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[Jonathan Barrett]

I get different answers all the time about passing cam feeder through a dimmer rack (ETC 24ch) to power up Audio or video on a separate distro. Basically Audio and lighting sharing the same service panel . I always keep Audio far away from lighting and hate doing this but does it matter just passing through? I have to do this on occasion when the gig only offers a single service for all departments.

Some say that it won't cause hum, buzz, or sag issues and some say use an Iso Transformer? What's the best practice here?

 

[Keith Broughton]

I have had many occasions where audio was powered from the same disconnect and the audio distro was "passed through" the dimmer rack.
No problem( in my case...) as long as the lighting load is not close to the capacity of the feed.

[Brian Hancock]

I'll say that depending on your power source, generator, generator feeding stepdown transformer, or grid power you may/may not have some issues with certain devices.

The way the sensor racks chop the sine wave to dim vs alternate dimmer technologies have created issues.  Certain stepdown transformers have harmonic balancing hardware that can help alleviate this ... and without going into the literally zillions of combinations of loads and equipment there is no real way to tell ... I'll say this it's only been an issue about 3 times in my 10 plus years but those times were real frustrating situations and each time we tracked the issues to a 24 way sensor rack and 3 phase power ... separated services and everything was happy again

[Tim McCulloch]

An an IATSE entertainment electrician working in arenas... I continue to be amused by the "lights and sound have to be separate" folks.  Why?  Well since you asked... at our downtown arena there are 2 primary show power vaults, each has a very large 480 delta to 120/208 wye step down transformer that feeds each disconnect switch or company switch in the vault (and a couple switches that are elsewhere in the arena).

I can hook up sound, lights, video, automation/rigging to separate switches but all the juice comes from the same secondary windings.  So far nobody has complained and I don't tell them it's all on one transformer.  Why make my day suck?  In the nearly 10 years this facility has been open we've not had any electrical service issues that were legitimately attributed to mixed use of the same transformer power.

The arena this one replaced had separate transformers for each disconnect, with separate ground rods for each.  The ground rods were not linked together to from an grounding electrode array (as Code requires) and this caused more issues that it fixed.  As this older facility was built back when Pin 1 issues were "caused by dirty power" or simply "electrical voodoo", I saw this power setup as a smoke and mirrors way of the venue addressing a problem that largely was not theirs.  The original disconnect switch installation had all the grounds terminating to one place.

That said, some older dimmers, along with variable speed (frequency) motor drives can dump harmonic content back into the neutral and that can cause a handful of problems via that neutral contamination.  The only solution for that is for those loads to be on their own transformer that does not connect to neutral on the primary side.  Mostly automation these days - I can give them a direct 480 delta connection but usually they want to run their own step up transformer (no neutral) to avoid contaminating the 480v service.

[Jonathan Barrett]

An an IATSE entertainment electrician working in arenas... I continue to be amused by the "lights and sound have to be separate" folks.  Why?  Well since you asked... at our downtown arena there are 2 primary show power vaults, each has a very large 480 delta to 120/208 wye step down transformer that feeds each disconnect switch or company switch in the vault (and a couple switches that are elsewhere in the arena).

I can hook up sound, lights, video, automation/rigging to separate switches but all the juice comes from the same secondary windings.  So far nobody has complained and I don't tell them it's all on one transformer.  Why make my day suck?  In the nearly 10 years this facility has been open we've not had any electrical service issues that were legitimately attributed to mixed use of the same transformer power.

The arena this one replaced had separate transformers for each disconnect, with separate ground rods for each.  The ground rods were not linked together to from an grounding electrode array (as Code requires) and this caused more issues that it fixed.  As this older facility was built back when Pin 1 issues were "caused by dirty power" or simply "electrical voodoo", I saw this power setup as a smoke and mirrors way of the venue addressing a problem that largely was not theirs.  The original disconnect switch installation had all the grounds terminating to one place.

That said, some older dimmers, along with variable speed (frequency) motor drives can dump harmonic content back into the neutral and that can cause a handful of problems via that neutral contamination.  The only solution for that is for those loads to be on their own transformer that does not connect to neutral on the primary side.  Mostly automation these days - I can give them a direct 480 delta connection but usually they want to run their own step up transformer (no neutral) to avoid contaminating the 480v service.

Thanks for the replies

So, question on what you said with the older facility, power issues arose from incorrect grounding arrays between the separate transformers per service panel but the original disconnects were ground terminated in one place? I see how the different grounds would be an issue, was it installed that way due to an older code standard or just done all out incorrect?

My takeaway so far is that mixing dimmer racks with Audio/Video distros (downstream a single service panel) is less of a concern then it should be if carefully investigating the service panel sources and types of dimmer racks.

Highlighted in your quote is/was my rational for not mixing dimmers with Audio/Video.

[Tim McCulloch]

Thanks for the replies

So, question on what you said with the older facility, power issues arose from incorrect grounding arrays between the separate transformers per service panel but the original disconnects were ground terminated in one place? I see how the different grounds would be an issue, was it installed that way due to an older code standard or just done all out incorrect?

My takeaway so far is that mixing dimmer racks with Audio/Video distros (downstream a single service panel) is less of a concern then it should be if carefully investigating the service panel sources and types of dimmer racks.

Highlighted in your quote is/was my rational for not mixing dimmers with Audio/Video.

The grounding scheme I described was installed almost 40 years ago when Audio Pin 1 (XLR ground) Problems were blamed on "dirty power" and "contaminated grounds" from the AC service itself.  Voodoo bullshit, we have since learned that poor device design and lack of a standard in handling audio with equipment safety grounding led to the problem.  The independent grounding scheme was a venue response to the loudest and most persistent advice of the day.  At the time it might not have violated Code because each rod was for an individual transformer secondary - a "separately derived source".

As for old dimmers and VSD controls, most of the audible problems ascribed to them went away as older audio equipment was replaced with gear that didn't have the Pin 1 Problem.  Part of that was better audio gear and part was better electrical installation and grounding practices.  I won't tell you it's problem free, I'll only say that after posting on the LAB since it's web inception, I can only recall hearing of dimmer or VSD noise once or twice.  Systems that need to be low noise, hum free (touring theatre comes immediately to mind) it's common to see a 1:1 power transformer for the audio dept - no connection to the supply neutral but the grounding connection passes through.  These are specified by the show's sound designer because it means the audio system is isolated (to a certain degree) from the venue AC service as well as any show automation drives.

My suggestion is to try and see.

[Jonathan Barrett]

The grounding scheme I described was installed almost 40 years ago when Audio Pin 1 (XLR ground) Problems were blamed on "dirty power" and "contaminated grounds" from the AC service itself.  Voodoo bullshit, we have since learned that poor device design and lack of a standard in handling audio with equipment safety grounding led to the problem.  The independent grounding scheme was a venue response to the loudest and most persistent advice of the day.  At the time it might not have violated Code because each rod was for an individual transformer secondary - a "separately derived source".

As for old dimmers and VSD controls, most of the audible problems ascribed to them went away as older audio equipment was replaced with gear that didn't have the Pin 1 Problem.  Part of that was better audio gear and part was better electrical installation and grounding practices.  I won't tell you it's problem free, I'll only say that after posting on the LAB since it's web inception, I can only recall hearing of dimmer or VSD noise once or twice.  Systems that need to be low noise, hum free (touring theatre comes immediately to mind) it's common to see a 1:1 power transformer for the audio dept - no connection to the supply neutral but the grounding connection passes through.  These are specified by the show's sound designer because it means the audio system is isolated (to a certain degree) from the venue AC service as well as any show automation drives.

My suggestion is to try and see.

Sure I can experiment but id rather understand the situation and make an educated decision. Interesting...So, I guess you could say the aversion towards mixing the two departments power sources derived from a time when equipment had poor standards and no one ever questioned it after the equipment standards improved...that's never happened before 

I see the 1:1 transformers on almost every...cough....show. Sometimes it didn't even improve the situation, but when you say it's specifically used on low noise shows how low are we talking? Ideally we would all want low noise and zero hum so is this voodoo in corporate live events or is there merit behind dragging the huge thing around?

[Tim McCulloch]

Sure I can experiment but id rather understand the situation and make an educated decision. Interesting...So, I guess you could say the aversion towards mixing the two departments power sources derived from a time when equipment had poor standards and no one ever questioned it after the equipment standards improved...that's never happened before 

I see the 1:1 transformers on almost every...cough....show. Sometimes it didn't even improve the situation, but when you say it's specifically used on low noise shows how low are we talking? Ideally we would all want low noise and zero hum so is this voodoo in corporate live events or is there merit behind dragging the huge thing around?

For corpies someone with more experience in six/seven-figure AV budget shows will have to comment on the necessity or benefits, real or perceived, of AC iso.

As I noted, the ground is either pass-thru (transformer frame) or the secondary ground is a separate run to "somewhere."  As a arena electrician I've not see that; in at least 1 touring theatrical they used a steel water pipe.  It wasn't my job to measure for a voltage differential between the 2 grounds but I recall the venue TD chatting with the touring head electrician and the A1.  I only worked that show on the load in so I don't know if they had to change it but my guess is they did not.  Was it important, needed or a placebo?  I don't know.

Low noise is in the ear of the system designer.  If an AC iso solved a noise problem once, it's going on every show that designer does and files in the 'better safe than sorry' folder.  I think your experience with "Sometimes it didn't even improve the situation..." points to that as well. In the time available to put together a big corporate event there isn't a lot of time to chase down individual sources and solutions if a preemptive "bigger band aid" *might* give the same result.

{vague historical recollection alert}
There are some genuinely odd venues or locale infrastructure that can do odd things; years ago there was an article or blog post about a Manhattan recording studio with AC-born problems that defied all conventional power engineering explanations - until they found inductive coupling between AC services buried together in the alley.  One of the services went to the opposite building and whatever they were doing there was very load-reactive and by induction wreaked havoc in the studio power.  I vaguely recall this related to Dean Jensen, but I could be wrong. {/vague historical recollection alert}

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