Control/Dimming Flickering Lights

One thing that every electrician I've talked to has made very clear is that the ground and neutral should only be bonded at the first shutoff.

Correct! Generally, at the main service panel coming off of your service entrance. From that point on, the ground is no longer a current carrying conductor. Any secondary bonding introduces current in the ground as it would then be in parallel with the neutral.

As to if a complete fix of the ground problem will fix the original complaint, we shall have to wait and see. Since pin 1 of the DMX is the electronic ground, usually bound to the case through a low resistance path, getting rid of ground currents is a good idea from a data standpoint.

Getting rid of neutral/ground mix-ups is ALWAYS a very good thing from a safety point of view. (To say the least.)
 
So we fixed all the Sky Cycs that had ground and neutral issues. Rehung them. No luck. Still flickering as bad as ever on the left leg. We do still have some remaining continuity across ground and neutral, so we'll keep chasing that down and continue to look for other solutions.

Despite what I said about waiting and seeing, I actually did get my hopes up that this was the solution. I just hope one of these days I can stop dealing with these technical problems and start paying some attention to the actual lighting design again.
 
for what it worth, last time I saw flickering in a very decent theater, it was because of a high DMX rate.
once I set the DMX rate from 10ms to 20ms, no flickering at all...
good luck.

++
 
for what it worth, last time I saw flickering in a very decent theater, it was because of a high DMX rate.
once I set the DMX rate from 10ms to 20ms, no flickering at all...
good luck.

Thanks. One of the first things we experimented with was DMX rate, with no luck. More importantly, we later duplicated the problem on local control from the C21 processor with all DMX sources disconnected.
 
Here's an interesting thought.....since you are working with an electrician, what is the possibility that they could swap the phases of power feeding the rack? If the problem moves to the other phase, then you know you have either a power problem or a C21 processor/power problem. If the problem stays in the same place in the rack, you have a rack/C21 processor problem.

Might be another easy test.

David
 
Found this thread quite interesting, and decided to join CB to share my thoughts.

At first I was thinking about a possible heat problem, the ambient temperature in the rack being too high. When I was in college, we had two ETC Sensor48s and a Unison mounted about 12-15' above the stage on a mechanical platform, with the roof about 8' above that. In the summer if the A/C wasn't on, it would get hot enough up there to cause the racks to overheat, and any lights on would start to flicker. This would usually start to happen when the temperature up there hit 88-90 degrees. The only way to stop the flickering was to make sure the A/C was on, and/or set up a box fan blowing on the racks. We had it checked, everything was done right, CEMs swapped out, yet we would still have the flickering problem. At my current theatre (and others that I've worked in) we have a Sensor 48 rack and its been much hotter but have never had any problems. I know this obviously isn't the issue thelatinist is having at this point, but I figured I'd share that in case anyone else has experienced that kind of problem.

Now, on to my thoughts and questions for the OP.

Do you have a Gam Chek? It would be a very useful tool for you to use for troubleshooting. You can do a lamp check on all instruments, if on one the check fails, but you know the lamp is good, I would take apart the connector and check for proper wiring. The check will only pass if the hot and neutral are connected correctly, and will fail if the neut/gnd are reversed. Use the Power Check to test all of your connectors in your outlet boxes. While it won't check if neutral and ground are reversed, you can rule out the possibility that hot and neutral aren't reversed, which could be energizing the fixture and pipe via the ground, especially since some of your instruments had the neutral/ground reversed in the pin plug. Last, you can check all of your stage cable for continuity, to make sure that hot and ground aren't reversed. I worked as ME for a show at a local college, grabbed a jumper, and was trying to figure out why the pipe was buzzing. So much of the equipment there was in such bad shape, and some moron kid decided to try to fix it and just hooked the wires back up to any screw in the connector. Hot/ground reversed. Glad I wasn't working on that instrument while the dimmer was hot.

Did you shorten the wires in the connectors where the insulation was cut back too far?

Have you tried plugging those cyc lights into a different circuit, maybe put at least one on a circuit that was flickering with another fixture, to see if the cyc light flickers?

Have you had a chance to open up the outlet boxes? Check for any loose connections, any cuts in wires? Do all the circuits share a neutral, is it loose/bad connection? Are there non-dims also in the same outlet box? Are the neutrals crossed, ie the neutrals from the dims going to the subpanel that feeds the non dims, or vice versa?

You did mention moving the outlet boxes recently, did the flickering start after you moved them, or had it been happening before that?

Did you check the wires in the lugs on the bad phase? Does it show signs of arcing? I would try what others have mentioned before, and swap the "phases" to see if the problem changes at all.
 
Okay, so we've been through and checked all of our instruments both visually and with a multimeter and have found no further wiring issues in the fixtures. However...

We used an outlet tester to test all of our stage-pin and Edison outlets. We have discovered the following:

1. Three of our stage-pin outlets have reversed polarity (hot-neutral swap).

2. Four of our Edison outlets have reversed polarity (hot-neutral swap).

3. With all channels dimmed, we're getting 104-107 volts on each of our stage-pin outlets.

We're going to get an electrician with theatrical lighting experience in to fix these issues as we're well beyond our qualifications.
 
...3. With all channels dimmed, we're getting 104-107 volts on each of our stage-pin outlets. ...
Between what and what?
"Dimmed" to what level?
Are you using a true-RMS voltometer?
And, do you have at least a 100W load on the dimmed circuit you're testing?
.
 
I'm sorry, I wasn't clear. This is with all channels at zero. We are getting 107 volts between hot and neutral on every stage-pin outlet. We first noticed that even dimmed to zero our stage-pin outlets were lighting our polarity tester. So we put a multimeter on them and found 107 volts on each stage-pin outlet. I do not know if it is a true-RMS voltmeter. We did not have a load on the circuits when we were testing them.
 
you need to have a load on when you test the output volts of the sockets, you will always get a high volts reading on no load dimmers, you absolutely must get rid of all earth currents, have you got earth/neutral swaps in the sockets, the electrician who did all this wouldn't be an electrician any more if he lived here
 
Although the Hot / Neutral swaps should be corrected, they would not induce a problem. Ground / Neutral swaps would. Time to meter the ground and neutral at the source again.


Here's an interesting thought.....since you are working with an electrician, what is the possibility that they could swap the phases of power feeding the rack? If the problem moves to the other phase, then you know you have either a power problem or a C21 processor/power problem. If the problem stays in the same place in the rack, you have a rack/C21 processor problem.

Might be another easy test.

David

Dave has got a good point here. It would take the power leg issue off the table if the results were unchanged and would allow you to focus all efforts on in-house issues. Of course, if the problems DID switch legs, then you would know the issue is not yours. Although checking lugs/bugs/ and other connections eliminates a lot, failures do happen inside transformers and circuit breakers. Not common, but time to eliminate the possibility.
 
Although the Hot / Neutral swaps should be corrected, they would not induce a problem. Ground / Neutral swaps would. Time to meter the ground and neutral at the source again.

Yes, we are aware that the Hot/Neutral swaps would not cause our flickering issue; we just found them while looking for other issues. So far the only Ground/Neutral swap we have found was in the stage-pin connector on one of our Cycs. We have tested all of our instruments for continuity across Hot/Ground and Hot/Neutral and eliminated the possibility of problems there. We have also tested all outlets for Hot/Neutral swap, Hot/Ground swap, and open Ground or Neutral. We do not have the equipment or knowledge to trace down any Ground/Neutral swap, so we'll have to get an electrician in.

Dave has got a good point here. It would take the power leg issue off the table if the results were unchanged and would allow you to focus all efforts on in-house issues. Of course, if the problems DID switch legs, then you would know the issue is not yours. Although checking lugs/bugs/ and other connections eliminates a lot, failures do happen inside transformers and circuit breakers. Not common, but time to eliminate the possibility.

Yes, that's on the list of things to have the electrician do first thing.

you absolutely must get rid of all earth currents, have you got earth/neutral swaps in the sockets, the electrician who did all this wouldn't be an electrician any more if he lived here

Yes, that's why we're going to bring in an electrician. We've done everything we know how to do, and we don't have the knowledge to search out a ground/neutral swap or extraneous bonding. As for the electrician who wired our theater (beyond the dimmer rack; the rack itself was wired by Century Lighting Service on behalf of Strand), it was not our usual electrician, but the theater contractor who designed our lighting and sound system; interestingly, he is no longer in business...
 
One simple way of checking for ground/neutral swaps is to turn on some loads on one phase leg only. This forces a bit of a neutral drop. (~ 1 or 2 volts) This voltage will appear on the neutral of all branch circuits irrelevant of the phase leg since the neutral is common. Unless there is a serious bridge still, then measuring between the building frame and the ground pin will yield 0 volts and measuring between neutral and the building frame would yield 1 to 2 volts. If bridging still exists, you may find some voltage on the ground pin but it should be lower then the voltage on the neutral pin. If they are exactly the same in some locations, then you are real near a ground/neutral short. If they are the same in all locations, then you either have a serious bridge somewhere, or there is a problem with the earth ground / building frame bonding.
 
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We're having trouble with flickering from six of our dimmer-controlled lights for the past month. The issue happens seemingly at random except that it is far more likely to happen when multiple instruments are adjusted at the same time. These instruments need not be the instruments that flicker: it seems to happen most often when we are adjusting our cyc lights using faders, and none of those channels is involved in the flickering. Sometimes it will be a brief flicker; at other times it will persist for 1-2 minutes or until we adjust the levels. There have been no electrical changes except that we moved a couple of pipe-mounted outlet boxes.

Our setup:

Strand C21 48-module rack with single C21 Processor unit
48 Strand Dual 2.4kW Standard dimmer modules
Compulite VectorOrange Control Console

We also have a programmable bypass wall panel for our house lights (channels 95 and 96), but that does not seem involved.

I have read all the posts in this interesting thread, and I have a few observations:

1. As a general rule, when the cause of a problem like this is described as "It's bad power", it usually isn't bad power. This is based on my 40-odd years of experience chasing this type of issue.

2. One of the most important clues is that the problem appears to be triggered by "self pollution" of the system, and the amount of load connected and energized. The fact that flicker occurred at repeatable levels when fading the whole rig (14 and 19%) offers another important clue.

3. The negative PQ analysis is a third important clue.

4. The fact that the entire C21 processor was replaced and the problem stayed does not eliminate the processor and its software as the source of the problem. Why? Because line-regulated phase control dimming is notoriously difficult to test under a wide variety of conditions. Sometimes, a software problem will only occur:
A. At a certain combination of phase voltages
B. At a certain combination of dimmer phase angles
C. A a certain combination of energized dimmers

The fact that your rack is running single-phase also puts it in the small minority of the universe of installed C21 racks. This increases the likelihood that you might have discovered a software problem--one that only shows up on single-phase configurations.

Now, I can hear lots of you saying "Really? Not likely!"

Let me repeat a story I have told in another CB thread that now I can't find. It's germane to this thread. About three years ago we replaced a system of about 1000 dimmers in Las Vegas. It was manufactured by one of the "big three" dimmer manufacturers, not the one in this thread. It had been running a show for many years, apparently without problems. Then a new show, with different loads and cues moved into the showroom. The system exhibited intermittent, uncontrolled harmonic self-pollution flickering failures. They were so severe that a show had to be cancelled. There was nothing wrong with the power or the rack installation. The failure was caused by a software bug that had been in the product since the first day it began shipping. However, it only showed up under a very specific combination of conditions, one that had not been duplicated in any other installation or in the product testing prior to release.

The same type of condition might be happening here--don't rule it out until you rule it out!

ST
 
The same type of condition might be happening here--don't rule it out until you rule it out!

ST

Steve
As a software engineer, I am curious how you could figure out that this was a software issue. Did you just rule out everything else, or were you able to reproduce using test data.

If you we're able to reproduce, could you share how you did it?

Just curious.
 
Steve
As a software engineer, I am curious how you could figure out that this was a software issue. Did you just rule out everything else, or were you able to reproduce using test data.

If you we're able to reproduce, could you share how you did it?

Just curious.

No guarantee that it's software--just my experience of many years with many types of regulated digital dimming systems. There are so many possible permutations of conditions that are hard to duplicate that systems are often under-tested for real-world conditions. Of course, it might be hardware too--for instance a zero-crossing detector that falls over at a particular level of THD-V, but perhaps only for a single set of voltage regulation tables.

My favorite way to reproduce this type of problem is to put the entire system, as fully loaded as possible, on the wheel-- and just fade up one bit at a time. Amazing what that exposes under serious load!

ST
 
the simplest way to check neutral earth swap is to clamp ammeter around earth cable near bond and read it off as circuits are raised.You should have no more than 30ma
 

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