chain to a rack, and then the different cables go out to power intels, LEDs, rotators, scrollers, and all that other shinanigans?


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## epimetheus (Jun 10, 2010)

starksk said:


> The third and fourth rows look like they are twist-lock connectors, possibly running to the amp and equipment racks that are pictured in the second picture but looking at the the gender, they are either reversed, or drawing power not providing power... I will leave that to the sound boys (and girls) to answer.
> 
> The next row looks like those used by snakes that run to consoles, but again I have been away from the sound side of the industry for too long to remember exactly what use those connections serve.



The third and fourth rows look like 20A or 30A twist locks that probably provide power to powered speakers or perhaps amp racks. The panel mount connectors do seem to be the wrong gender though for power outlets (the panel mounts look like males, which are generally power inputs). The fifth row looks like 50A connectors which would be most likely be power feeds for amp racks, though it's hard to determine what gender they are from the pictures.


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## epimetheus (Jun 10, 2010)

Shawncfer, I dig the fact that you're asking these questions! There was a time when I was asking these questions myself; I decided I liked the answers so much that I decided to get an electrical engineering degree!


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## jstroming (Jun 10, 2010)

I always hit this one convention center in the northwest that the house electrician WON'T tie in tails for you. Take a guess what that disconnect looks like after 5 years of random dudes tieing things in HAHA.

I agree with epimethius above, and am thankful the rules were changed so things like this can be addressed. Years ago (and still!) I would ask a shade-ster old roadie friend of mine, and god only knows how accurate that would be! This opens things up a bit. Happy their seems to be a common ground here where some of these topics can be addressed safely.


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## JD (Jun 10, 2010)

Most sound companies that I have known and worked with like to have everything separate; Distros, feeders, and even a separate company switch if possible! 

Outside of sine-wave dimmers, current lighting equipment is brutal on the waveform! Don't blame them myself.


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## Shawncfer (Jun 10, 2010)

So can power go to a distro and then daisy chain to a dimmer rack and then daisy chain to another rack??? And does power go from the distro to power multi paramater fixtures? Or what does it power?


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## cdcarter (Jun 10, 2010)

Shawncfer said:


> So can power go to a distro and then daisy chain to a dimmer rack and then daisy chain to another rack??? And does power go from the distro to power multi paramater fixtures? Or what does it power?



Yes, though a distro isn't always needed/used. The distro rack that is included can power anything. Usually this means your movers, but in some cases it can be your electricians phone charger (if an extra 120V circuit is open). 

The distro can usually supply both 120V (phase-neutral) power and 208V (phase-phase) power, but that depends on who made it, and what purpose it is being used for. A rental house near me has one distro that only has 9 208V circuits on it, is mounted to an old wooden board, and had cobwebs on it one time I used it.


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## derekleffew (Jun 11, 2010)

Shawncfer said:


> What does power supply for moving lights look like? Like does anyone have any pictures? ...



Here's three:

DL's Photobucket
On these models, the main circuit breaker, Cam-Lok inputs, and pass-thrus are on the back; and the branch circuit breakers, 120V convenience outlets, and 208V Socapex outputs are on the front.


Motion Labs - Power distribution and motor control systems

Motion Labs, Lex Products, and Entertainment Power Systems, are three of many manufacturers of touring distros.


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## Shawncfer (Jun 12, 2010)

So Derek,

do the passthroughs go straight into the dimmer rack? Or can they atleast? And if so, how do you know you have enough power for the dimmer rack?

Second. I know we use Socapex to run from dimmers to conventionals more for touring groups and stuff like that. But does power run from the distro to the moving lights through socapex as well? And, I've worked with some moving lights before, but do more expensive intels, (Martins, High ends, etc.) have edison outlets or are they just daisy chained for power. And if so, with what kind of cable are they daisy chained?

Third. In your picture, there are three types of cables I see. In the bottom right are camlocks, the green, blue, white, and red cables. Right? What kind of cables are those really thick ones on the left side and what do they usually power? And then what are those really skinny ones in the middle conencted to the little boxes, and what do they usually power?

Derek, I know I'm probably annoying you with so many questions. But I really appreciate the help all of yall give!


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## mstaylor (Jun 12, 2010)

Shawncfer said:


> So Derek,
> 
> do the passthroughs go straight into the dimmer rack? Or can they atleast? And if so, how do you know you have enough power for the dimmer rack?
> 
> ...


I am just catching up on this thread so I will multible responces. As far as doing tie-ins, even if you carry an electrical certification or even a license, most houses won't let you tie in yourself. As one that aps shows all the time, I require the show guy to come check it before I hot it. Many guys will say it's OK they have meters on their gear, I still insist they look first. 
How the power gets in your system is really a by-product of how your company set it up. Many times there is a distro that feeds power to various pieces of equipment. Other times it will go straight to the rack. Some dimmer racks have passthroughs to pass power along. 
What do distros look like is kinda like what do cars look like. There many flavors and depends on use and size of show. Some are small and are simply a distro to break up the power in different directions. Others have mover power on it also. Some have motor controls tied in with but that is rare. I did a show with rental gear and the motor distro was on one side and mover power on the other. Mover power can be sent through Soca but the power end has to go to straight power not a dimmer. Once on the truss most will use a Soca to edison breakout.


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## derekleffew (Jun 12, 2010)

Shawncfer said:


> ... do the passthroughs go straight into the dimmer rack? Or can they atleast? And if so, how do you know you have enough power for the dimmer rack? ...


Power may by daisy-chained using a rack's (dimmer or ML PD) pass-thrus in any order, but most put the rack with the largest load first in the chain. As for "how do you know you have enough power for the dimmer rack?", the master electrician does a load calculation. See Power for rig, Power for rig2, for examples.


Shawncfer said:


> ... Second. I know we use Socapex to run from dimmers to conventionals more for touring groups and stuff like that. But does power run from the distro to the moving lights through socapex as well? And, I've worked with some moving lights before, but do more expensive intels, (Martins, High ends, etc.) have edison outlets or are they just daisy chained for power. And if so, with what kind of cable are they daisy chained? ...


Larger moving lights are normally run at 208V. Socapex is plugged into a 208V distro and an L6-20 breakout is used on the truss. One never daisy chains power at the fixture (except for some LED units).


Shawncfer said:


> ... Third. In your picture, there are three types of cables I see. In the bottom right are camlocks, the green, blue, white, and red cables. Right? What kind of cables are those really thick ones on the left side and what do they usually power? And then what are those really skinny ones in the middle conencted to the little boxes, and what do they usually power? ...


In the picture, the 8 cables on the left vertically are the 208V Socapex for moving lights, VL3000s, IIRC. 
The thin cables in the middle are all DMX cables plugged into to opto-splitters. 
The ones in the lower right are Cam-Lok 4/0 feeder.


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## JChenault (Jun 12, 2010)

STEVETERRY said:


> There are effectively three dimming engines in a three-phase control module, each synced to the zero-crossing point of the three phases. Dimmers on each phase must be "told" which dimming engine to operate from. That's the 3-phase/single-phase setup in a CEM.
> 
> 
> ST




Steve

I'm trying to build a mental model of how this works, and what a dimming engine does. ( The last time I really studied a dimmer wiring diagram was about 40 years ago looking at a Kliegl wiring diagram). 

I think that what you are saying is that the DMX signal comes to the CEM. The CEM decides when to send the 'Switch On' signal a dimmer based on it's understanding of what phase the dimmer is getting, and where the crossing point is. IE based on the DMX value it will wait X microseconds after the crossing point to tell the dimmer to turn on. This implies that the dimmer modules are pretty simple, and the interesting logic is in the CEM. 


From a high level is this how it works or am I missing something.

Thanks


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## JD (Jun 13, 2010)

JChenault said:


> IE based on the DMX value it will wait X microseconds after the crossing point to tell the dimmer to turn on. This implies that the dimmer modules are pretty simple, and the interesting logic is in the CEM.



Pretty darn close! As the waveform crosses the 0 volt point, the ZVD produces a logic pulse and the CEM starts counting. It is the CEM that calculates how to translate various dimmer curves. The CEM outputs a firing signal which goes through an opto-isolator to turn on the SSR, so yes, there is not much going on in the power module outside of filtering and protection. 

As Steve said, there are three ZVD circuits, one for each phase leg. The CEM also takes care of mapping which ZVD pulse should be used on which virtual dimmer counter. Everything between the ZVD pulse and the firing pulse is more or less software programed into the CEM. 

In the case of ETC, line voltage is also inputed as data to the CEM so that pulse timing can be altered to compensate for fluctuation in the line voltage and keep the output at the same level.


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## Shawncfer (Jun 14, 2010)

Okay,another question. So when a dimmer rack is hooked up to three phase, its set up so that 1/3 of the rack is able to run on one phase, another 1/3 one the second phase, and the last third on the third phase. Now I get that and that makes sense. And I also know thats possible because of the ground. Now, I just dont get that concept. Like I know it does it, I just dont know why. And also, do you have to have three different grounds? I mean I know you don't, but I dont see why not. Hope someone can help...


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## MNicolai (Jun 14, 2010)

The transformers have three phases outputting on them. The neutral picks up any imbalances between the phases. If all are loaded equally, no current is driven into the neutral. The more the phases are out of balance, the more current ends up on the neutral. The more out of balance a transformer is, the less efficiently it operates; it takes more power in to transform less power out.

Transformers for dimming systems are built and designed to handle a large imbalance given that's the nature of entertainment lighting. We have no idea in a given preset if the loads will be distributed evenly or if all lights will end up on a single leg.

You do not have to have three separate grounds, just as you do not need three separate neutrals. I'll defer further discussion on grounding to someone far more knowledgeable on the matter.


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## JChenault (Jun 14, 2010)

Shawncfer said:


> Okay,another question. So when a dimmer rack is hooked up to three phase, its set up so that 1/3 of the rack is able to run on one phase, another 1/3 one the second phase, and the last third on the third phase. Now I get that and that makes sense. And I also know thats possible because of the ground. Now, I just dont get that concept. Like I know it does it, I just dont know why. And also, do you have to have three different grounds? I mean I know you don't, but I dont see why not. Hope someone can help...



I think there is a confusion here about neutral -vs- ground they are not the same thing.

While there are others on this group more able to define this, I will take a shot at it.

When you are powering any electrical device, the electrons need a complete path. In an AC system ( What is in common use today) you have the hot leg and the neutral leg. In a stage pin plug, these are the outside pins. In an edison plug they are the blades. The neutral leg is intended to be the return leg for the electricity back to the generator that produced the power. The neutral leg is designed to have electrons moving along it all the time.

The ground leg, on the other hand, is a safety device. It connects the case of the device ( say your leko) to the ground. The idea is that if your wiring in the fixture becomes frayed, and the hot wire touches the case, the electrons will travel down that wire instead of through you when you touch the fixture. The ground is not intended to carry current unless there is a problem. This is the reason that the ground pin on a stage plug, or the ground plug on an edison is just a bit longer than the hot and neutral. It will connect first so if there is a problem with the fixture it will be grounded before it is powered.

Let's look at a simplified example. Consider a dimmer rack that has three 20 amp dimmers, and is powered by three phase power. There are four wires coming into the rack. Phase A, Phase B, Phase C, Neutral, and Ground. If I put dimmer A up full, I will see 20 amps moving through Phase A, and 20 amps through Neutral, and 0 amps on Ground. If I power up dimmers A, B, and C to 20 amps each, I will see 20 amps moving through each of the phases, 0 amps through neutral, and 0 amps through ground.

_(Ok - the above is not completely accurate as the SCR dimmer adds some inductive load which is part of the reason the neutral needs to be oversized )_


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## Shawncfer (Jun 14, 2010)

JChenault said:


> If I power up dimmers A, B, and C to 20 amps each, I will see 20 amps moving through each of the phases, 0 amps through neutral, and 0 amps through ground.
> 
> _(Ok - the above is not completely accurate as the SCR dimmer adds some inductive load which is part of the reason the neutral needs to be oversized )_



Thank you first of all.

Second, How come when all three are on theres no power in the nuetral?

Third, how is the nuetral oversized. THATS what through me off. I read in the book, "Electricity for the Entertainment Electrician and Technician" that the nuetral needs to be over sized but I didn't understand why?


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## sstolnack (Jun 14, 2010)

I think there's no power in neutral because if all 3 are at equal power, they cancel eachother out because they are shifted 120', right?


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## JChenault (Jun 15, 2010)

Shawncfer said:


> Thank you first of all.
> 
> Second, How come when all three are on theres no power in the nuetral?
> 
> Third, how is the nuetral oversized. THATS what through me off. I read in the book, "Electricity for the Entertainment Electrician and Technician" that the nuetral needs to be over sized but I didn't understand why?




Let me try to explain it 

Let's start by thinking about the electron flow alternating current. In DC the electrons flow from '-' to '+'. IE if you have a flashlight powered by a battery, the electrons are moving from the '-' terminal of the battery to the '+' terminal. The electrons move in one direction only. With AC things get a bit more interesting. In AC power the direction of the electrons alternates. The electrons flow in a sine wave similar to this. http://www.controlbooth.com/forums/glossary-m-s/12227-sine-wave.html So if you have a single light which is being driven by AC power, and you look at the electrons moving in the hot leg, you will see them moving one way, reversing, and moving the other way 60 times a second. If you look at the neutral leg, you will see the same thing. IE moving back and forth 60 times a second.

For the sake of this discussion, let's call the direction of the electron flow positive if it is above the center of the graph, and negative if it is below the center of the graph. The number of electrons moving is proportional to the distance from the center of the graph. 

For simplicity sake, let's consider a single phase, three wire system. (The example is easier to follow and the same underlying principals apply to three phase). In this system you have a single neutral, and two hot legs - and each hot leg is 180 degrees out of phase with the other. IE when the electrons on phase A are moving positive, the same number of electrons are moving negatively on phase B. _Another way to think of this is to draw another sine wave moving in the opposite direction to the first. _

BeanAnimal's Bar and Grill - Electricity and the Aquarium

There are 120 volts between phase A and neutral, and 120 volts between phase B and neutral. _(And 240 volts between Phase A and Phase B - but we don't care about that now)_

Let's hook up a light to phase A that takes 1000 electrons to power it up. (It's a thought experiment ). This means that the number of electrons moving through the wire will vary from 0 to +1000 to 0 to -1000 and repeat 60 times a second. If we look at the neutral wire we will see the same number of electrons passing through it.

Now lets hook up a another light of the same power to phase B. 
From phase A we are sending to the neutral. 0 +1000 0 -1000 0 ..
From phase B we are sending to the neutral. 0 -1000 0 +1000 0 ...

So if we look at the electrons at any point passing through the neutral wire, we see that they cancel out and there are no electrons going through the wire as long as the two circuits are identically loaded.

Now three phase uses the same concept, except the picture is harder to draw and wrap your head around. IE when phase A is at +1000, phase B+C together make -1000 but it is not as intuitive.

Basic Electricity

The neutral needs to be oversized because dimmers are an inductive load. What that means is the the dimmers can 'Shift' the time of the returning electrons a bit. ( This is one of the things the choke does in a dimmer - also the filament in the lamp ). So it is possible in a system with inductive load to have more electrons going through the neutral than a single phase. We handle this by oversizing - IE we put in a wire which is physically bigger and can handle more electrons without heating up for the neutral wire.

Good questions - hope that helps. If someone who really understands this stuff better than I do has a correction please feel free to clarify.


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## Shawncfer (Jun 15, 2010)

JChenault said:


> The neutral needs to be oversized because dimmers are an inductive load. What that means is the the dimmers can 'Shift' the time of the returning electrons a bit. ( This is one of the things the choke does in a dimmer - also the filament in the lamp ). So it is possible in a system with inductive load to have more electrons going through the neutral than a single phase. We handle this by oversizing - IE we put in a wire which is physically bigger and can handle more electrons without heating up for the neutral wire.



I Understood you perfectly up until that...


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## derekleffew (Jun 15, 2010)

Just roll with it, Shawncfer. And take comfort in the fact that you are not alone.


STEVETERRY said:


> Here is some real Neutral History in the attached file. I did this around 1981. We were having weekly Code revision meetings at See Factor (boy, were we young and full of piss-and-vinegar!). Mitch Hefter said to me: "SCR dimmers can generate neutral currents in excess of the maximum phase current." "Impossible!" said I, and then went back to my shop to prove it. Oops! I proved Mitch's statement, and the gory details are attached.
> 
> This was the start of the work we did in the 1987 Code to require upsized neutrals on phase-control dimmers. I don't think many people in the electrical community were even aware of "harmonics" or "non-linear loads" at that point. ...


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## STEVETERRY (Jun 15, 2010)

JD said:


> Pretty darn close! As the waveform crosses the 0 volt point, the ZVD produces a logic pulse and the CEM starts counting. It is the CEM that calculates how to translate various dimmer curves. The CEM outputs a firing signal which goes through an opto-isolator to turn on the SSR, so yes, there is not much going on in the power module outside of filtering and protection.
> 
> As Steve said, there are three ZVD circuits, one for each phase leg. The CEM also takes care of mapping which ZVD pulse should be used on which virtual dimmer counter. Everything between the ZVD pulse and the firing pulse is more or less software programed into the CEM.
> 
> In the case of ETC, line voltage is also inputed as data to the CEM so that pulse timing can be altered to compensate for fluctuation in the line voltage and keep the output at the same level.



JD is doing fine with this, and I'm in England at the ITEAC conference, so I'll let him keep going!

ST


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## Shawncfer (Jun 15, 2010)

Okay so I reread it a couple times and was able to grasp what you were saying and I get it now.

However,

JChenault said:


> There are 120 volts between phase A and neutral, and 120 volts between phase B and neutral. (And 240 volts between Phase A and Phase B - but we don't care about that now)



So even though there are three legs, they're all connected to one nuetral, which is the same as them all being connected to their own nuetral. They just share one. Therefore each 1/3 of the rack is 120v because phase to nuetral is 120v. Right?


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## JChenault (Jun 15, 2010)

Shawncfer said:


> Okay so I reread it a couple times and was able to grasp what you were saying and I get it now.
> 
> However,
> 
> ...



(This post may be confusing as I could not get my original quote in there. )

The section you quoted is talking about single phase three wire. In this type of feed there are only two hot legs, not three. But your understanding of a common neutral is correct.


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## JChenault (Jun 16, 2010)

So there seems to be some confusion as to the 'why' of the neutral needing to be oversized. Here is my understanding. 

It's all Physics, magnetism, and the relationship between electricity and magnetism.

If you have current ( electrons) moving through a wire and the current is changing direction, the current generates a magnetic field. Similarly if you have a piece of wire moving through a magnetic field, it generates a current flow. ( This is how generators work).

In an SCR dimmer, the wave form of the output is pretty abrupt when you turn the dimmer on. The sharp start of current can make lamps sing and vibrate. ( A filament is a coiled piece of wire. When the power is applied, it generates a magnetic field which makes the filament vibrate ).

The way we get around this is with a choke. The ETC website talks about it a bit here.  (Note that I believe the page is incorrect as to the cause of the sing, my understanding is that it is not heat, but magnetism. 

The choke is a ferrite (Iron) core with a wire running around it. The output from the SCR passes around and around and ... the ferrite core. When the power turns on from the dimmer, the electrons generate a magnetic field in the choke. This field slows up the movement of the electrons and smooths out the waveform. This delays the electrons moving in the circuit, and this delay is greatest when the dimmer is at about 50%.




Remember that the reason that the neutral ( in a perfect, evenly loaded world) would not be carrying any electrons is that the positive and negative currents canceled out. If you have one phase at full ( minimal delay from the choke) and the other phase at 50% ( maximal delay at choke ) you can have more electrons passing through the neutral than through any phase.


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## JD (Jun 16, 2010)

Regarding the Neutral; 
When your dimmers are at 100% or at 0%, the neutral is caring very little current. However---- Think about how dimmers work. They work by chopping the waveform going to the load. The SSR in the dimmer is either in conduction or open circuit. Nothing between! At 100%, it is in conduction through almost the entire waveform. At 50%, it turns on halfway through the waveform. Now, think carefully about what is happening if all your dimmers are set to 33% on a three phase source. (we will assume it is a liner curve.)

While the SSRs on phase leg A are on, the SSRs on B and C are open circuit.... No Balancing is occurring!

While the dimmers on leg B are on, A and C are open circuit.

Likewise, while the dimmers on leg C are on, A and B are open circuit.

In other words, because there is no offset balance, the Neutral is conducting 300% more current then any of the phase legs. It shouldn't be a problem as all the dimmers are only at 33%, right? Unfortunately, tungsten lamps do not have a liner relationship between voltage and current draw. Because of this, the current on the Neutral actually exceeds the rated capacity. Between 30% and 50% is a real bad area, even though there is some overlap on phases past 33%. The current ripple of each phase on the neutral is occurring at a different time for each phase leg. Because of this, the ripple current is three times the frequency of the line (in America, that's 180 cps) thus the term "Triplen Harmonics." (google- Triplen Harmonics three phase wye) 

To make life real interesting, some transformers now come with Triplen Filters. Good idea? Nope! Long story short, dimmers need a solid neutral so the filter has the same effect as a loss of neutral! You won't have anything blow up, but the zero voltage cross detectors will be thrown for a loop causing all sorts of crazy problems!

EDIT: One last little sting- Since the peak power draw is occurring during the last 1/3 of the waveform, the "Power Factor" is considered to be very low/bad (dependent on the actual dimmer setting.) So, when power companies start billing based on Power Factor Compliance (see "cap & trade"), theaters may be especially hard hit on the billing end.


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