Dimmers and ground currents

From: snipped-for-privacy@none.com (audio=A0ferret)

I am trying to fix a sound system noise problem, and have some questions that require electrical engineering expertise. I hope that someone here can shed some light. Sound and lighting systems are installed in a large church sanctuary. The lighting dimmers are fed by a 400 amp delta-wye transformer, whose

480v feeders run in slab, under the sound mix position. The 3 phases and ground are in PVC. The distance between the transformer and service entrance is about 250-300'. With all dimmers running at 50%, we get a substantial amount of signal contamination in the mixer's mic inputs, largely at 180Hz, though other harmonics are present. When the dimmers are at 50%, we measure over 50 amps on ground, at the transformer. The transformer is also bonded to building steel. The lighting folks say that this in reasonable, and not excessive. I have read the large amounts of harmonics are byproducts of SCR dimming, and I know that the draw across phases varies constantly, because lighting conditions change a great deal through a service. My questions to you are: Is 50 + amps on ground, acceptable with regard to code? Is there a way to remediate the noise? thanks

--------------------------------------------- So, what you are saying is: when you dim the lights at church to 50% output you get too much noise from the sound system, mainly from your mic/mixer., are they Wireless ? do they distort any with no dimming on the lights...?

Have you tested it by adjusting the lighting with reference to the noise coming from the sound system ?

It may just need proper shielding & attenuation to over come the environment and ambient energy dispelled by the nearby (slab) 3ph lighting system, or more clearly, probably just relocation to a better area away from any dimmers & other electrical circuittry.

It's sounds to me like a your ground is suspect too, the problem maybe a short or dirty ground in the lighting circuits..or..a bad connection.

I've worked on church circuitry & I was amazed at the crazy things others had done to the electrical system. I found dozens of shorts and had to restore several antigue cathedral fixtures to normal op again after decades of disrepair.

more info please: like, How many dimmers ? check them one by one against the problem & you may find out more.

Do you also know, How many Amps at the Tr with no dimming and fully dimmed? Is there a substancial difference in current?

Are you using Wireless mikes ? magnetic flux from power cables will affect these if physically too close, they'll affect the harmonic signal from the mikes carrier.

Roy Q.T. Urban Technician [I don't make em, I just fix em]

I can give only general answers - but first: Something does not sound right - 50 amps on ground can make you very very dead.

Did the system ever work properly? If so, something has gone bad/been changed that causes the problem. If not, it's a design problem or one (or more) of the parts of the system is defective. From your description, one assumes the source of the noise is the dimmed lighting. Is that proven (either by all lights full on or all lights full off)?

Break the problem down into elements: Source identification (which seems to be the lights) Determine which lighting circuits affect the noise level - maybe you have a circuit or circuits that are bad Determine the point(s) and hows of noise ingress: conducted? radiated? Are you sure *ingress* is at the mic inputs? (For example, a radiated signal could come in on speaker wires and make its way back to the mic inputs)

Remedies: shielding, isolating, filtering, elimination at source

Ed

First, is there any chance you are confusing the terms "ground" and "neutral"? Is the 50 amps on ground mostly mostly fundamental vs. higher harmonic content?

You can expect small, in the wall, dimmers to be thyristor dimmers that dim by means of phase control that intrinsically generates noise from the switching operation. Third harmonic currents from the separate phases will add up rather than cancel each other in the neutral. It is possible to get filters to remove the harmonics, but that is probably an expensive way to go.

You might consider wireless microphones so that induction between long wire runs is minimized.

The real solution to your problem would be to use variable transformers or an induction regulator or some kind of magnetic amplifier device to lower the lighting voltage. These do not generate harmonics (except possibly some magnetic amplifier devices) at all. That solution may be rather bulky and expensive.

Bill

| I am trying to fix a sound system noise problem, and have some questions | that require electrical engineering expertise. I hope that someone here | can shed some light. | | Sound and lighting systems are installed in a large church sanctuary. | The lighting dimmers are fed by a 400 amp delta-wye transformer, whose | 480v feeders run in slab, under the sound mix position. The 3 phases and | ground are in PVC.

What is the output voltage of the transformer? 208Y/120 or 480Y/277? If the latter, how are you getting 120 volts for the sound system?

| The distance between the transformer and service entrance is about | 250-300'. | | With all dimmers running at 50%, we get a substantial amount of signal | contamination in the mixer's mic inputs, largely at 180Hz, though other | harmonics are present. | | When the dimmers are at 50%, we measure over 50 amps on ground, at the | transformer. The transformer is also bonded to building steel. The | lighting folks say that this in reasonable, and not excessive. I have | read the large amounts of harmonics are byproducts of SCR dimming, and I | know that the draw across phases varies constantly, because lighting | conditions change a great deal through a service.

By "ground" do you mean the wire (with white or gray insulation) that is wired up to actually carry current, or do you mean the wire (with green insulation or bare) that is wired up to provide grounding protection? At what point are you taking this measurement that gets 50 amps?

| My questions to you are: | | Is 50 + amps on ground, acceptable with regard to code?

Depends on what you mean. If what you really mean is the neutral wire, then it may well be in compliance with code given a sufficient lighting load. But the harmonics may still be a serious safety issue if parts of the system are not properly rated for it.

| Is there a way to remediate the noise?

Yes. But it may be costly. Lots more information is needed to know what the various options are. One possible option is to operate the lights and/or the sound system from a line-to-line voltage and not involve the neutral. But getting all this to be quiet on three phase is harder than it would be on single phase. If I had been consulted at the beginning I would have recommended three single-phase 120/240 volt output transformers (each connected to different 480 volt phases coming in). This would allow better balancing of lights and better neutralization of harmonic content. But transformers are expensive to change out, so this isn't likely an option, though supplementing the power system with a single small 480 volt or 208 volt (input to

120 volt (output) transformer complying with NEC article 647 might help a good bit.

There is no chance that I am confusing ground and neutral. The 50 amp current is on the ground wire.

These are not small, wall mounted dimmers. These are 3Kw theatrical dimmers:

Replacing wired mics, with wireless, is impractical on this scale. I have over 130 channels of mic lines between the stage, house, monitor, and recording splits.

Thanks, but I am not sure that replacing the dimming system is practical.

The lighting is indeed a source of the noise. Yes, it is has been tested with dimmers at full, 50%, and off.

We had hoped to find that a hot and neutral had been reversed at the lighting pin connectors, but that was not the case.

We have the ability to disconnect the console from the amplifier rack and related equipment, and still had noise show up at microphone inputs.

Seems to me that the only way to shield from local EM would be to put feeders in rigid conduit. That isn't going to happen.

The sound system is fed by a separate transformer.

Green wire ground. Measured at the Delta/Wye transformer.

I do not mean neutral wire. This is on the GREEN wire.

That's an interesting idea. Thanks.

This is an indication of serious wiring problems. You really need to solve that problem before others.

It is not immediately clear what technology is used for the actual dimming. I am not interested enough to go on a wild goose chase. The site seems to be mostly puffery.

Bill

These are SCR dimmers. The site does contain data sheets and manuals, but they are a bit of chore to retrieve.

In wall or not, SCR's at half intensity means noise!

Have you found out how much current is in you neutral? Have you looked at the waveform of your "ground" current?

Bill

Dimmers create harmonics. If the entire load on that transformer is dimmers at 50%, well, others have discovered that a neutral wire must be sometimes double the size required by code due to harmonic and reactance problems. But this should make it obvious. A substantial voltage increase would be seen on one or some 300+' wires between transformer and load side.

Second, get a field strength meter to learn what those electric and magnetic fields are above the pvc pipe; measured with all lights off, 100% on, and 50% on. When they worried about childhood leukemia from electricity, it was not those big transmission towers that created the hype. That hype was created by local newsreporters who never bothered to first look at the numbers. Threat was fields from wire such as the

400 amps inside that pvc conduit. What are your wires doing especially when dimmers create so much harmonics? That may explain sound system problems.

As others have noted, 50 amps on the ground wire is well more than excessive. Measure for significant voltage increases between both ends of each wire in that 300' pvc conduit. These problems can be aggravated by harmonics and reactive loading that is largest when lights are 50% dimmed.

50 amps > I am trying to fix a sound system noise problem, and have some questions

50 amps on the grounded (green) conductor spells big trouble to me.

• As in A Danerous Ground Fault ..!.. Some where they must have used a Ground for a Neutral., Perhaps on a segment of Lighting Circuitry or to an Outlet Circuit with some small resistive appliance connected to it.

Just Where is this Church Located ?

Roy Q.T. Urban Technician [I don't make em, I just fix em]

The web page mentions different rise times available. Can this be adjusted by you, and if you adjust it does it make any difference?